Energy and nitrogen balance of lactating dairy cows given mixtures of urea-treated whole-crop wheat and grass silage

Energy and nitrogen balances were carried out with four multiparous Holstein/Friesian cows offered four diets in a Latin-square experiment to evaluate urea-treated whole-crop wheat as a partial grass silage replacement for lactating dairy cows. Grass silage (GS) was produced from the primary growth of a perennial ryegrass sward. Spring wheat (cv. Axona) teas harvested at 603 g dry matter (DM) per kg and preserved with 20 (WCW-20) or 40 (WCW-40) kg urea per t DM. The diets were 6 kg DM of a dairy concentrate daily with one of four forage treatments offered ad libitum. The forage treatments were GS alone, a 2:1 DM ratio of GS with WCW-40 (2 :1 40), or a 1 :2 DM ratio of GS with WCW-20 (1 :2 20) or WCW-40 (1 :2 40). Each period lasted 4 weeks with energy and nitrogen balances being carried out in respiration chambers over 6 days in the last week. Replacement of GS by diets containing WCW resulted in significant increases in DM intake (P < 0.01). Changes in milk yield and composition were small and non-significant but yields of milkfat and protein were higher on WCW diets than on GS diets (P < 0.05). With increasing proportions of WCW in the diet there were significant linear falls in apparent digestibility of DM (P < 0.001), organic matter (P < 0.001), neutral-detergent fibre (P < 0.01), acid-detergent fibre (P < 0.01), starch (P < 0.001) and nitrogen (P < 0.01). Gross energy intakes (P < 0.01) and faecal (P < 0.001), methane (P < 0.05) and milk (P < 0.05) energy outputs were higher with the WCW diets than with GS but urine energy and heat losses were unaffected. In consequence there were no significant differences among the diets in digestible or metabolizable energy (ME) intakes. However dietary ME concentrations (MJ ME per kg corrected DM) fell with increasing WCW inclusion from 11.54 on GS to a mean of 9.96 on the 1:2 diets (P < 0.001). It was calculated that the ME concentration of the WCW was only 8.1 MJ/kg corrected DM at maintenance intake, considerably lower than values used conventionally. There were no significant diet effects on the partition of ME or on the partial efficiency of ME utilization for milk production (k(l)). The increasing inclusion of WCW increased N losses in urine (P < 0.05) and faeces (P < 0.01) with no net effect on N digested or retained though there teas a small increase in milk N output (P < 0.01). It is concluded that low digestibility is the major cause of the small milk response to the partial substitution of urea-treated WCW for grass silage with no evidence of a reduction in the efficiency of utilization of ME.