The effect of cereal type (barley versus oats) and rapeseed meal supplementation on the performance of growing and finishing dairy bulls offered grass silage-based diets

A 3 × 2 factorial design with growing dairy bulls offered a grass silage-based diets was used to study the effects on animal performance of (1) cereal type (flattened barley versus flattened oats) and (2) inclusion of rapeseed meal (RSM) in the diet. Two feeding trials comprised a total of 42 Finnish Ayrshire and 18 Holstein-Friesian bulls. The animals were housed in a tie-up barn and fed individually. All bulls were offered grass silage (686 g digestible organic matter in kg dry matter (DM)) ad libitum. The target for average concentrate level during the experiment was 400 g/kg DM for all treatments. Three cereal feeding treatments were flattened barley, flattened barley + flattened oats (1:1 on DM basis) and flattened oats, fed either without RSM (RSM−) or with RSM (RSM+). In the RSM− diets the crude protein (CP) content of the concentrate was 132 g/kg DM. Rapeseed meal was given so that the CP content of the concentrate was raised to 160 g/kg DM in the RSM+ diets, which increased the CP content 21% with RSM supplementation. The mean initial live weight (LW) of the bulls was 257 ± 26.6 kg and the mean final LW 687 ± 30.9 kg. Increasing the proportion of oats in the diet decreased the live weight gain (LWG) (P < 0.05). Linearly impaired LWG was a consequence of decreased metabolizable energy intake (P < 0.05) with increasing oats proportion. Because there was no difference in DM intake, also feed conversion efficiency (kg DM/kg LWG) reduced (P < 0.05) with increasing oats proportion. There were no effects of treatments on the dressing proportion, carcass conformation score or carcass fat score. The RSM supplement had no effect on performance parameters, and there were no significant cereal type x RSM interactions for any of the measured parameters. In conclusion, the LWG and feed conversion of growing dairy bulls reduced with increasing oats proportion. Since rapeseed meal did not affect animal performance, there is no reason to use RSM for finishing dairy bulls when they are fed good-quality grass silage and grain-based concentrate.