Feeding an energy supplement with white clover silage improves rumen fermentation, metabolisable protein utilisation, and milk production in dairy cows

Six rumen-fistulated Holstein-Friesian cows were used in a Latin square design to test the hypothesis that more frequent feeding of a high energy supplement to cows consuming high-protein white clover silage would improve microbial protein production, resulting in greater N retention and higher milk yields. The white clover silage (10.7 MJ metabolisable energy (ME)/kg DM) was fed to cows either alone (WCS) or with 4.5 kg DM of rolled barley grain (12.1 MJ ME/kg DM). The grain was offered either 24 times (WCS/24B) or twice daily (WCS/2B, at 0800 and 1700 hours). Cows offered the supplements, regardless of feeding frequency, had higher (p < 0.05) organic matter (17.3 nu. 16.0 kg/day) and estimated ME (208 nu. 189 MJ/day) intakes than cows offered white clover silage alone. Mean daily ruminal fluid pH (P < 0.05) and ammonia-N concentrations ( P < 0.05) were lower in the supplemented treatments, with total VFA concentrations being highest ( P < 0.05) in the WCS/2B treatment. Nitrogen intake and output in the faeces were similar for all 3 treatments. However, nitrogen excretion was lower ( P < 0.05) in urine ( 174 nu. 218 g/day) and higher ( P < 0.05) in milk (115 nu. 93 g/day) of cows offered the supplements. The crude protein consumed by cows on all 3 diets was estimated to be well in excess of cow requirements. The supplements reduced the calculated net losses of ammonia-N from the rumen from 25% of total crude protein intake for WCS to 14% in the 2 supplement treatments, and increased the metabolisable protein supply available for milk production. Increases in metabolisable protein were estimated to be due to a higher microbial crude protein contribution in the supplemented treatments compared with the WCS treatment. Grain supplements increased (P < 0.05) milk yield (22.4 nu. 19.6 kg/day) and although there were no significant differences in milk fat and protein concentrations between treatments, the latter tended to increase with grain supplementation. Milk yield was higher in the WCS/24B treatment than in the WCS/2B treatment, but neither the calculated nor the measured rumen variables were sufficiently different to explain this effect of frequency of feeding the grain. One possible explanation for the difference was the marked fluctuations in key rumen variables throughout the day in the WCS/2B compared with the WCS/24B treatment. Such fluctuations in the rumen environment are not accounted for in theoretical calculations since associative effects are not considered. The benefits of a higher milk production as a result of more frequent feeding of the supplement to cows should be considered in context of the additional effort or costs associated with more frequent feeding.