The effect of rate of substitution of processed, urea-treated whole-crop wheat (pWCW) for grass silage on intake, performance and whole-tract digestibility was evaluated using 44 dairy cows. Cows received 10.5 kg of concentrates per day and one of the following forage mixtures (dry matter (DM) basis): grass silage alone (W-0); 0.75 grass silage, 0.25 pWCW (W-25); 0.5 grass silage, 0.5 pWCW (W-50) or 0.25 grass silage, 0.75 pWCW (W-75). Forage DM intake increased linearly with inclusion rate of pWCW from 9.7 kg DM per day in cows fed W-0 to 14.6 kg DM per day in W-75. By contrast, milk and protein yield (kg/day) were higher (P , 0.05) in cows receiving W-25 compared with W-0, but there was no effect (P . 0.05) of treatment on fat yield (kg/day). From week 11 of the experiment onwards, body condition score increased with rate of inclusion of pWCW ( P , 0.05). Whole-tract apparent digestibility of organic matter (OM) and fibre (kg/kg), decreased linearly with rate of inclusion of pWCW. Assuming a constant digestibility of starch in the other diet components, the apparent digestibility of starch in pWCW was 0.95 kg/kg and was not affected by rate of inclusion (P . 0.05). Four continuous culture vessels were used to determine the effect of rate of inclusion of pWCW on ruminal metabolism in four periods, each of 14 d duration with sampling conducted on days 9 to 14. Vessel ammonia concentration increased linearly ( P , 0.05) with rate of inclusion of pWCW whilst mean pH tended (P ¼ 0.06) to decrease. The ratio of acetate to propionate increased from 2.5 in vessels receiving W-0 to 3.2 in those receiving W-75 (P , 0.001). There was no effect (P . 0.05) of treatment on digestibility (g/g) of OM, fibre or starch or microbial protein flow (g/day). It is concluded that forage DM intake increased linearly with rate of inclusion of pWCW, but there was no further improvement in milk yield from inclusion rates above 0.25 of the forage DM, with body condition score increasing instead. Increasing the inclusion rate of pWCW resulted in a more ketogenic volatile fatty acid profile but did not affect the efficiency of microbial protein synthesis when determined in vitro.

The development of a forage processor that grinds the grains in whole-crop wheat (WCW) prior to ensiling has been shown to improve whole tract digestibility of the starch component and improve the efficiency of forage utilisation (Jackson et al. 2004). This allows wheat to be harvested over a much wider harvest window than was previously possible. Further work (Bond et al., 2004) demonstrated that animals fed processed, urea-treated WCW harvested at 700 g dry matter (DM) per kg had a significantly higher milk yield than those fed fermented WCW harvested at approximately 450 g DM/kg, or urea-treated WCW harvested at approximately 850 g DM/kg. Previously, processed, urea-treated WCW for dairy cows has been included at 0.67 of the forage DM intake, although, the optimal inclusion rate of processed WCW is unclear. The objective of the current experiment was to investigate the effect of rate of inclusion of processed, urea-treated WCW on the performance and apparent digestibility in dairy cows.