Incorporation of rapeseed meal extracted by pressure in a diet for growing fattening bulls .1. Intake, digestibility and fermentation in the rumen

Intake, digestibility, degradation kinetic and fermentation were studied with bulls offered fattening diets with four different incorporation rates of rapeseed meal obtained by pressure; the ether extract content was 8.6% and the glucosinolate content 17,6 mu molg. The control diet was made of sugar beet pulp, cereals, soya bean meal and linseed meal. In the three other diets, nitrogen from rapeseed meal provided 33, 66 or 100% of nitrogen from soya bean meal or linseed meal. The intakes of the control diet were 94.3 +/- 7.3 g/kg metabolic body weight. Intakes were maintained high in the diet with 33, 66 or 100% of protein from rapeseed meal at 93.5 +/- 7.7; 93.0 +/- 5.0 and 93.5 +/- 7.7 g/kg, respectively. The apparent digestibility of rapeseed meal in the ration containing only rapeseed meal was high at 74.9 +/- 1.9% for dry matter, 76.5 +/- 2.1% for organic matter, 69.9 +/- 3.9% for nitrogen, 59.1 +/- 4.8% for ADF and 74.6 +/- 6.9% for ether extract. Dry matter of rapeseed meal was less degradable (57.8 +/- 5.0%) than that of soya bean meal (65.5 +/- 2.6%) or of linseed meal (61.6 +/- 4.2%) but was not as well protected than that of protected soya bean meal (43.6 +/- 2.6%) (P < 0.001). Similar trends were observed for the degradability of nitrogen (P < 0.001). Ether extract of rapeseed meal was more degradable than that of linseed meal (P < 0.001). There were no significant effects of the incorporation rate of rapeseed meal on the pH in the rumen. The fermentations were mainly of the acetate type with usual concentrations of propionate, butyrate, total volatile fatty acids, glucose or alpha amino nitrogen. Ammonia concentration was higher with the control diet and lower with the diet with 100% of protein from rapeseed meal. Fatty acids of rapeseed meal were more protected against saturation by the rumen microorganisms than in linseed meal.