The effect of the replacement of grass silage by increasing proportions of urea-treated whole-crop wheat on food intake and apparent digestibility and milk production by dairy cows

Six Holstein-Friesian cows in mid lactation were used to examine the effects of replacing increasing proportions of grass silage (GS) by whole-crop wheat (WCW) in the diet of lactating dairy cows. Grass silage was a first-cut perennial ryegrass. The WCW was harvested at 645 g dry matter (DM) per kg and was preserved with either 20 (WCW-20) or 40 (WCW-40) kg urea per t DM. The diets were 6 kg DM dairy concentrate daily with one of six forage treatments offered ad libitum. The forage treatments were GS alone, GS with either WCW-20 or WCW-40 in a 2:1 DM ratio, GS with either WCW-20 or WCW-40 in a 1:2 DM ratio, or WCW-40 alone. The experiment consisted of an incomplete change-over design with three 4-week periods. DM intakes were highest with the mixtures (P < 0.05) but milk yield was not significantly affected by the treatments although it tended to be highest with the 1:2 ratio. Milk composition responses were generally small and non-significant, but yields of fat and protein were highest with the mixtures (P < 0.05). The apparent digestibility of DM and organic matter (OM) decreased linearly (P < 0.01) with increasing WCW inclusion but digestible DM and OM intakes were not significantly affected. Apparent digestibility of neutral-detergent fibre tended to be lower with the forage mixtures (quadratic P < 0.10). Apparent digestibility of starch was higher with GS than with diets containing WCW and decreased linearly with diets containing increasing proportions of WCW (P < 0.01). Intakes of starch and digestible starch rose with increasing WCW inclusion but so also did faecal starch output such that 0.75 kg starch per day was excreted on WCW-40 alone. Water intake from food and water excretion in urine decreased linearly with increasing WCW inclusion but water excretion in faeces and milk was not significantly affected by diet. The difference between measured water intake and output (water deficit) increased from 38 kg/day on GS alone to 58 kg/day on WCW-40 alone (linear P < 0.001). The only effects of level of urea treatment were to increase milk urea concentration (P < 0.05). It is concluded that the small size of the milk yield response to the increased forage intake obtained when up to two-thirds of the GS is replaced by WCW is due to the reduced digestibility of the OM and particularly of the starch component resulting from the egestion of whole wheat grains in the faeces.