PROTEIN DEGRADABILITY IN PRAIRIES FOR ROTATIONAL GRAZING IN THE COAST OF CANTABRIA AND ITS PREDICTION - G. Salcedo Diaz

Department of farming technology at I.E.S. “La Granja, 39792 Heras, Cantabria, Spain

ABSTRACT

During 1994 and 1995 the ruminal degradability of crude protein in prairies of Lolium perenne (cv. Tove) and Trifolium repens (cv. Grassland Huia), used for rotational grazing with milk cows from March to November were studied. The results showed a high effective degradability (73.67 percent), the protein solubility (43.7 percent), a rate of time degradability of 0.125 and a potential degradability of 88.12 percent. The highest degrading peaks took place in autumn and the lowest in summer. The content of neuter detergent fibre is the variable in closer relationship with effective degradability of protein (Depb)=126.8 - 1.07 NDF. The relationship degrading nitrogen in rumens per MJ of metabolizing energy (NDR/MJ of ME) is higher in autumn (2.26 g), lowest in summer (1.96 g) and intermediate in spring (2.03 g); its prediction is closely influenced by the content of crude protein in pasture.

Keywords: rotational grazing, ruminal degradability, protein, prediction, Cantabria

INTRODUCTION

Green forages of prairies (mainly gramineous) is the most common food given to milk cows along the Cantabria coast, administered in grass, ensilage or hay. The new feeding systems in milk cows, praise the importance which has the degradation of protein in rumen, in order to calculate the true protein intake in the small intestine (Satter and Roffler, 1975).

The degradability of protein in the forage for ruminants depends on forms of protein reserves (Wallace et al., 1987); where physical and chemical features of forage, which may undergo a ruminal fermentation are located in the cellular walls (Tamminga, 1983). Ellis et al., (1988); season of the year Le Goffe et al., (1993); degree of lignification (Deinum, 1984); vegetal species, Le Goffe et al., (1993) and type of conservation (Vik-Mo, 1989; Salcedo and Sarmiento, 1997). Most of the methods used to measure the values of degradable nitrogen are based on the value of the quantity of amino acids the intestine can absorb.

When milk cows are given low degradability food, a relation degradable protein is obtained in rumen per MJ of metabolizing energy (NDR/MJ of EM) which does not allow microbial growth. The ARC (1980) recommends a relation of 1.25 g of NDR/MJ of EM to improve the synthesis of microbial protein, which would cause ruminal alkalosis on diets based mainly on forages (Salcedo, 1997).

The aim of this work is to determine the ruminal degradability by means of the “in sacco” technique of grass used in rotational grazing and its prediction using laboratory data (NDF).

MATERIAL AND METHODS

The experiment was carried out at the I.E.S. “La Granja”, Heras, (Cantabria), on a grazing field of 8 ha in rotational grazing. The annual mean fertilization meant the contribution of 250-100-90 uf/ha of N-P-K.

The measurements “in vivo” were carried out on two cows in lactation fitted with ruminal cannulas of 10 cm Æ, fed with 80 percent pasture and 20 percent of concentrate. To establish the ruminal degradability we used the “in sacco” technique (Mehrez and Orskov, 1977), using nylon of 11 x 6 cm and 45 mm pores. Incubations were carried out during the first two weeks of April 1996, the first for the grass of 1994 and 1995 for the second; accordingly, the second incubation was carried out during the first fortnight of June to avoid the effect the ruminal factor could have on degradability. The bags were introduced in the cows rumen at 0800 hours and removed after 2, 4, 8, 16, 24, 48 and 72 hours. Once incubated they were washed in cold water in a washing machine for three periods of five minutes each, changing the water and dried at 60ºC using a fan-operated hot heater for a period of 48 hours.

The estimates of N missing from rumen followed the model described by Orskov and McDonald (1979): y=a + b [ 1 - e^{-(c x t)}], y being the percentage which disappears in time t; a, the soluble fraction or easily degradable; b, the fraction potentially degradable and c the fractional rate of fraction b. The factors a, b and c in this model are obtained by non-linear decline using the PROC (NLIN) of SAS (1988). The effective degradability (De) from N and DM was calculated for a speed of ruminal emptying of 0.06^h-1, using Orskov and McDonald formulae (1979): De = a + (b . c)/c + k.

RESULTS AND DISCUSSIONS

Table 1 shows the results of cinetic degradation, finding important differences between months, years, interaction year x month and animals.

The soluble fraction in gross protein (apb) showed no important differences between years Y, but it did between months, year x month and animals. The average value was 43.74 percent minimum 38.17 percent in June and maximum 53.3 percent in November. These results are lower than those found by Alfageme (1995) in dactilo, Italian and English ryegrass in the gleaning period at 10 cm and in the leafy stages of oats but similar to the beginning of gleaming (Salcedo, 1997) and higher than those mentioned by (Van Vuuren, 1993 and AFRC, 1993) in green forage.

When relating the solubility of crude protein with contents in FND, we see (r² =0,81), Table 2. Tamminga et al. (1991) they obtain better comparisons in field hays with crude protein and cellular wall, but in multiple regressions protein contribution does not improve importantly due mainly to the high relationship with NDF. Van Soest (1982) concludes that as the plant gets old the solubility of crude protein decreases because of the enlargement of the cellular wall, similar facts are obtained (Van Vuuren et al., 1991; Salcedo, 1997).

In the slowly degradable fraction (bcp) we observed important differences at year and month level but not between animals, Table 1. The mean value was 44.44 percent, minimum in August (40.6 percent) and maximum (48.76 percent) in October, lower than those mentioned by AFRC (1993) in green forages, due to a higher content in the soluble fraction of the protein in the forages in our experience. The mean value time degradation rate (cpb) was 0.125, higher than those mentioned by the AFRC (1993) of similar digestibility of dry matter.

Table 1. Degradation of protein in forage in grazing conditions.

a: fast degrading fraction; b: slowly degrading; c: time-degrading pattern; De: effected degradability for a step K=6 percent; Dp: potential degradability.

Table 2. Prediction of degrading parameters of crude protein (Y percent) from the content in NDF.

*** P<0,001

The effective degradability of CP (Depb) was 73.67 percent. In autumn (85.14 percent) the highest and lowest 68.35 percent in spring, due to a higher percentage of soluble protein, as More and Simon (1985), producing a lower proteosynthesis related to a low temperature and solar irradiation.

The values obtained corresponded to those of Amrane and Michalet-Doreau (1993) in the leafy stages of Italian ryegrass of similar content in cellular wall and lower content of crude protein; similar to those mentioned by AFRC (1993) of higher content in the slowly degradable fraction, motivated by a compensating effect of Depb in relation to bpb and apb, although when the latter is higher, the level of N-ammoniacal in rumen is higher too (Verité et al., 1984); and lower than those obtained by Le Goffe et al. (1993b) in green forages. Higher results are obtained by (López et al., 1990), in field grass, although estimates are meant for a rate of K=3 percent. Alvir and González (1992) concluded that degradation of matters that contain nitrogen found in forages is determined among other factors, by the relationship forage/concentrate of the portion, which has an effect on both the time food stays in rumen and the intensity of microbic action; in conclusion, in diets rich in concentrates the Depb decreased, increasing the slowly degrading fraction but decreasing the soluble protein.

CONCLUSIONS

The prairies used under rotational grazing conditions in a leafy state show high percentages in the degradability of their protein, being the content in NDF the variable which better explains this variation.

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