Effect of grain and straw supplementation on marginal milk-production responses and rumen fermentation of cows grazing highly digestible subterranean clover pasture

Abstract

The marginal milk-production response to cereal grain supplementation of dairy cows grazing irrigated subterranean clover (Trifolium subterraneum L.) pasture in spring was investigated. We also determined whether the addition of a perennial ryegrass (Lolium perenne L.) straw supplement would improve that response. There were 8�replicated treatments in a factorial design with 2 amounts of cereal grain pellet [0 and 5.0 kg dry matter (DM)/cow.day] and 4 amounts of straw (0, 0.5, 1.0 and 2.0 kg DM/cow.day), 48 cows in total. Cows in all treatments grazed subterranean clover at an allowance (measured to ground level) of 19 kg DM/day. The barley and wheat pellet and straw had a neutral detergent fibre (NDF) concentration of 164 g/kg DM and 704 g/kg DM, respectively. Cows from all treatments consumed pasture with an in vitro DM digestibility of 743�g/kg DM and NDF concentration of 313 g/kg DM, compared with the pregrazed pasture on offer of 707 and 360, respectively. Pasture DM intake was not affected (P>0.05) by supplementation with either grain or straw, and averaged 12.7�kg�DM/cow.day. Cows supplemented with grain achieved a marginal milk-production response of 1.0�kg�fat-corrected milk (FCM)/kg DM grain. However, with increasing amounts of straw, responses of 0.0, 0.9 and 0.5 kg FCM/kg DM grain were observed. Fat-corrected milk yields were not different (P>0.05) between straw treatments and averaged 25.8 kg FCM/cow.day. Grain feeding significantly (P<0.05) reduced milk fat concentration and increased (P<0.05) milk protein concentration, although offering increasing amounts of straw had no significant (P>0.05) effect on either milk fat or milk protein concentration. Average ruminal fluid pH was significantly (P<0.05) lower when cows consumed cereal grain pellets than with those treatments without cereal grain (5.82 v. 6.09). There were no differences (P>0.05) in ruminal fluid pH among cows that consumed different amounts of straw. When cows were managed to limit intake of subterranean clover by restricting pasture allowance, supplementation with cereal grain resulted in acceptable marginal milk-production responses of 1.0 kg FCM/kg DM cereal grain. Under these circumstances, providing additional NDF as straw did not improve the milk-production response to cereal grain, with NDF ranging from 290 to 337 g/kg DM when 0–1.7 kg DM straw was consumed.