© copyright mla 2003(with edits by hutton oddy, une, 2006) · web viewlearnt more about the...

13. Lean Meat Yield© Copyright MLA 2003

(with edits by Hutton Oddy, UNE, 2006)

Learning objectivesAt the conclusion of this unit you should:

Understand the relationship between lean meat yield and the profitability of prime lamb businesses.

Have learnt the practical measures that the sheep industry uses to assess lean meat yield, Learnt more about the application of genetics and management of growth through nutrition to

manage a lamb enterprise to comply more closely with market specifications.

13.1 Introduction

How does the lamb industry assess and improve lean meat yield?This unit is designed to help you to understand the relationship between lean meat yield and the profitability of a prime lamb business. It aims to provide you with the necessary skills to understand lean meat yield, analyse feedback and to begin to identify methods for improving lean meat yield and the effects of yield on profitability in a prime lamb business.

These aims will be delivered through the following steps:

Describing Lean Meat Yield, its impact on profitability and its importance to your business. Identifying and understanding the key factors that influence Lean Meat Yield. Analysing carcase (weight and fat) and VIAscan® feedback sheets. Calculating differences in value between animals which meet target market specifications and

those which fail. Identifying the effect of genetics on animal performance and Lean Meat Yield. Identifying the nutritional needs of different classes of animals and the influence of nutrition on

growth rates and Lean Meat Yield. Developing a strategy to analyse feedback for your animals and listing possible methods for

improving Lean Meat Yield and profitability in your production system.

13.2 Impact and importance of lean meat yield

As the Australian prime lamb industry has developed, there has been a steady move towards the adoption of Value Based Marketing systems. The use of weight and fat grids in Over the Hooks trading is now widespread throughout the industry. Performance feedback provides producers with information that can be used as a basis for improved management decisions. The introduction of VIAscan technology has given abattoirs the opportunity to accurately predict the Lean Meat Yield of carcases. This provides new opportunities for producers through the provision of more detailed performance feedback. Increasing Lean Meat Yield is desirable from an industry view point because it will contribute to increased national output and value; for individual enterprises it will assist in improving enterprise productivity.

ANPR420/520 Sheepmeat Production and Marketing - 13 - 1©2009 The Australian Wool Education Trust licensee for educational activities University of New England

13.3 Understanding lean meat yieldHow is Lean Meat Yield (LMY) different from saleable meat yield (SMY) and dressing percentage?Cutting specifications vary within boning rooms and between individual companies. To provide industry uniformity and understanding, the concept of a ‘standard yield’ was investigated. Lean Meat Yield provides an easily understood, transparent and comparable industry standard.

The following definitions are important.

Lean meat yield (kgs): This is the weight of all muscle tissue (lean meat) once it is separated from the fat and bone components of a carcase using basic commercial techniques.

Lean meat yield (%): This is the Lean Meat Yield (kgs) expressed as percentage of the initial Hot Standard Carcase weight.

Saleable meat yield (kgs): This is the weight of saleable meat produced from a carcase, to a set of specifications. Specification sets vary greatly, even within a boning room, therefore yield comparisons are only relevant within a specification set. Because of this great variability, Saleable Meat Yield is NOT used for VIAscan predictions.

Dressing percentage: This is the weight of a dressed carcase expressed as a percentage of the liveweight of the animal from which it was processed. THIS IS NOT YIELD! ‘Dressing percentage’ is useful for the pre-slaughter estimation of carcase weight. Carcase weight can be estimated by applying ‘dressing percentage’ to live weight Through the analysis of feedback, producers will get to know and understand the ‘dressing percentage’ of their own animals.

Lean meat yield expressed as % or kgs: It is easy to be misdirected by focusing too much on Lean Meat Yield (%). Lean Meat Yield (kgs) will generally deliver greater value to producers and add value to the industry. To further understand this, refer to Table 13.1.

Table 13.1 Source: MLA, (2003).ean Meat Yield

13 - 2 - ANPR420/520 Sheepmeat Production and Marketing©2009 The Australian Wool Education Trust licensee for educational activities University of New England

13.4 Why have customer specifications?Modern consumers are sophisticated and discerning. They require consistently tasty and tender lamb products that will provide a wide range of meal options and product choices. They expect to purchase leaner and more versatile products that represent good value for money.

People are eating differently today:

They don’t always sit down at a table to eat. They don’t always use a knife and fork. Many don’t separate eating from other activities – they eat whilst working, watching television,

when driving and when walking down the street.

In Australia today:

It is estimated that 30% of all food is consumed outside the home. It is claimed that about $1 million per day is spent on diet related products (this includes fitness

and exercise programs). At 4:00p.m. in the afternoon, a majority of food shoppers don’t know what they will be eating for

dinner that night. Half of teenagers accompany an adult for grocery shopping at least once a fortnight and half of

these influence brand or food choice.

Consumers will ignore products that fail to meet their expectations.These consumers include retail, export, foodservice and fast food customers.

Consumers want high quality, more satisfying productsConsumers require tasty and tender lamb. Modern consumers have a much wider range of choices and if the product they want is not available they will choose another.

Consumers want less fat: Consumers are clearly indicating that they require foods which they perceive to be healthy and that excess fat on lambs is unacceptable.

More lean meat and more versatile and convenient products: Modern lamb cuts require larger, leaner carcases because they are often produced from single muscles or two muscle groups (Trim Lamb Topsides, Rounds, Rumps etc) rather than multi–muscled cuts such as a traditional leg of lamb.

High value export markets are emerging for leaner, meatier cuts: Markets in North America, Asia and Europe need larger and leaner lambs to provide the carcases which best suit their product requirements.

Profitability: Consumers are willing to pay more for the products which they most desire. Producing large, leaner lambs is more profitable than producing fat lambs.

Measuring lean meat yieldLive weight and animal fatness are the prime indicators of Lean Meat Yield in animals and carcases. Good live animal assessment skills will result in accurately estimated carcase weights, fat scores and skin values. Accurate assessment skills require the use of live weight scales and good fat scoring (palpation) technique.

WeightCarcase weight contributes more to Lean Meat Yield than any other attribute. Carcase weight can be determined by knowing the live weight and understanding and knowing dressing percentages.Live weight can be accurately measured using a modern set of lamb weighing scales.

FatAfter, carcase weight the most significant influence on Lean Meat Yield is fatness (or leanness!)


13.5 AUS-MEAT fat classesA simple (1-5) scoring system is used to estimate the amount of fat cover on both the live lamb and carcase. Fat scores vary from 1 (leanest) to 5 (fattest). Scores are based upon the tissue thickness (both fat and lean tissue) at the GR site. Assessing fat score, “over the fence”, without handling lambs can be misleading. For successful live animal fat scoring, have the lambs standing in a relaxed state, preferably in a race, small pen or on liveweight scales.

GR SiteThe GR site is 100mm from the carcase midline over the 12th rib. This site is used as a reference point because it is easy to measure on the hot carcase and provides a good indication of the overall fatness (and thus yield) of the whole carcase. On the slaughter floor, GR is usually measured with a ‘cut and measure’ knife or AUSMEAT probe. Fat class is sometimes determined by manual palpation.

Figure 13.1 Source: MLA, (2003).


13.6 VIAscan measurementHow does the VIAscan Sheep Carcase System work?

The VIAscan Sheep Carcase System is a fully automated system which is located at a fixed station on the slaughter floor.

Each system consists of:

• A colour video camera which captures images of carcases at line speed. • A computer that receives, stores and processes these images. A series of linear measurements and reflected light components are processed to determine

the proportional dimensions, fat density and fat distribution of each image.

These elements are then analysed and computed to determine a VIAscan predicted Lean Meat Yield (LMY). These computations are based on a series of cutting trials that included a wide representation of breeds, crosses and gender. In these trials, the bone-out and yield data capture procedure was based on a traditional three-way carcase cut – forequarter pair, loin pair and leg pair. This has allowed data to be analysed for each primal as well as for the whole carcase. Loins were further separated to provide precise detail on rack and eye of loin performance.

As well as traditional methods of delivering feedback such as hard copies mailed or faxed, VIAscan is able to deliver yield based feedback via the Internet. VIAscan is currently the only tool available to predict lean meat yield. High yielding carcases (low waste) improve the efficiency for all participants in the marketing chain. Because meat quality implications in a yield based system must not be overlooked, the system has the capability to discriminate carcases of poor conformation and insufficient fat cover (Fig. 13.3).



13.7 Managing feedback from abattoirs?Six steps to using feedback

1. Maintain a feedback file: File feedback sheets in a ring binder or on a spreadsheet.2. Analyse the feedback.: Calculate performance indicators such as percentage compliance with

specification, average fat score, average weight, average LMY (kg), margin (selling price per kilogram minus cost of production).File these calculations in a binder or on a spreadsheet.

3. Compare data against past performance.: Refer to your files to track improvement.4. Use your feedback as a guide to change your breeding, feeding and management

activities if necessary.: Using your feedback analysis, develop suitable strategies to improve5. performance.6. Utilise support information to assist with your management decisions.: Use resource

documents and/or undertake training to reinforce your capabilities.7. Use this information as a basis for increasing financial returns to your operations:

Analysing and using feedback will enable you to benchmark and improve your carcase weight, yield and overall performance.

13.8 Managing lean meat yield on farmLThe key tools for improving lean meat yield are:

• Improved genetics. • Improved management of feed supply and growth. • Better animal health. coupled with feedback.

The effect of genetics: Growth rate, muscle development, fatness and therefore lean meat yield are heritable. Genetically faster growing and leaner rams and ewes breed faster growing and leaner lambs. Faster growing lambs also reach heavier weights earlier.

A question frequently asked by producers is; “Which breed is best for yield”?The politically correct answer is that no one breed is better or worse for lean meat yield. However, in a breed comparison conducted by Meat and Livestock Commission in the UK (Kempster et al, 1987) the Texel breed had an average lean meat yield greater than the other breeds compared at that time. Some of the key breeds used in Australian prime lamb industry were included in that trial (with the exception of Merino). Since then there has been intense selection for growth and yield traits in meat sheep (in Australia and around the world) and it is generally thought that the variation in growth, muscling and fatness within a breed is equal to or greater than the variation that exists across breed types. There is no doubt that improvements in lean meat yield have resulted from careful selection and breeding from animals superior for growth and muscle and against fatness within each breed.

Seedstock breeders (studs) are under significant pressure to improve the elite animals that they can provide to clients to improve lean meat yield. Single trait selection for growth, leanness or muscling independently offer very little for the seedstock breeder aiming to improve yield. Accordingly, sheep genetic improvement programs (in Australia, LAMBPLAN) provide a selection index that contains a balance between growth (weight for age) and eye muscle depth and fat depth measured at the C site. Generally, selection for improved growth and leanness will give the greatest response in lean meat yield. Lambs sired by rams with higher growth will be proportionally leaner than other lambs when compared at the same age and weight. They will also have a higher yield of lean meat than lambs bred from lower growth sires. This suggests that lambs sired by rams with higher Estimated Breeding Values (EBVs) for growth may be leaner and will have higher lean meat yields, at the same age and weight, compared to lambs sired by rams with average or lower growth EBVs. Selection against fat at a constant weight, using LAMBPLAN EBVs for fat, will improve lean meat yield. The addition of Eye Muscle Depth (EMD) to an index that contains growth rate and fat score information will lead to a minor increase in yield, but will give significant improvements in eye muscle area and loin weight. Selection for EMD alone will not improve LMY.


As described above, lean meat yield (LMY) is a measure of the total meat; it is not a measure of where that meat is located or what the value of that meat will be. For this reason, lamb breeders are also conscious of the need for improved muscling and location of the muscles within the carcase (sometimes referred to as shape). This is particularly important for the loin from which the higher priced cuts are derived, more so than the hind quarters and forequarters.

Key point: Higher lean meat yields will result from a combined approach of selecting for higher growth, leanness and improved muscling. (Kempster, et al., 1987)

Maternal sires. The dam provides half the genes of every prime lamb (and everything else as well). There is considerable genetic variation in maternal breeds and this contributes to the variation in growth, muscling and fatness that contributes to variation in lean meat yield. Genetic improvement of the dams is an important source of systematic improvement in meat yield. The traditional maternal breed for the Australian prime lamb industry is a “purebred” Merino or a first cross from the joining of Merino ewes to a specialist maternal sire such as Border Leicester. Industry is increasingly experimenting with different breeds of maternal sires (including East Freisan, Coopworth and dual purpose Merino). Within Australia, a Maternal Central Progeny Test (MCPT) has been used to derive genetic parameters for Prime Lamb dams and to illustrate the benefits of selection for maternal traits that contribute not only to reproductive success (a key profit driver) but to the component traits that affect lean meat yield (growth, eye muscle and fat depth) of their progeny.

The selection indexes that are used by maternal breeders using LAMBPLAN have appropriate weighting on growth, leanness and muscling to improve yield, as well as placing pressure on improving maternal performance and reproductive ability.


Lean meat yield is influenced by a combination of the animal’s genetics and the environment in which it is grown.

Therefore: Performance = Genes + Environment (P = G + E)

This is a simplified form of the statement that phenotype = genes + environment. Here phenotype is the measured performance of the offspring of the sire and dam. You will have learnt the detailed principles that underpin this statement in previous genetics courses, and the concept is presented here only as an illustration. Performance is what you can see or what you can measure.


For example: Carcase weight in kilograms, fat and eye muscle depth in millimetres.

Genes in the above equation refers to the inherited contribution passed to the progeny by both the sire and dam. Environment means the environmental and management effects that are not passed onto progeny.

For example: birth type- single or twin, date of birth, seasonal feed conditions.

If we correct performance for these environmental effects we are left with genetic effects. These are animal traits that can be measured and calculated into EBVs.


Estimated Breeding Values (EBVs)This is presented as a simplification of the previous notes provided in Module 3 (Genetics and Breeding) and is included here for completeness. LAMBPLAN information is provided in the form of Estimated Breeding Values (EBVs). These provide a simple picture of the value of an animal’s genes for a production trait. EBVs are based on the animal’s own measured performance and that of its relatives. For each trait the EBV is shown as a positive or negative difference from the breed base which is set at zero. The average EBVs for different traits change over time as a breed makes genetic progress. Current breed average and top 20% EBVs are therefore more relevant for benchmarking than the zero base.

LAMBPLAN EBVs for three commercially important traits:

Post weaning weight (pwwt)Weight EBVs describe the animal’s genetic merit for growth rate (post weaning weight EBV is calculated as weight corrected for a standardized age, in this case post weaning or approx 225d after birth although the measurements may be taken from 5 to 11 months). The higher an EBV, the greater the genetic potential of that animal to grow quickly. This will mean that animals with a higher weight EBV will be heavier at a constant age.


Post weaning fat depth (pwfat)Fat depth EBVs describe the fat depth of an animal at constant weight. A negative EBV means a genetically leaner animal. Measurements are taken by accredited LAMBPLAN scanners, using ultrasound technology. The units are millimetres. The EBV for post weaning fat depth is adjusted to a constant liveweight of 45 kg. A reduction in 1mm pwfat EBV is estimated to increase Lean Meat Yield by 0.5% - 1.0%.

Post weaning eye muscle depth (pwemd)Eye muscle depth EBVs describe each animal’s genes for eye muscle depth at constant weight. A positive EBV means a genetically heavier muscled animal and one that will have slightly more of its lean tissue in higher priced cuts. The units are millimetres. EBVs are calculated at constant liveweight of 45kg. Eye muscle depth and eye muscle area are highly correlated. Accordingly measurement of eye muscle depth is an effective estimate of eye muscle area.

Single trait selectionLean meat yield is less sensitive to selection for eye muscle depth, than to fat thickness. This is because the muscle and fat depth EBVs are presented as deviations from the mean and in units of mm. The mean values for PWEMD and PWFat are typically 2-3 fold different. Typical mean values are 15mm for PWEMD and 5 mm for PWFat. Accordingly, if we compare two rams with post weaning eye muscle depth EBVs of -2 and +2, this will give a difference of 2mm, which will only give a 1.2% increase in LMY (at the same PWFat EBV).However, by reducing the fat EBV from +2 to -2, this will give a difference of 2mm of fat at the GR site, which will in turn increase LMY by 1.8% (at the same PWEMD EBV).If selection were based on both traits equally and simultaneously (PWEMD and PWFat) it is possible to increase LMY by the sum of the contributions of each (1.2% + 1.8% = 3%).

How are EBVs calculated?



How are EBVs incorporated into indexes?Selecting animals usually involves several traits (characteristics). Traits can be combined into what is referred to as a ‘selection index’. A selection index combines EBVs for several traits to give the best overall basis for selecting breeding stock to achieve production goals. Weighting EBVs by the relative importance of each trait produces a selection index.

For example, the Carcase Plus index places:

• 60% of emphasis on weight or increasing growth rate. • 20% on reducing fatness. • 20% is placed on greater muscling.

The Carcase Plus index is calculated using post weaning information which has been shown as the age where the majority of lambs are turned off. This selection index rates animals on their ability to breed heavy, lean and well-muscled progeny suitable for export weight and fat specifications. The correlation between the Carcase Plus and LMY index created for terminal sires is also very high. As a result selecting rams with high Carcase Plus indexes will have a very positive effect on the LMY of the progeny.

Note: The Carcase Plus index is the one used in the recommendations in Table 13.2.

Other indexes include the LAMBPLAN 80:10:10 index. In 1990 the Index was set at 100, however, due to genetic progress, this has increased as shown in the following graph.



Table 13.2 shows the average, top 20% and top ram in LAMBPLAN across all terminal sires for the 2002 analysis. Source: MLA, (2003).


An index of 105 indicates that the ram will produce $5 extra lamb value for every ewe joined. Over four years, a ram will produce 200 progeny, with the extra value produced by that ram is $5 x 200 = $1000.

The Maternal central progeny test has demonstrated a $35 difference per ewe per year (using only $1.95/kg cwt with price grid discounts for over-fat and under-weight). Despite this, the major driver of this would be fertility traits such as number of lambs born. The selection of maternal sires for 1st X ewe production can however, have an enormous influence on the 2nd X or terminal lamb’s ability to produce a carcase with high lean meat yield. The following graph from the Maternal central progeny test sire evaluation shows the huge variation which existed in the carcase trait EBVs of first cross wethers out of different Border Leicester Sires. This evaluation has demonstrated that there is more variation within breed than there is across breeds and has emphasised the importance of selecting superior sires to produce ewes for prime lamb production.


Table 13.3 shows the average, top 20% and top ram in LAMBPLAN across all Border Leicester sires for the 2002 analysis. Source: MLA, (2003).


13.9 Effects of environment on lean meat yieldThere are three major environmental influences that have a bearing on lamb growth, muscle development and fatness, and lean meat yield.

Birth type Nutrition Disease (principally intestinal parasite burden, and to a lesser extent a number of protozoan

and bacterial infections).


Unlike genetics where decisions are made annually on choice of sire, and less frequently for the dam, management of nutrient supply and disease state are ongoing throughout the annual production cycle. Although nutrition and disease effects can sometimes have profound impact on yield and productivity most producers have in place well established systems in which they manage their stock. Moreover, these systems are invariably intended to maximize number and weight of lambs that reach slaughter, and hence whole of system profitability, rather than maximize yield per head. Decisions about nutritional management are made to maximize whole of flock productivity, usually without consideration of nutritional effects on lean meat yield.

Lean meat yield is a function of growth rate, muscle development and fat deposition. These components of yield are subject to variation from nutritional inputs throughout the life of the lamb.

Some of the variation in a lambs growth and yield can be attributed to in-utero effects. Birth type, single, twin, triplet, and nutrition of the dam during pregnancy affect birth weight. Moreover, birth type and ewe nutrition also affects the amount of milk that is available for each lamb. Birth weight, in particular, is a major determinant of subsequent growth rate.

Birth Type. Major factors affecting birth weight are parity (i.e. single, twin or triplet born), and nutrient supply to the dam in the last trimester. Individual twins are typically born 15% lighter than singles and subsequently grow 10% slower. Individual triplets may be born up to 30% lighter than singles and subsequently grow 20% slower. Multiple births are associated with changes in fatness, initially multiple born lambs are less fat because they are each able to obtain less milk from their mother than a single born lamb. Later in life, and particularly if they have been fed to catch up, twin and triplet lambs may become slightly more fat (and thus have lower lean meat yields) than single born lambs.

Nutritional management of lambs - effects on growth, fatness and yield.Most lamb production systems are pasture based. This makes use of detailed nutritional calculations to meet requirements for growth (as for example described in Module 3 – Nutrition of Meat Sheep) difficult to implement in practice. Approaches to simplify the complexity of prediction of pasture intake by sheep with different genetic potential, at different weights and that predict growth rate, are available as computer based models such as GrassGro and GrazFeed. These are seldom used because producers (and advisors) rarely have sufficient of the required data to the run the simulations. Nonetheless, it is important to understand the principles upon which they are based. A good overview of the principles upon which nutritional requirements of grazing sheep are calculated is given by M. Freer (2002) in Chapter 16 “The nutritional management of grazing sheep” (pp357-375) of Sheep Nutrition (eds M Freer and H Dove) CAB International & CSIRO Publishing. The basic principles for pasture based systems are:-

Feed intake is a function of an animal’s capacity to eat (set in part by genetic potential, age and weight of the animal, and past growth) and the amount of pasture and the nutrient availability in that pasture. Heavier animals, more pasture availability and better quality pastures are associated with increased nutrient intake.

Growth rate is determined by the ingestion of nutrients (predominantly energy and protein) relative to the maintenance needs of the animal. Higher growth is associated with nutrient intakes much greater than required for maintenance.

Fat deposition is determined by the weight of the animal (relative to mature weight) and growth rate. Faster growth is associated with increased fat relative to protein deposition.

In practice these are converted into general rules of thumb such as the following example,

Growing lambs can eat from 3-4% of their body weight (from 1.2 -1.6 kg dry matter /day).

Growth rate is determined largely by the nutrient density of the feed (as shown in Table 13.6). The amount of protein in the diet should be sufficient to

a) Maximize microbial protein synthesis in the rumen. b) Make up for any shortfall in requirements between microbial protein produced in the rumen

and that required by the animal's tissues.


Minimum amounts of dietary protein are determined by the energy density of the diet (higher energy density, greater rates of fermentation in the rumen) and the proportion of dietary protein degraded in the rumen (more degradable, lower dietary protein requirement). Except in the case of lambs less than 20 kg (and these are likely to be suckling mum), and around weaning when intake will be affected by stress at separation from mum, and by the rapid development of rumen, the minimum dietary crude protein contents shown in Table 13.4 are likely to be sufficient to meet both rumen and tissue needs for maximum growth.

Table 13.4. Relationship between Energy and Protein content of feed and estimated growth rate of a 40kg lamb*. Source: MLA, (2003).Estimated Lamb Growth rate (g/day)

M/D (MJ metabolisable energy / kg DM)

Minimum Crude Protein % in dry matter+

<150 9 11.6~180 10 12.9~220 11 14.1~275 12 15.4>300 13 16.7

* Assumed between 40 - 50% of mature size + based on degradability of dietary protein in the rumen of 70%

However, maximizing growth rate may not be conducive to maximizing lean meat yield. Growth rate (predominantly determined by energy density of the diet) has a major effect on the relative proportions of fat and protein deposited in the carcase (Figure 13.12).

Figure 13.12. The effect of feed intake relative to maintenance on protein and fat deposition in the carcase of lambs. Castrate males lambs from Border Leicester X Merino ewes and

Poll Dorset sires were fed different amounts of a pelleted ration with M/D =10 MJ ME/ kgDM. The lambs were approximately 6 months old and weighed 52 kg at the start of the

experimental period. Source: Hegarty et al, (1999).

Effect of energy intake on carcass protein and fat deposition in lambs

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10152025303540

0 0.5 1 1.5 2 2.5

Intake (multiple of maintenance requirement)

g/d carcass protein gain

carcass fat gain


Figure 13.13 The effect of liveweight gain on the ratio of fat gain to protein gain in the carcase of lambs. Castrate male lambs from Border Leicester X Merino ewes and Poll

Dorset sires were fed different amounts of a pelleted ration with M/D = 10 MJ ME/kg DM.. The lambs were approximately 6 months old and weighed 52 kg at the start of the

experimental period. Source: Hegarty et al, (1999).

Liveweight gain affects ratio of fat to protein deposition in carcass

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Figures 13.12, 13.13 and 13.14 illustrate that higher growth rates achieved by eating more of the same feed, or by eating more of nutrient dense feedstuffs are associated with increased rates of fat and protein fat deposition in lambs near finished weights. Higher growth due to nutrient intake is associated with much more fat than protein deposition

Figure 13.14 Effect of diet energy density on ratio of fat to protein gain in the carcase of lambs. Data from castrate male lambs from Dorset Horn sires over Merino X Border

Leicester ewes approx 6 month of age, weighed approx 35 kg at the start and 50kg at the end of the feeding period. Source: Oddy and Sainz, (2002).

M/D of diet v's ratio of fat / protein gain

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The amount of protein relative to energy available to the tissues of the animal also affects the ratio of fat to protein deposition in the carcase, and thus lean meat yield. At energy intakes close to maintenance, increasing rumen bypass protein content of the diet facilitates muscle growth while reducing fat deposition (Figure 13.15, see Bell and Bower, 1990 for a practical example).

Figure 13.15 Effect of increasing amino acid supply to the intestines at 0.8 x maintenance energy intake on carcase protein and fat gain. Data from castrate males lambs from Border

Leicester X Merino ewes and Poll Dorset sires fed different amounts of a pelleted ration with M/D =10 MJ ME/ kgDM. The lambs were approximately 6 months old and weighed 52 kg at

the start of the experimental period. Source: Hegarty et al, (1999).

Effect of increasing amino acid supply to the intestines on carcass protein and fat gain. Data

shown for energy intake 0.8 maintenance

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In summary, nutritional management of overfatness (to increase yield) can be achieved by reducing the energy density of the feed, by slowing growth rate and by increasing dietary protein supply relative to energy.

Guide to weight gainsAs seen above, there are many variables that will influence the performance of lamb growth and fattening. In practice farmers monitoring lamb growth during finishing by regularly weighing lambs. Fat scoring will also provide an indication of growth rates.

Influence of pasture intake on LMYManaging nutrition for growth rate is a sophisticated and complex undertaking. The most cost-effective way is to produce lambs on quality pasture. Lamb intake of pasture and hence growth rate is highest on green pasture with an availability of about 1,500kg per hectare.

As seen above the nutrient density of pasture is critical to lamb growth. Growth rates can drop considerably as pastures mature. This is because digestibility (and therefore energy and protein content of pasture decreases), as demonstrated in Figures 13.16 and 13.17.


(Note the legend to Figure 13.17 should show the top value for digestibility should be 80% not 50% as shown.)

DigestibilityDigestibility is a useful practical measure of pasture quality. It is directly related t the energy density of the diet (high digestibility = high energy density, see Figure 15)

To achieve fast lamb growth of 250grams per day or better, a green pasture of 70% or higher digestibility and 1500kg per hectare green availability is required.

Young green pasture is the most digestible. The legume content will increase pasture quality by increasing feed intake and digestibility.

Ideal pastures contain about 30% legume content.

Managing for growth rate and Lean Meat YieldThe following concepts are key elements to be considered when managing for growth rate and Lean Meat Yield:

Target growth rates for lambs. Pasture quality is influenced by digestibility and the proportion of legume. Animal production will be determined by pasture intake. Intake is determined by herbage mass (the amount of pasture) and the digestibility of the

pasture. Digestibility decreases with plant maturity. Digestibility is positively related to energy and protein content of pasture. Digestibility of a pasture will be influenced by pasture species, stage of growth and legume

percentage. Pasture benchmarks are a prediction of the minimum herbage mass of green pasture that a

particular class of livestock can graze to and still meet their production requirements. Herbage mass and digestibility interact to influence intake.

Body composition in the lambAssuming that growth is occurring, the growth of the young lamb can be regarded as having four phases:

Milk feeding (very efficient). Weaning (may involve a growth check). Pre-finishing. Finishing (fat deposition rapidly accelerates).

Throughout these phases, there is a relatively constant rate of muscle deposition in the body and it is the rate of fat deposition that is highly variable until, in the finishing phase it becomes the dominant process. Females will enter the finishing phase at a younger age than males of the same genetic potential. Carcases weighing 25kg from ewes typically contain 2-3kg of fat more than males and this will reflect in a lower LMY% for females, consistent across breeds. Castrates (wethers) will be fractionally leaner than ewe lambs. For a typical lamb, most of the early weight gain development is muscle tissue. However, as the animal approaches maturity, muscle gain per day begins to noticeably decline while fat deposition rapidly increases. Therefore, as lambs get heavier, the proportion of fat increases significantly, whilst the proportion of bone declines and the proportion of muscle slightly declines.

Once a lamb reaches maturity, the growth rate also slows whilst its’ feed intake increases. This means that feed conversion ratio also declines and it will be eating greater quantities of feed per kilogram of weight gain. Younger lambs will tend to have faster growth rates and better feed-conversion efficiencies than older lambs. It will be the final carcase weight and growth rate during finishing that principally determines LMY% of a lamb, except where lambs suffer a growth check in the first 6 weeks. In order to support high growth rates during finishing without producing over-fat, lower LMY lambs, careful selection of sire and dam genetics for breeders with a high propensity for lean growth is essential. Selection of high growth rams will produce lambs that are heavier at the same age than the average lamb. Therefore, the genetically superior lamb will reach heavier weights before reaching maturity and before it enters the fattening phase.


Manipulating growth rate pathwayNutrition affects LMY (weight and percentage) by influencing the final carcase weight, but also by affecting the composition of the carcase at any given weight. Alterations in growth rate arising from differences in the overall plane of nutrition can alter body fat and protein content and the eating quality of the meat produced.

In general:

Slower growth rates can result in higher lean meat yields, due to less fat deposition in the carcase, however if growth is too slow this may reduce eating quality and decrease profit.

Faster growth rates towards the end of finishing are associated with faster rates of fat deposition, which can lower the lean meat yield.

Animals that have an early growth check due to inadequate nutrition early in their life (prior to weaning) end up being fatter, because their long-term capacity to grow muscle and bone has been reduced. Despite this, animals that have a late growth check (during the finishing phase) will compensate for this when sufficient nutrition is supplied. Animals that have been held back try to catch up by growing muscle and this ‘catch-up’, or compensatory growth is normally leaner than normal growth because the body puts the priority on growing carcase muscle in preference to laying down fat.

Fat development may then be delayed until muscle growth has caught up to that appropriate to the animal’s maturity. Once lambs enter the finishing phase, there is little change to the ‘muscle: bone’ ratio, and therefore most of the impacts of feeding on LMY percentage, can be assessed by monitoring lamb fatness.

Table 13.5 Source: MLA, (2003).

Example: To ensure optimal lamb growth, with single or twin rearing ewes, they would require 1500kg/ha green dry matter and a legume content of 15%.


Table 13.6 the growth rates in brackets are based on a weaned four month old crossbred lamb of about 32kg liveweight, on a pasture where the green component was 73% digestible, there was 1500kg DM/ha of green herbage and 15% legume. Source: MLA, (2003).

Percent of Potential Minimum pasture requirementBenchmarks provide “ball park” estimates for the minimum green herbage mass to which stock can graze and still maintain satisfactory levels of production. There are a number of courses available to lamb producers to help them develop pasture assessment skills and pasture management skills. Prograze is one of these courses which looks at grazing management and the affect of pastures on livestock.

Effect of intestinal parasites on lamb performance and lean meat yield.Infection with intestinal parasites occurs routinely in lambs. The effects of infection are usually greatest shortly after weaning. At this time, lambs are under nutritional and behavioural stress, and well as reaching the point where their immune systems are becoming capable of mounting a response to intestinal parasites. In practice, management of lambs by weaning onto pastures with a low worm burden, and treatment with anthelmintics is used to reduce the chances of intestinal parasitism affecting growth (and survival) of the lambs. Where treatment is not effective (increasingly the case as resistance of parasites to anthelmintics is increasing), infected lambs may not gain weight as quickly as uninfected, and this may impact on subsequent capacity to grow and on future lean meat yield. Examples of the impact of parasitism are shown in the following table. One of the major effects of intestinal parasitism is a reduction in feed intake. However, the reduction in growth and in particular growth of protein during infection is disproportionately more than the reduction in feed intake.

Table 13.7 Growth of lambs and of fat and protein during a three month period of parasitism (intestinal – Trichostrongylus colubriformis, abomasal – Ostertagia circumcincta). ALI = ad-libitum intake, infected, ALC = ad-libitum intake, control uninfected, PF = pair fed uninfected, fed same amount as eaten by ALI. Source: Sykes and Coop, (1998).

ALI ALC PFIntestinal parasitesGrowth rate (g/d) 94 194 151Fat gain (g/d) 39 74 55Protein gain (g/d) 6.3 18.8 16.2

Abomasal parasitesGrowth rate (g/d) 78 156 124Fat gain (g/d) 36 63 52Protein gain (g/d) 8.9 18.1 12.0


Other factors influencing lean meat yield

Trace element deficiency The major trace element deficiencies in prime lamb producing districts of Australia include Vitamin B12 (cobalt), copper, selenium, Vitamin A and Vitamin E.Producers should test the blood of fast growing young animals for these deficiencies. Testing is recommended to determine the extent of the problem so that appropriate strategies can be developed. Trace element deficiencies can reduce growth rates and influence fertility levels in ewes and rams.

Health Sheep diseases and/or parasites can compete for nutrients, depress food intake and reduce feed conversion efficiency. Follow correct vaccination recommendations for your area. Remember not to vaccinate lambs and ewes on the body as this reduces skin quality, only vaccinate in the upper neck. As lambs approach marketable weights it is important to be mindful of the withholding periods of the drenches, lice and fly treatments you may have given your lambs. There are withholding periods of 3 to 180 days for chemicals used on animals that are for sale for meat. There may be even longer export slaughter intervals set by the importing country.

Condemnations at slaughter Condemnations at slaughter can cause significant losses of lean meat yield, due to removal of parts of the carcase prior to the hot weight scales. This means lower carcase weights and consequently lower lean meat yield (kg).Reasons for trimming include arthritis, pleurisy (secondary pneumonia), broken ribs, cheesy gland, vaccination abscesses, dog bites and bruising.

The trimming for these conditions occurs before the carcase is weighed, and therefore not only do you lose the weight trimmed from the carcase, you may also drop down in weight to a lower price per kg. A lamb with arthritis gets it’s leg chopped off below the affected joint, a lamb with pleurisy will get one or two sides of it’s rib cage removed, and a lamb with cheesy gland or abscesses in the hind leg will sometimes lose the whole back leg. Trimming for this type of condition could amount to 1-5kg loss in carcase weight per lamb, and therefore a high incidence of pathological problems at slaughter could result in lower carcase values. A number of these conditions can be prevented if better management and hygiene practices were employed throughout the lambs’ life – starting at lamb marking

Readings The following readings are available on CD.

1. Gardner, G.E., Pethick, D.W., Hopkins, D.L., Hegarty, R.S. Cake, M.A., Boyce, M.D. and Allingham, P.G., 2006. 'The impact of carcase estimated breeding values on yield and quality of sheepmeat. International Journal of Sheep and Wool Science, vol. 54, pp. 33-39.

2. Hegarty, R.S., Neutze, S.A. and Oddy, V.H., 1999. 'Effects of protein and energy supply on the growth and carcase composition of lambs from differing nutritional histories. Journal of Agricultural Science, Cambridge, vol. 132, pp. 361-397.

3. Kempster, A.J., Croston, D., Guy, D.R. and Jones, D.W., 1987. 'Growth and carcase characteristics of crossbred lambs by ten sire breeds compared at the same estimated sub-cutaneous fat proportion.' Animal Production, vol. 44, pp. 83-98.

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topics as your assignment. Short answer questions appear on WebCT. Submit your answer via WebCT


ReferencesBell, A.K. and Bower, S. M., 1990. Effects of a cottonseed meal and barley based diet on the

carcase quality of prime lambs. Proceedings of the Australian Society of Animal Production, vol. 18, pp. 455.

Freer, M., 2002. The nutritional management of grazing sheep. In Sheep Nutrition (M. Freer and H. Dove, eds) CABI publishing, CSIRO publishing ISBN 0 85199 595 0

Gardner, G.E., Pethick, D.W. Hopkins, D.L., Hegarty, R.S., Cake, M.A.., Boyce, M.D. and Allingham, P.G. 2006 The impact of carcase estimated breeding values on yield and quality of sheep meat. International Journal of Sheep and Wool Science. Vol. 54, pp. 33-39.

Hopkins, D.L., 1994. Predicting the weight of lean meat in lamb carcases and the suitability of this characteristic as a basis for valuing carcases. Meat Science, vol. 38, pp. 235-241

Hall, D.G. et al, 2001. Growth and carcase composition of second cross lambs, effect of sex and growth path on pre- and postslaughter estimates of carcase composition. Australian Journal of Agricultural Research, vol. 52, pp. 859-867.

Hegarty. R.S., Neutze, S.A. and Oddy, V.H., 1999 Effects of protein and energy supply on the growth and carcase composition of lambs from differing nutritional histories. Journal of Agricultural Science, Cambridge. Vol. 132, pp. 361-375

Kempster, A.J., Croston, D., Guy, D.R. and Jones, D.W., 1987. Growth and carcase characteristics of crossbred lambs by ten sire breeds, compared at the same estimated sub-cutaneous fat proportion. Animal Production, vol. 44, pp. 83-98.

LAMBPLAN website : http://www.sheepgenetics.org.au/lambplan/Meat and Livestock Australia Ltd., 2003. Data supplied courtesy of MLA, Sydney.Oddy, V.H. and Sainz, R.D., 2002. Nutrition for sheep-meat production. In Sheep Nutrition (M.

Freer and H. Dove, eds CABI publishing, CSIRO publishing ISBN 0 85199 595 0Prograze Manual, 2000 reprint.Producing and Marketing Lambs to Specifications in NSW, NSW Agriculture 1997, ISBN 0 7310

9830 7Sykes, A.R. and Coop, R.L., 1998. Chronic Parasitism and Animal Efficiency. ARC Research

Review, vol. 3, pp. 41-46.


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