Effect of level of grain supplementation on milk production responses of dairy cows in mid–late lactation when grazing irrigated pastures high in paspalum (Paspalum dilatatum Poir.)

Two grazing experiments tested the hypothesis that a cereal grain-based supplement, fed to cows that are in mid–late lactation and grazing low metabolisable energy and high neutral detergent fibre content paspalum (Paspalum dilatatum Poir.)-type pastures, will increase milk yield, but that this response will diminish with successive increments of supplement. A further objective of this research was to investigate some of the factors, such as altered rumen fermentation pattern, that might vary the point at which diminishing returns start to occur. Cows grazed irrigated perennial pasture at an allowance of either 25 (experiment 1) or 31 (experiment 3) kg of dry matter (DM) per cow per day in late summer–early autumn (mid–late lactation) and were supplemented with cereal grain-based concentrates up to 11 and 7 kg DM/cow.day in experiments 1 and 3, respectively. In experiment 1, there were 3 replicates of 6 treatments (3 cows in each treatment group) that involved the feeding of either 0, 3, 5, 7, 9 or 11 kg DM/cow.day of supplement for 50 days. Experiment 3 was conducted over 35 days. There were 2 replicates of 4 treatments (4 cows per treatment group) that involved the feeding of either 0, 3, 5 or 7 kg DM/cow.day of supplement. A further experiment (experiment 2), associated with experiment 1, examined the effects of offering cereal grain-based concentrates up to 7 kg DM/cow.day to dairy cows consuming 10 kg DM/day of herbage with a high paspalum content on aspects of rumen fermentation. Incremental responses of 40 g/kg fat-corrected milk (FCM) to increasing concentrate intake diminished with increasing concentrate intake, with the level of supplementation at which diminishing returns occurred dependent on herbage allowance and, therefore, herbage intake. At a supplement intake of 3 kg DM/cow.day, the response in FCM was 1.1 kg/kg concentrate DM in both grazing experiments. There were no further increases in milk production with additional increments of concentrates in experiment 3 where the pasture allowance was highest. In experiment 1, where concentrates were offered to a level of 11 kg DM/cow.day, and the pasture allowance was lower, diminishing returns were not as pronounced as in experiment 3 until the highest levels of concentrate intake. Substitution of supplement for herbage was a major factor in causing the diminishing returns in both experiments, but especially in experiment 3, where pasture intakes were higher. Milk fat content was significantly (P<0.05) reduced (41.8 v. 32.5 g/kg) when concentrate intake increased from 9 to 10.4 kg supplement DM/cow.day in experiment 1. It was hypothesised that this reduction in milk fat content was probably due to the effects of subclinical rumen lactic acidosis. This hypothesis was supported by the trend to lower rumen pH for longer periods as supplement intake increased as well as a more variable milk yield at the highest level of supplement intake. We conclude that responses of FCM of 1.1 kg/kg DM cereal grain-based concentrates can be achieved when they are fed twice daily up to 3 kg DM/day to dairy cows grazing restricted amounts of paspalum-type pasture in autumn. Beyond 3 kg DM/day, marginal responses diminished with increasing concentrate intake, with the level of supplementation at which diminishing returns occurred being dependent on herbage allowance and, therefore, intake.