Protein supplementation of grass silages of differing digestibility for growing steers

Previous studies have demonstrated that protein sources which are primarily degraded in the rumen such as rapeseed meal or soyabean meal may be as eff ective as a less rumen degradable protein source such as fishmeal in suppor ting higher levels of animal performance in young steers fed on grass silag e. However, the response to type of protein supplement is likely to be infl uenced by the composition of the basal diet. This study has examined the ef fect of supplementing silages prepared from early (EH) and late (LH) harves ted grass with two protein sources of differing rumen degradability, rapese ed meal (RSM) and fishmeal (FM) or a mixture of the two (M), thus creating eight experimental diets of LH, LHRSM. LHFM, LHM, EH. EHRSM. EHFM and EHM. Silage was offered ad libitum and supplements were included at 100 g fresh weight/kg silage DM intake. The RSM and M diets were made isonitrogenous wi th FM diets by the addition of urea. Animals remained on diets for 18 weeks and liveweights were monitored for a further 13 weeks while the animals we re at pasture. Dry matter (DM) intakes and liveweight gains were higher wit h EH than with LH silage (P < 0.001). Response to type of protein supplemen t was dependent on silage quality. On LH silage. higher intakes were noted on FM and M (82.4 and 82.8 g DM/unit metabolic liveweight/day, respectively ) relative to silage only (75.8 g DM/unit metabolic liveweight/day) and thi s contributed towards higher liveweight gains (P < 0.01). Liveweight gains tended to be higher on LH silage supplemented with FM compared to RSM (0.76 v. 0.67 kg/day, respectively, P = 0.08). In comparison, on EH silage, rela tive to EH unsupplemented, the intake was highest on M (89.2 v. 96.6 g DM/u nit metabolic liveweight/day), but liveweight gains were not significantly different between supplements. On turnout to pasture, those animals fed on silage alone exhibited compensatory growth (P < 0.025) with the result that those fed on LH silage only grew faster and achieved the same liveweight a fter 13 weeks at grass as those supplemented with RSM or M. There was a ten dency for those fed on FM to maintain an advantage in liveweight after the period at pasture. On EH silage, at the end of the grazing period no signif icant differences in liveweight existed between the different supplements a nd on average were 23 kg heavier than EH silage unsupplemented. In conclusi on. silage type (stage of harvest and quality) and protein supplementation influenced animal performance. On late harvest and poorer digestibility sil age, there was some indication that feeding fishmeal was better than rapese ed but this was less evident on early harvest and higher digestibility sila ge. However, considering the price differential between these two supplemen ts and small difference in animal performance it is concluded that rapeseed meal is as effective as fishmeal when used as a protein supplement for gro wing cattle fed on grass silage.