evaluation of bull prolificacy on commercial beef cattle ranches using dna paternity analysis

A. L. Van Eenennaam, K. L. Weber and D. J. Drakeanalysis

Evaluation of bull prolificacy on commercial beef cattle ranches using DNA paternity

doi: 10.2527/jas.2013-7217 originally published online April 21, 20142014, 92:2693-2701.J ANIM SCI

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Evaluation of bull prolificacy on commercial beef cattle ranches using DNA paternity analysis1,2

A. L. Van Eenennaam,*3 K. L. Weber,* and D. J. Drake

*Department of Animal Science, University of California, Davis 95616; and University of California Cooperative Extension, Yreka 96097

AbstrAct: SNP-based DNA testing was used to assign paternity to 5,052 calves conceived in natural service mul-tisire breeding pastures from 3 commercial ranches in northern California representing 15 calf crops over 3 yr. Bulls present for 60 to 120 d at a 25:1 cow to bull ratio in both fall and spring breeding seasons in ~40 ha or small-er fenced breeding pastures sired a highly variable (P < 0.001) number of calves (Ncalf), ranging from 0 (4.4% of bulls present in any given breeding season) to 64 calves per bull per breeding season, with an average of 18.9 13.1. There was little variation in Ncalf among ranches (P = 0.90), years (P = 0.96), and seasons (P = 0.94). Bulls varied widely (P < 0.01) in the average individual 205-d adjusted weaning weight (I205) of progeny, and I205 var-ied between years (P < 0.01) and seasons (P < 0.01) but not ranches (P = 0.29). The pattern for cumulative total 205-d adjusted weaning weight of all progeny sired by a bull (T205) was highly correlated to Ncalf, with small differences between ranches (P = 0.35), years (P = 0.66), and seasons (P = 0.20) but large differences (P < 0.01) between bulls, ranging from an average of 676 to 8,838 kg

per bull per calf crop. The peak Ncalf occurred at about 5 yr of age for bulls ranging from 2 to 11 yr of age. Weekly conception rates as assessed by date of calving varied sig-nificantly and peaked at wk 3 of the calving season. The distribution of calves born early in the calving season was disproportionately skewed toward the highly prolific bulls. The DNA paternity testing of the subset of those calves born in wk 3 of the calving season was highly predictive of overall bull prolificacy and may offer a reduced-cost DNA-based option for assessing prolificacy. Prolificacy of young bulls in their first breeding season was positively linearly related (P < 0.05) to subsequent breeding sea-sons, explaining about 20% of the subsequent variation. Prolificacy was also positively linearly related (P < 0.05) to scrotal circumference (SC) EPD for Angus bulls that had SC EPD Beef Improvement Federation accuracies greater than 0.05. Varying prolificacy of herd bulls has implications for the genetic composition of replacement heifers, with the genetics of those bulls siring an increased number of calves being disproportionately represented in the early-born replacement heifer pool.

Key words: beef cattle, bulls, DNA, markers, paternity, prolificacy

2014 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2014.92:26932701 doi:10.2527/jas2013-7217

INtroDuctIoN

In the commercial cow-calf sector, the principal de-terminants of income are the number of sale animals and the value per sale animal (Garrick and Golden, 2009). In

that regard, a herd bull has 2 qualities of value to com-mercial producers. One is his ability to impregnate as many cows as possible, and the other is the ability to pass genes for superior performance on to his offspring. In the absence of the former, the latter is moot. Natural service breeding is the predominant practice for beef cattle operations in the United States, but few studies have examined the variation in the number of calves sired in multiple-sire breeding pastures and the consis-tency of an individual bulls performance over time.

Few genetic tools exist for selecting bulls with superior breeding performance. Holroyd et al. (2002) found that there were breed differences in a variety of traits related to calf output (e.g., scrotal circumference,

1This work was supported by National Research Initiative Competitive Grant No. 2009-55205-05057 (Integrating DNA information into beef cattle production systems) from the USDA National Institute of Food and Agriculture.

2The authors gratefully acknowledge the cooperation and labor provided by the 3 collaborating ranches (Cowley Family Ranch, Kuck Ranch, and Mole Richardson Farms).

3Corresponding author: [email protected] October 3, 2013.Accepted March 22, 2014.

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Van Eenennaam et al.2694

testicular tone, dominance, libido score, and semen quality), but that those traits explained only 3557% of the phenotypic variation in the number of progeny sired.

The objectives of this study were to use SNP-based DNA testing to quantify variation in prolificacy of bulls in multisire breeding groups using data from 3 large commercial beef ranches in northern California, and to investigate the potential uses of this information for man-agement decisions in commercial beef cattle production systems. Bull age and breed association EPD were exam-ined to determine their relationship with bull prolificacy. Additionally, various calf subsampling strategies were ex-amined to determine their accuracy as as an approach to avoid the costs associated with sampling DNA from the entire calf crop.

MAtErIALs AND MEthoDs

Three commercial ranches, designated A, B, and C and located in the Shasta Valley of northern California, were evaluated for bull performance using DNA test-ing to determine parentage. Registered Angus sires had been used extensively during the previous 10 yr on these 3 ranches, making the cow herds primarily Angus. Ranches raised their own replacement heifers.

Ranch A had a spring calving herd of 550 cows and a fall calving herd of 350 cows. Breeding seasons were 60 d in length and included several breeding pastures typically involving 2 to 5 bulls and a cow to bull ratio of approximately 25:1. Breeding pastures were fenced and generally less than 40 ha in size. Bulls used included predominately Angus (n = 64) plus a small number of South Devon (n = 2) and South Devon Angus cross (n = 6) bulls bred and raised on the ranch.

Ranch B had a 200-cow spring calving herd and a fall calving herd of about 300 cows. On this ranch breed-ing seasons were 90 d in length with several breeding pastures made up of 2 to 5 bulls and a cow to bull ratio of approximately 25:1. Breeding pastures were less than 40 ha in size. Bulls used include predominately Angus (n = 19) and a small number of Horned Hereford (n = 2).

Ranch C had a fall calving herd of about 700 cows. The breeding season was approximately 120 d. Breeding pastures tended to be bigger than on the other 2 ranches, with a larger number of bulls, 5 to 9, in each pasture, but a cow to bull ratio of approximately 25:1 was main-tained. Bulls used included predominately Angus (n = 37) and a small number of Red Angus (n = 3), Horned Hereford (n = 1), and Polled Hereford (n = 1).

Before being joined with the cows, a breeding sound-ness examination (bsE) was conducted on all bulls, and only bulls passing the exam were used. Breeding groups consisted of replacement heifers as a single group and mature (all other) cows. The cows were not assigned

to the same breeding group each year, but rather were assigned on the basis of practical considerations (e.g., amount of feed available in different pastures) at the judgment of the ranch manager. Cows in the various fall or spring herds generally stayed with those breeding herds. Bulls were observed on a daily basis or several times per week during the breeding season and were removed from the breeding pasture for injury or poor body condition. When bulls were removed, replacement bulls were most frequently obtained from other breed-ing groups as idle substitute bulls were not typically available. The replacement bull selection decision was based on the judgment of the rancher when considering a variety of practical factors, including reassigning bulls from breeding pastures that had a slightly lower cow to bull ratio, selecting bulls that were observed to be very actively breeding cows, and making selections to avoid known bull dominance issues such as a history of ob-served aggressive behavior between specific individuals and avoiding the comingling of older bulls with young inexperienced bulls. Replacement heifers were bred to younger (generally less than 3 yr of age), lighter-weight bulls that had typically been purchased for calving ease. These bulls were shifted from replacement heifer to ma-ture cow breeding pastures as they became older and heavier after a couple of years.

Birth dates and dam identification were obtained at calving, and calves were individually identified (n = 5,052). Birth weights were taken only on Ranch B. At marking time, electronic ear tags (Destron Fearing, St. Paul, MN) were applied to calves, and hair samples were obtained for DNA testing. Calf genotypes (~100 SNP) were obtained as part of the Bovine SeekSire genotyp-ing service (GeneSeek Inc., Lincoln, NE). Parentage SNP genotypes for bulls were extracted from Bovine SNP50 BeadChip genotypes (Illumina, San Diego, CA), which had been acquired for these bulls in the course of a previous project. Bull genotypes were compared with calf genotypes using the SireMatch program (J. Pollak, U.S. Meat Animal Research Center, Clay Center, NE). Bull prolificacy was defined as the number of calves as-signed to each bull by DNA paternity testing (Ncalf) for an entire calf crop.

Individual calf weights were obtained at approxi-mately 205 d of age. Weights were adjusted using Beef Improvement Federation (bIF) linear adjustments for cow age and calf age (BIF, 2010), although some of the calves fell outside the recommended age range on weigh day because of practical constraints associated with calves going to summer pastures such that they were not accessible for weighing during the BIF prescribed age range. Weights for each calving group were obtained on consecutive days when it was not possible to weigh the entire group on 1 d. Individual 205-d weights (I205)


Commercial beef bull prolificacy 2695

were also adjusted for sex differences by least squares using a model that included fixed effects of ranch, year, season, and calf sex. Total adjusted 205-d weight of progeny (t205) for each bull was defined as the sum of the I205 of all his progeny for each calf crop. Prolificacy of young bulls (age less than 3 yr) was defined as that observed for their first breeding season and was com-pared to the average of their prolificacy recorded in sub-sequent breeding seasons

Prolificacy and EPD Relationships

To determine the relationships between American Angus Association EPD and bull means for Ncalf, I205, or T205, these variables were evaluated using scatter-plots for each EPD. Because of the use of some young low-accuracy bulls, all analyses were restricted to bulls with BIF EPD accuracies greater than 0.05 for the trait being evaluated. Average phenotypic values that showed a probable linear relationship with EPD (P < 0.20) using linear regression were selected for further evaluation.

Prolificacy and Calving Distribution

The beginning of the calving season (d 1) for each calf crop was assigned as the date the fifth calf was born on a given ranch. Birthdates were grouped into 7-d intervals representing weeks of the calving season. Bulls were categorized into 3 equal-sized prolificacy groups, high (hP), middle (MP), and low (LP) prolifi-cacy, based on the total number of progeny produced in each calf crop. Additionally, the prolificacy grouping of young bulls (age less than 3 yr) for their first breeding season was compared to their average prolificacy group-ing in subsequent breeding seasons.

Alternative Prolificacy Assessment Methods

As a less costly alternative to parentage-testing all calves for assessment of prolificacy, smaller subsets of calves were used to sort bulls into prolificacy groups HP, MP, and LP. Alternative subsets included calves from a sin-gle week (wk 3) of the calving season or the sum of calves from 3 wk (wk 2, 3, and 4). The original prolificacy assess-ments based on the entire calf crop were compared to the assessments based on these subsets using a chi-squared test.

Statistical Analyses

Analyses of Ncalf were restricted to bulls present for the full duration of the breeding season and were ana-lyzed using a model that included fixed effects of ranch (R), year (Yr), and season (Sn). The contributions of Ncalf and I205 to T205 were estimated by comparing R2

from regressions with T205 as the dependent variable and inclusion of Ncalf and I205 together or singularly (Systat). Correlations were estimated with Pearson cor-relations. Ncalf repeatability for bulls that were present for more than two seasons was calculated for all bulls, and for mature bulls older than 3 years of age using the method of Lessells and Boag (1987), which accounts for among-group and within-group variance for unequal group size. Effect of bull age on Ncalf was evaluated us-ing the linear mixed effects model (LMEr) function of R and a model that included fixed effects R, Yr, Sn, and bull age and random effect of individual bull.

Linear regression of selected breed association EPD and average prolificacy were evaluated for P levels. Individual bulls were examined in scatterplots, and sus-pect outliers based on a very high Cooks distance (Di) and studentized residual (P < 0.05) were removed from the analyses if their inclusion alone obfuscated a statisti-cally significant linear relationship that was present in the absence of that data point.

The mean number of calves born each week for bulls with 2 or more calf crops was calculated by least squares using a model that included fixed effects of R, Yr, Sn, and week (W), and mean separation significance was correct-ed by Bonferroni adjustments. Mean weekly calving rate for each group was calculated by least squares using a model that included fixed effects of R, Yr, Sn, W, prolifi-cacy group, and prolificacy group by week.

Prolificacy for the first breeding season for young bulls was compared to the average value in subsequent breeding seasons by regression. First season prolificacy grouping (HP, MP, or LP) was compared to the average grouping in subsequent breeding seasons by Pearson, rank order correlation, and chi-squared analyses. Because more than 20% of the cells in the chi-squared analysis were below 5, a likelihood ratio chi-squared analysis was also conducted.

rEsuLts

Paternity Assignments

A total of 5,052 calves were assigned paternity on the basis of DNA from 15 calf crops and 275 bull natural breed-ing opportunities (Table 1). Reproductive failure, meaning that no calves were produced, occurred in 4.4% of the bull seasons (12 out of 275 bull breeding season opportunities). In 40% of the calf crops at least 1 bull sired only 1 calf and at least 1 bull sired more than 50 calves. DNA information was unable to uniquely assign paternity to an average of 3.8% of progeny across all ranches, with 2.6% on Ranch A, 3.0% on Ranch B, and 5.7% on Ranch C.



Prolificacy

The overall mean Ncalf was 18.9 13.1 progeny, with little variation among ranches, 18.6 6.0, 19.9 3.8, and 21.1 13.2 (P = 0.90) for Ranches A, B, and C, respectively. Similarly, differences between years (19.9 3.8, 20.1 1.9, and 19.7 7.7; P = 0.96) and sea-sons (spring, 20.5 12.2; fall, 19.2 5.0; P = 0.94) for Ncalf were small. Additionally, Ncalf across the 15 calf crops showed little variation (P = 0.51), ranging from 14.4 5.7 to 26.5 14.4 (Table 1). However, the mean Ncalf per bull varied widely (P < 0.01), ranging from a mean of 3.3 6.3 to 39.1 10.9 (Fig. 1). Repeatability of Ncalf for bulls 3 or more years of age was 0.37, and it was 0.33 when all bulls were included in the analysis.

I205. Individual calf 205-d weight did not vary signif-icantly between ranches (232 10.1, 235 6.4, and 279 18.0 kg; P = 0.29) but showed significant year (225 2.4, 233 2.3, and 227 2.4 kg; P < 0.01) and season (spring, 237 2.5 kg; fall, 220 2.0 kg; P < 0.01) differences. Again, bulls varied widely in mean progeny I205 (P < 0.01), ranging from means of 196 to 262 kg (Fig. 1).

T205. Total contribution of calf weight (sum of indi-vidual sex-adjusted 205-d wt) per bull showed a pattern similar to that of Ncalf alone (Fig. 1). Total adjusted 205-

d weight of progeny per bull was similar across ranch (4,687 1,890, 5,713 1,077, and 3,425 1,404 kg; P = 0.35), year (4,762 388, 4,410 368, and 4,654 386 kg; P = 0.66), and season (spring, 4,882 414 kg; fall, 4,336 304 kg; P = 0.20) but varied widely be-tween bulls (P < 0.01), ranging from mean totals of 676 to 8,838 kg. The mean number of calves per bull was highly related (P < 0.01) to total production, explain-ing 96.9% of the variation (R2 = 0.969), with each calf contributing 220 2.6 kg. Mean I205 was also related to total production (P < 0.05) but by itself explained only 2% of the variation (R2 = 0.02). Similarly, Ncalf was highly correlated (r = 0.98) to T205 per bull compared to the lower correlation (r = 0.15) to I205.

Prolificacy and Bull Age. There was no significant re-lationship between Ncalf and bull age. As shown in Fig. 2, bulls of increasing age tended to have a higher maximum Ncalf, resulting in a higher variance in Ncalf. Young bulls in their first breeding season (N = 24) ranged in age from 1.4 to 2.9 yr (mean of 2.4 0.3 yr). The mean number of calves per young bull ranged from 1 to 40 (mean of 14.9 9.8). Prolificacy in subsequent breeding seasons was posi-tively linearly related (P < 0.05) to first breeding season prolificacy, explaining about 20% of the subsequent varia-

table 1. Average bull age at the beginning of the breeding season and number of calves produced per natural service bull in multisire breeding pastures on 3 commercial ranches (A, B, C) in northern California in 20092011

Ranch

Year

Calf crop

No. of sires

Minimum bull

age, yr

Maximum bull

age, yr

Mean bull age

SEM, yr

Total

no. calves

Per bullMinimum no.

calves1Maximum no.

calvesMean no. calves

SEMA 2009 Spring 18 2.3 6.9 4.3 0.3 353 3 47 19.9 3.8A Fall 19 2.4 4.6 3.5 1.4 346 1 47 19.6 18.2A 2010 Spring 22 1.9 5.9 4.3 0.9 435 3 45 19.8 3.8A Fall 19 2 5.6 3.9 1.2 328 1 48 18.4 22.1A 2011 Spring 17 2.4 5.9 4.7 1.2 402 4 53 24.2 4.7A Fall 19 2 6.6 4.2 1.8 286 1 33 16.8 14.5B 2009 Spring 8 1.4 9.8 4.3 0.3 141 1 45 16.7 10.0B Fall 10 2.1 9.6 4.3 0.3 214 10 50 21.8 9.3B 2010 Spring 8 2.3 5.9 3.0 1.1 142 3 30 16.5 7.4B Fall 12 2.1 10.6 4.3 0.3 247 4 44 20.2 12.9B 2011 Spring 4 3.4 6.9 4.3 1.4 110 18 42 26.5 14.4B Fall 12 2.1 11.6 4.6 1.6 266 3 51 22.8 6.2C 2009 Fall 30 2.4 6.5 4.3 1.2 642 2 54 20.3 3.0C 2010 Fall 27 2.5 6.6 4.6 1.7 567 1 52 19.9 3.8C 2011 Fall 38 2.4 8.5 5.5 0.9 573 1 64 14.4 5.7A 20092011 114 1.9 6.9 4.1 0.7 2,150 1 53 18.6 6.0B 20092011 54 1.3 11.6 4.8 0.5 1,120 1 51 19.9 3.8C 20092011 95 2.4 8.5 3.9 1.6 1,782 1 64 21.1 13.2A, B, C 2009 85 1.4 9.8 4.3 0.3 1,696 1 54 19.9 3.8A, B, C 2010 88 1.9 10.6 4.3 1.0 1,719 1 52 20.1 1.9A, B, C 2011 90 2 11.6 4.2 0.6 1,637 1 64 19.7 7.7A, B, C Spring 77 1.4 9.8 4.0 0.7 1,583 1 53 20.5 12.2A, B, C Fall 186 2 11.6 4.5 0.2 3,469 1 64 19.2 5.0A, B, C 20092011 263 1.3 11.6 4.4 1.7 5,052 1 64 18.9 13.1

1Where bulls produced at least 1 calf. In 4.6% of the breeding seasons (12 out of 275) bulls produced no progeny.



tion. The correlation between first breeding season pro-lificacy and mean subsequent prolificacy was 0.45 with a rank order correlation of 0.47. Young bulls categorized into 3 equal groups, HP, MP, and LP, based on their first-year prolificacy were not related to subsequent categoriza-tion by chi-squared analysis (P = 0.20) or likelihood ra-tio chi-squared analysis (P = 0.20), although the sample size of this young sire group was relatively small (n = 24). Young bulls categorized by their first breeding season as HP tended to remain HP (3/8 or 4/8), and only 12.5% (1/8) fell to LP. Similarly, young bulls initially categorized as LP mostly remained as LP (62.5%, 5/8), with 25% (2/8) changing to MP but only 12.5% (1/8) improving to HP.

Prolificacy and EPD. Scrotal circumference (sc) EPD was not significantly related (P = 0.16) to prolifi-cacy all bulls were included in the data set. However, a single outlier bull (from Ranch C) had a large Cooks dis-tance (Di) and studentized residual. When that bull was removed from the analysis, SC was related (P = 0.04) to prolificacy (Fig. 3), where

Ncalf = 15.2 + 8.27 (3.0) SC (R2 = 0.13, SE = 8.7).

The equation for Ranches A and B combined (without Ranch C) was within the confidence intervals for the combined equation of Ranches A, B, and C (without the single outlier bull).

Carcass weight (cW) was negatively related (P = 0.03) to prolificacy, where

Ncalf = 27.7 0.354 (0.158) CW (R2 = 0.09, SE = 10.1).

Carcass weight and the $Beef (bE) index value were also both negatively related (P < 0.05) to prolificacy, likely because CW is a component in BE and is there-fore highly correlated (r = 0.81) to it. Additionally, when CW and BE were regressed together, only CW was sig-nificant. When both SC and CW were regressed together against prolificacy, only SC (P < 0.05) remained signifi-cant compared to CW (P = 0.28). Scrotal circumference and CW were not correlated (r = 0.01).

Regression responses similar to those seen for Ncalf were seen for SC and CW when regressed on T205, where

T205 = 7,723 + 4,189 (1919) SC (P = 0.04, R2 = 0.13, SE = 4,345),

T205 = 14,097 179.7 (77.7) CW (P = 0.03, R2 = 0.10, SE = 4,987).

The similar regression responses for T205 and Ncalf were not surprising because of the high correlation (r = 0.98) between these 2 variables. No other EPD exam-ined were related to prolificacy.

Individual 205-d weight was positively correlat-ed with weaning weight (P < 0.01), yearling weight (P < 0.01), CW (P < 0.01), $Feedlot index value (P < 0.01), and BE (P < 0.01) EPD but not to any other EPD. Individual 205-d weight was not highly corre-lated with either Ncalf (r = 0.22) or T205 (r = 0.26). The importance of Ncalf compared to that of I205 on T205 is demonstrated by comparing the R2 (0.9793) for the regression of Ncalf (P < 0.01) on T205 to the R2 (0.9812) for the combined regression of Ncalf (P < 0.01) and I205 (P = 0.09) on T205, showing the very small improvement resulting from the inclusion of I205 in the regression.

Figure 1. Mean number of calves (Ncalf; left axis), calf 205-d sex-adjusted weight (I205), and total 205-d sex-adjusted weight (T205)/20 (right axis) per natural service bull present in a multisire breeding pasture. Only bulls that were present for the entire length of the breeding season and that were in use for more than a single breeding season are included.

Figure 2. Calves per bull (Ncalf) per calf crop vs. age of the bull for natural service bulls present in multisire breeding pastures on 3 northern California commercial beef ranches (A, B, and C).



Calving Distribution

Calving distribution showed the preponderance of calves being born early in the calving season (Fig. 4). The largest number of calves born in a single week oc-curred in wk 3 in 12 of the 15 (80%) calving seasons evaluated. Pooled across ranches and adjusted for ranch, year, and season, peak calving occurred in wk 3. In the 3 seasons where peak calving was not during wk 3, it oc-curred in wk 2 twice and wk 1 once.

The HP bulls sired more calf births per week during the early part of the calving season than the MP or LP bulls (P < 0.01; Fig. 5). Bulls siring more progeny (HP) had a disproportionately higher percentage of calves born early in the calving season. Low prolificacy bulls tended to have a consistently low number of calves born throughout the calving season. These data suggest that high prolificacy is associated with the breeding of a greater than expected number of cows early in the breed-ing season, ultimately leading to a larger total number of progeny for the calf crop.

Alternative Prolificacy Assessment Methods

As an alternative to assessing bull prolificacy by de-termining parentage of all calves, progeny from only a single week (wk 3) or a subset of weeks (wk 2, 3, and 4) were modeled and compared to DNA sampling and test-ing all calves. Prolificacy assessments based on either a

single week or the sum of several weeks were closely related (P < 0.01) to prolificacy assessment based on the total calf crop. No HP bulls were reassessed to LP using the subset of calves born in wk 2, 3, and 4, and only 1.4% were reassessed using only those born in wk 3 (Table 2). Between 17% and 19% of the HP bulls were reassessed to MP using the subsets. Nonetheless, either of these reduced assessments (wk 2, 3, and 4 or only wk 3) offers a reduced sampling (and thus cost) method of determining prolificacy. Reassessment of LP bulls to HP also occurred at a low rate (3% for both subsets).

DIscussIoN

DNA testing can be used to accurately assign pa-ternity and has been documented here and by others (Holroyd et al., 2002; Van Eenennaam et al., 2007; Gomez-Raya et al., 2008), providing an opportunity to investigate bull performance in commercial multisire breeding pastures. We have shown that prolificacy is the main driver of bull productivity as measured by total weight of calves weaned per bull.

The results of bull prolificacy studies conducted in varying environments and management systems consis-tently reveal that prolificacy varies considerably among individual bulls. Holroyd et al. (2002) also used DNA paternity in multisire commercial ranches but with Bos indicus or Bos indicus cross bulls in extensive condi-tions in northern Australia and found very similar results to our temperate intensive production systems, including a 6% frequency of reproductive failure, i.e., bulls that sired no calves in a given breeding season, compared to the 4.4% observed in the current study. They found sev-eral phenotypic traits related to bull performance with little practical application because of the limited amount

Figure 3. Mean calves per bull (Ncalf) vs. scrotal circumference (SC) EPD of Angus natural service bulls present in multisire breeding pastures on 3 northern California commercial beef ranches (A, B, and C). Bull 648 was classified as an outlier on the basis of a high Cooks distance (Di) and studen-tized residual (P < 0.05).

Figure 4. Adjusted calves born per week of the calving season across all 15 calf crops show that peak calving and implied peak conception occur during wk 3 of the calving (and breeding) season. Means that differ (P < 0.05) are noted with different letters. Error bars represent SEM.



of variation explained by these traits. Bamualim et al. (1984) found bull age (between 2 and 5 yr of age) was not significant on pregnancy rate of Bos indicus cross bulls; however, they may not have had sufficient cow numbers to adequately challenge bull fertility capacity.

A significant relationship was found between SC EPD and bull prolificacy in the current study, but no other breeding group management activities explained a signifi-cant amount of variation in calf output. Scrotal circumfer-ence measurements have been previously associated with Ncalf (Coulter and Kozub, 1989), although Holroyd et al. (2002) generally found no relationship between actual SC measurements and prolificacy with the exception of 5/8 Brahman bulls. This relationship between SC EPD and prolificacy during a natural service breeding season has not been previously reported.

Scrotal circumference EPD have been positively as-sociated with sperm motility and total BSE score (Moser et al., 1996). Favorable influence of scrotal circumfer-ence and SC EPD on heifer maturity has been reported (Brinks et al., 1978; Toelle and Robison 1985; Smith et al., 1989; Moser et al., 1996; Martnez-Velzquez et al., 2003). Scrotal circumference is also a component of the breeding soundness examination that has been related to bull fertility (Kealey et al., 2006), and SC estimates testicular tissue volume, which impacts semen quantity. The most valuable SC measurements are those taken at approximately 1 yr of age.

Difficulties in assessing fertility in both male and fe-male cattle are well documented, and genetic improve-ment is further hindered by the low heritability of fertil-ity traits (


that required to sample the entire calf crop in this study. Because of age and source verification marketing, birth-dates are often known, and samples could be collected at marking time from a designated group of calves. Even without whole-herd birthdate records, strategic planning during the third week of calving could include record-ing or marking calves born during that time period for later sampling. Given the moderate repeatability of pro-lificacy, this type of reduced sampling could provide an approach to identify bulls with the greatest likelihood of being either highly or lowly prolific. The costs involved in DNA collection include not only the costs of the tests (currently ~$15/head for parentage testing; www.neo-gen.com/, accessed January 14, 2014), which are likely to continue to decrease in the future, but also the costs associated with unique animal identification, labor to process each animal and collect the DNA sample, and costs required to manage the records and integrate the DNA information back into herd management decisions.

The value of parentage information needs to out-weigh the costs of genotyping. One study examined the value of DNA paternity identification on commercial beef cattle operations. The assumption of their model, based on 15 microsatellite loci, was that the informa-tion would be used to cull bulls that were producing low weaning weight calves (Gomez-Raya et al., 2008). Although this might be important if all bulls are produc-ing an equal number of progeny, as can be seen from the data in this study, some of the bulls that produced bulls with the lowest weaning weight were siring a large number of calves and hence were not the least profitable bulls. The benefit that could be derived from these par-entage data would conceptually be the removal of low prolificacy bulls and increasing the cow:bull ratio of the more prolific bulls, thereby saving on the costs associ-ated with maintaining an inactive or low prolificacy bull.

Using the data obtained in this study, it can be es-timated that if the entire calf crop were sampled to ob-tain prolificacy estimates, then the cost per bull to obtain prolificacy data would be approximately 20 times the cost of the test (1 bull + 19 offspring), or $300/bull in the case of a $15 test. Less expensive alternative sam-pling strategies could be envisioned, including sampling all bulls and only those calves born in wk 3 (~20% of the calf crop) or, alternatively, sampling only those offspring produced by young sires in their first breeding season based on the observation that young bulls categorized as either HP or LP tended to remain in those categories in subsequent breeding seasons. However, given the small number of young bulls involved in this study (8 each ini-tially categorized as HP and LP), care should be taken in over interpreting these results. In addition to prolificacy data, the DNA information could also be used to calcu-late genetic merit estimates of these commercial bulls

and identify those producing superior or problematic classes of calf (e.g., high birth weight calves).

These results reveal that highly prolific bulls sire a disproportionately large number of the preferred more valuable, early born calves (Funston, 2012) in well-man-aged herds that have a large numbers of females cycling early in the breeding season. In self-replacing beef sys-tems, replacement heifers are often selected on the basis of age to enhance the potential for conception early in their first breeding season. This study showed that only a small percentage of such replacement heifers would likely be sired from LP bulls, providing indirect selec-tion on male fertility. Using DNA paternity assignment to evaluate the relationship between heifer fertility and sire prolificacy would provide information of economic interest given the high costs of raising replacement heif-ers and the overriding importance of fertility to the beef enterprise (Melton et al., 1979; Melton 1995).

IMPLIcAtIoNs

The DNA paternity testing of the calf crop can be used to determine sire prolificacy, and testing of fewer calves offers opportunities for reduced costs. The num-ber of calves sired by natural service bulls in multisire breeding pastures is highly variable between bulls but similar across ranches and was positively associated with SC EPD. Varying prolificacy of herd bulls has implica-tions for the genetic composition of replacement heifers, with the genetics of those highly prolific bulls siring a lot of calves likely to be disproportionately represented in the replacement heifer pool. To be cost-effective, the costs of parentage testing need to be recouped by the value derived from this resulting information. One such use might be the cost savings associated with the remov-al of low prolificacy bulls, although the feasibility of this approach would depend on the continued ability of the more prolific bulls in one year to be able to successfully service an increased cow:bull ratio in the following year. Prolificacy was found to be moderately repeatable (0.33) in this field study of commercial herd sires.

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evaluation of bull prolificacy on commercial beef cattle ranches using dna paternity analysis

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