J Sci Food Agric 1994,64,337-339

The Effect of Field Drying on the Non-structural Carbohydrate Composition in Vetch Hay Jesus Treviiio,"* Carmen Centeno; Luis Ortiz' and Rafael Caballerob "Departamento de Produccibn Animal, Facultad de Veterinaria, 'Instituto de Alimentacibn Animal, Ciudad Universitaria, 28040 Madrid, Spain (Received 5 April 1993; revised version received 21 October 1993; accepted 5 November 1993)

Abstract: Common vetch (Yicia satiua L) was harvested and allowed to dry in the open air. The composition of fresh forage in non-structural carbohydrates varied markedly between years, the contents of sugars and starch in both leaves and stems being much higher in 1990-91 than 1991-92. Field drying produced a significant decrease in the amounts of these Carbohydrates. The total losses of sugars and starch in hay, expressed as losses of dry matter, ranged from 28 to 15 g kg-', depending on the year.

Key words: common vetch, Vicia satiua, haymaking, sugars, starch, carbo- hydrates.

INTRODUCTION

Common vetch (Vicia satiua L) is an important forage crop in Spain, grown under a wide range of climatic and soil conditions. It is generally cut for hay at the growth stage of early pod formation.

Losses of dry matter and quality during haymaking may be very large, and they include respiration, rain and mechanical losses. Soluble carbohydrates, which are highest in nutritive value, are most susceptible to loss, resulting in an overall reduction in forage quality in addition to the dry matter loss (Wilkinson 1981; Johnson et a1 1984).

The work reported here was undertaken to obtain information on both the composition of non-structural carbohydrates in vetch forage and the changes occurring in this fraction during the field drying to make hay under Mediterranean climatic conditions.

EXPERIMENTAL

Field trials were conducted during 1990-91 and 1991-92 on a sandy loam soil at La Poveda Experimen- tal Station near Madrid. Common vetch (Vicia satiua cv

* To whom correspondence should be addressed.

Comun tolerada) was sown in autumn at the rate of 100 kg ha-'. A basal dressing of 30 kg N, 60 kg P20 , and 60 kg K 2 0 ha-' was applied before sowing.

A completely random experimental design with three replications was used. The size of plots was 100 m2. Plants were cut on 16 and 8 May in the first and second trial, respectively, at the stage of very early pod forma- tion. Immediately after cutting, one forage sample of approximately 10 kg per plot was taken and sub- sampled into leaf and stem components; this fresh material was frozen at -24°C and then freeze dried. Vetch forage was set out in the field and allowed to dry in the open air (120 h in 1991 and 72 h in 1992). The weather during the field drying period was good in the two years of experiment (rainfall, 0 mm, mean maximum and minimum air temperatures, 25.4 and 55°C in 1991 and 27.7 and 9.2"C in 1992). After drying (> 80% DM), one sample of hay (ca 3 kg per plot) was taken and subsampled into leaves and stems. All samples were ground, kept in air-tight containers and stored at - 24°C until required.

Water soluble carbohydrates (WSC) obtained by exhaustive extraction with deionized water (60°C, 60 min) were measured colorirnetrically by the anthrone method (Yemm and Willis 1954); glucose and fructose give different molar responses, and therefore a sucrose standard was used as a compromise. For sugar analysis,

337 J Sci Food Agric 0022-5142/94/$09.00 0 1994 SCI. Printed in Great Britain

338 J Treviiio, C Centeno, L Ortiz, R Caballero

TABLE 1

the leaf and stem components before and after field drying" Non-structural carbohydrates (g kg- dry matter) in the whole aerial parts of vetch and in

wsc Glucose Fructose Sucrose Starch

Year 1990-91 Herbage

Whole aerial parts 103 f 2.9 38 f 2-5 15 f 1.2 34 f 2.9 50 f 2.1 Leaves 72 f 2.9 21 f 2.4 11 f 1.2 24 f 2.2 38 f 1.7 Stems 138 f 7.8 54 f 3.7 21 f 2.9 46 f 4.3 61 f 5.0

Hay Whole aerial parts 83 f 3.6 16 f 2.6 8 f 1.4 46 f 2.4 39 f 3.9 Leaves 67 f 2.2 9 f 1.4 6 f 1.2 34 f 1.6 34 f 4.7 Stems 108 f 2.9 21 f 1.9 12 f 1.7 57 f 2.4 44 f 4.3

LSDb (P c 0.05) 20 7 5 6 10

Year 1991-92 Herbage

Whole aerial parts 60 f 3.3 17 f 1.6 10 f 1.4 22 f 2.2 20 f 2.4 Leaves 48 f 2.6 10 f 0.9 8 f 1.6 20 f 1.4 11 f 1.5 Stems 81 f 3.8 29 f 2.8 13 f 1.2 26 f 2.6 31 f 3.1

Hay Whole aerial parts 52 f 3.2 10 f 1.4 7 f 1.2 26 f 2.9 11 f 1.7 Leaves 38 f 2.4 6 f 0.8 6 f 0.5 22 f 1.6 2 f 0-4 Stems 66 f 4.3 16 f 1.2 9 f 0.9 34 f 1.9 19 f 2 3

LSDb ( P < 0.05) 8 4 3 5 7

' Analytical values are means of duplicate analyses of three replicates f standard deviation. Least significant difference.

250-mg samples were mechanically shaken for 1 h in 100 ml of 80% (v/v) ethanol at room temperature and filtered through Whatman No 40 paper. The filtrate was evaporated to dryness under reduced pressure at SWC, diluted to 25 ml with acetonitrile/water (80 : 20 v/v) and passed through a Waters C , , Sep-PAK cartridge. Glucose, fructose and sucrose were determined by HPLC using a Carbohydrate Analysis Column (Millipore Iberica SA/Waters, Barcelona, Spain). Starch was determined colorimetrically after sequential hydro- lysis with a-amylase and amyloglucosidase (Karkalas 1985).

Data were subjected to analysis of variance, and dif-

ferences between means were tested by the least signifi- cance difference (LSD) test. The BMDP package was used (Dixon 1988).

RESULTS AND DISCUSSION

The carbohydrate content of the whole aerial parts of vetch and in the leaf and stem components are sum- marked in Table 1. For the aerial parts of the plant, the sum of glucose, fructose and sucrose constituted between 81.6 and 84.4% of the total soluble carbo- hydrates, glucose being the predominant sugar in 1990- 91 and sucrose in 1991-92. Sugar and starch contents

TABLE 2 Percentage losses of sugars and starch during the field drying of vetch

Year 1990-91 Year 1991-92

Whole Leaves Stems Whole Leaves Stems aerial parts aerial parts

Glucose 59.5 58.9 62.7 42.8 41.4 46.8 Fructose 48.8 47.7 46.3 31.7 26.7 33.3 Sucrose + 29.8 +36.4 + 18.2 + 14.4 +7.5 +26.0 Total sugars 22.7 15.8 29.0 14.8 12.6 16.4 Starch 24.8 14.7 31.1 41.8 82.3 40.9

Field drying and non-structural carbohydrates in vetch 339

were significantly higher in the stems than in the leaves (P < 0.05). In general, the non-structural carbohydrate composition of forage vetch was characterised by a con- siderable fluctuation between years; in practice, between plants harvested at the same stage of maturity in 1990- 91 and 1991-92, a two-fold difference was found for the content of combined sugars and starch in both the leaf and stem components. These differences found between years might be associated with weather conditions, especially rainfall and temperature, since other experi- mental conditions such as soil fertility or agronomic practices were similar for the two field trials.

The results obtained for vetch hay are also shown in Table 1. Field drying produced a significant decrease in the amount of soluble carbohydrates of forage (P < 0.05). However, the changes in the individual sugars did not occur in the same way, with the concen- trations of glucose and fructose decreasing and that of sucrose increasing (P < 0.05). The starch content also decreased significantly in hay compared with fresh forage in both experiments (P < 0-05). Separate leaf and stem analyses showed that the changes in the non- structural carbohydrates followed a similar pattern in both plant components, with decreases in the amounts of glucose, fructose and starch and increases in sucrose content. This increase in the amount of sucrose in hay was consistent between years and confirms the hypothe- sis of other researches (Melvin and Simpson 1963) that sucrose can be synthesised from glucose or fructose in plant tissues during the air drying of herbage.

There were losses of carbohydrates during the field drying of vetch forage. The total losses of individual sugars and starch, calculated as the percentage change in the quantity of the constituent between standing crop and hay, appear in Table 2. In general, these losses of carbohydrates were higher in the stem than in the leaf. This might be explained by the fact that leaves dry more rapidly than stems; thus, leaf tissue would reach dry matter concentrations high enough to limit respiratory

losses sooner than stem tissue (Harris and Shanmugal- ingam 1982; Collins 1985). When expressed as losses of dry matter, the total losses of sugars and starch for hay vetch were rather small, ranging from 28 to 15 g kg-'. This range of dry matter losses attributed to respiration and enzymatic activities is in agreement with the data reported by others (Pizarro and James 1972; Rotz and Abrams 1988) for various grass and legume hays made in good conditions.

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