The nutrition of lambs offered forage brassicas
Burnett, Fiona Ann
The nutritive value of a range of forage brassicas to lambs was described and the results used to test the extent to which the supply of nutrients from forage brassica crops could be increased and animal performance improved by supplementation. Some of the factors influencing the voluntary intake of forage brassicas by lambs were also investigated.The voluntary intake of OM of cabbage leaf, hybrid turnip leaf, stubble turnip leaf and bulb, rape leaf and stem, kale leaf and stem and swede bulb were found to range from 17 to 24 g OM kg'! W day“! with uniformly high level of digestibility of OM (0.77 to 0.94). The amounts of OM apparently digested in the rumen per kg OM and kg DOM intake were in the range 0.34 to 0.73 and 0.67 to 0.79 respectively, higher than those reported previously for other forages. Non-ammonia nitrogen flows at the abomasum ranged from 0.66 to 1.33 g N g~! N intake. It was estimated th at tissue gains in lambs grazing the bulb components were limited by the availability of nitrogen substrates in the rumen, whereas in lambs grazing the leaf and stem components, tissue gains were limited by the availability of energy substrates in the rumen.This latter hypothesis was tested by giving energy andenergy/protein supplements to lambs offered leaf components of rape andhybrid turnip leaf under restricted intake in an indoor study and byconducting a grazing experiment. Supplementation with a barley supplement as 23% of the diet increased the NAN flow at the abomasum by 30% in lambs given hybrid turnip but not in lambs given rape leaf. Supplementation also resulted in a 10% increase in the proportion ofdigestible OM apparently digested in the rumen with both crops. The results were interpreted as indicating that NAN flows at the abomasum were indeed limited by energy substrates in the rumen, the lack of response with rape leaf being attributed to the method and type of supplement not being optimum for the capture of the N potentially available in the rumen.Under grazing conditions, the substitution rates of rape leaf and hybrid turnip leaf by supplements ranged from 0 to 0.4 g OM forage g- * OM supplement intake. Lambs grazing hybrid turnip had lower intakes and carcase gains than lambs grazing hybrid turnip, but when supplemented, carcase gains were similar to those of lambs grazing rape leaf. The fat content of the carcase with lambs grazing hybrid turnip, irrespective of supplement treatment, was lower than that of lambs grazing rape leaf.The voluntary intakes of rape leaf were not found to be influenced by the presence of foam in the rumen or by the presence of additional tactile stimuli on the rumen wall. The infusion of aglucone products of sinigrin, the principal glucosinolate present in cabbage, into the rumen of lambs given a cabbage diet resulted in variable voluntary intakes. These findings suggested that glucosinolates may be involved in the low and variable intakes associated with forage brassicas.