Intake and digestion by lambs fed a low-quality grass hay supplemented or not with urea, casein or cassava meal

Ten Polwarth×Texel lambs (21 ± 3 kg live weight), housed in metabolic cages and fed ad libitum a low-quality grass hay (Cynodon sp.) were used in a replicated 5×5 Latin Square experiment to evaluate effects of supplement type on forage and total intake, rumen fermentation, microbial protein synthesis, digestibility and N retention. Supplements were a non-fibre carbohydrate (NFC) source (i.e., cassava meal), NFC plus a degradable true protein N (TPN) source (i.e., calcium caseinate), NFC plus a non-protein N (NPN) source (i.e., urea:ammonia sulphate, 9:1), NPN alone or no supplement (control). Hay and fibre intake, as well as fibre digestibility, rumen microbial protein synthesis and N retention were lowest (P<0.05) in lambs supplemented with NFC alone. Total organic matter (OM), digestible energy intake, rumen microbial protein synthesis and N retention improved (P<0.05) with supplementation with NFC plus either TPN or NPN. Rumen pH, as well as rumen concentrations of ammonia, sugars, amino acids and peptides was markedly affected by supplement type and time after feeding (treatment×time interaction, P<0.05). Hay utilization by lambs was not improved by any supplement and markedly depressed with NFC as the sole supplement. This effect, however, was likely not due a decreased rumen pH, given that the mean rumen pH values were near 7.0 in all treatments. Digestible energy intake, rumen microbial protein synthesis and N retention were improved only when supplementation included both NFC and degradable N sources, regardless of whether it was TPN or NPN.