Effects of casein and glucose on responses of cows fed diets based on restrictively fermented grass silage.

This study was conducted to investigate whether well fed dairy cows given restrictively fermented grass silage diet will respond to incremental glucose and amino acid supply at early stage of lactation. Four rumen-cannulated Finnish Ayrshire cows were used in a 4 x 4 Latin square experiment with 14-d periods. The cows were fed good quality restrictively fermented grass silage ensiled with a formic acid additive for ad libitum intake. A concentrate mixture consisting of barley (85%) and solvent extracted rapeseed meal (11.4%) was given at a rate of 9 kg/d. The four treatments were continuous abomasal infusions of water (control), casein 300 g/d, glucose 300 g/d, and casein 300 g/d + glucose 300 g/d. The infusions had only minor effects on feed intake, diet digestibility, or rumen fermentation pattern. Both casein and glucose infusions increased milk, protein and lactose yields the effects being partly additive on the combined infusion. Infused casein increased milk protein and urea as well as plasma urea concentrations. Both casein and glucose tended to increase plasma glucose concentration. Casein increased arterial plasma concentrations of essential amino acids (EAA), branched-chain AA (BCAA), and total AA (TAA). Both casein and glucose, although glucose usually less than casein, increased arteriovenous differences of EAA, nonessential AA, BCAA, and TAA. Extraction efficiencies of AA were higher for glucose than for casein. Mammary plasma flow was at highest on the control diet, but reduced owing to infused nutrients, the reduction being less with combined rather than separate infusions of casein and glucose. Based on the partly additive increases in milk production parameters and changes in plasma metabolites, it is suggested that glucose alone increased milk protein yield by sparing AA from hepatic utilization, while casein increased both supply of AA and glucose. It was concluded that cows at early stage of lactation fed diets comprising of restrictively fermented grass silage and a cereal-based concentrate suffer from both limited AA and glucose supplies.