Diet selection by growing lambs offered whole barley and a protein supplement, free choice: Effects on performance and digestion

Feeding whole barley and a protein supplement at choice to early weaned lambs (15 kg initial weight) was studied in four experiments. In a first experiment whole barley and a pelleted soybean meal-based supplement (348 g crude protein (CP)/kg dry matter (DM)) were given either at choice (CH) or mixed in a 75/25 ratio (M) and compared to a pelleted compound feed containing the same ingredients and supplemented with barley straw (C). The effect of allowing free access to straw in the CH treatment (CHS) was tested in a second experiment. Rumen fermentation was also studied in six lambs per treatment slaughtered after the growing period, and digestibility and allantoin excretion were determined in other four lambs per treatment. In Experiments 3 and 4 supplements containing 253–264 (L), 341–329 (M) and 424–427 (H) g CP/kg DM were offered at choice with whole barley to lambs penned individually (Experiment 3) or in a group (Experiment 4) and diet selection and lamb performance were recorded. The protein supplement was selected in proportions of 0.28 and 0.34 (S.E. 0.021) of concentrate intake with treatments M and CH (Experiment 1) and 0.45 and 0.52 (S.E. 0.033) with treatments CH and CHS (Experiment 2). The concentration of rumen ammonia increased (Pb0.05) to 28 and 56 (S.E. 8.6) mg/l compared to 12 mg/l in treatment C. Rumen pH also increased (Pb0.001) from 4.95 to 5.48 and 5.60 (S.E. 0.080) with treatments CH and CHS while the concentration of volatile fatty acids and the acetate to propionate ratio tended to decrease (Pb0.1). Organic matter digestibility and allantoin excretion were higher with whole barley feeding (0.82 vs. 0.75 (S.E. 0.18) and 13.1 vs. 7.7 (S.E. 1.06) mmol/d) but decreased with straw supplementation (0.78 and 10.2 mmol/d). Daily weight gain of male lambs (but not of females) tended (P=0.08) to be higher with whole barley than with the compound feed due to higher concentrate intake (Pb0.05), which was also depressed by straw supplementation of whole barley reducing the rate of gain (P=0.06). Incidence of hyperparakeratosis and ulceration of rumen wall was high with all treatments (0.80 and 0.36) but papillary clogging was less frequent (Pb0.01) with whole barley (0.16) than with the compound feed (0.70) and tended (P=0.01) to be reduced by straw supplementation. In Experiments 3 and 4 the proportion of protein supplements selected ranked in reverse order