Short and long term effects of diet on the concentration of protein in the milk of dairy cows

The effect of diet on the milk protein concentration from dairy cows was investigated in a series of experiments aimed at both short and long term changes. To study the effect of dietary energy source and protein level, four concentrates were offered at a constant forage:concentrate ratio to 12 animals in mid-lactation on a grass-silage based diet. These provided two sources of energy (barley and sugar beet pulp) and two levels of digestible undegradable protein. Milk protein concentrations from the four diets were 36.3, 35.2, 35.1 and 34.9 g/kg (SED 0.40) for barley high and low protein and sugar beet pulp high and low protein respectively. There was little difference in microbial protein yield between diets, based on estimates of urinary purine derivative (PD) excretion. In a second experiment, three concentrates were offered, which differed in metabolisable energy (ME) density and fat content, to 12 mid-lactation dairy cows on a grass-silage based diet. Mean milk protein concentrations were 31.4, 32.5 and 31.0 g/kg (SBD 0.45) for low ME high fat, low ME low fat and high ME high fat concentrates respectively. Milk protein yields did not differ significantly, although milk yields were significantly increased by the inclusion of fat in the diet due to increased lactose synthesis. PD excretion was correlated to fermentable ME intake. To investigate longer term effects of diet on milk protein concentration, 22 dairy cows were paired and offered a control diet of silage or restricted silage, ad libitum straw and 0.5 kg protein supplement (prairie meal) during the dry period. All animals were offered the same lactation diet and mean milk yields were the same from both groups. The mean milk protein concentration was significantly affected by dry period diet, with 29.4 and 32.0 g/kg (SED 0.40) for control and supplemented animals over the first 31 weeks of lactation. The experiment was repeated with 36 animals (18 pairs) using similar dry period diets. The over the first 18 weeks of lactation mean milk yields were significantly increased by the supplemented dry period diet, and mean milk protein concentration was only slightly increased (32.7 and 36.3 kg milk/d, SED 0.62; 30.7 and 31.5 g protein/kg, SED 0.28) with no significant difference in dry matter intakes with a proportion of animals. Two experimental lactation concentrates were fed to 6 pairs of mid-lactation animals which had also been offered the dry period diets to study the interaction effects. Mean milk protein concentrations were significantly increased by 2 g/kg with a 'milk protein enhancing' concentrate compared to a 'milk protein depressing' concentrate; milk yields were similar for both diets. No interaction effects were seen. It is concluded that the metabolisable protein supply to the mammary tissue is the most important factor in determining milk protein concentration. This may come directly from the diet, in which case maximising microbial protein efficiency by balancing the rumen degradable protein supply with fermentable ME is important. Avoiding a high acetate fermentation may also be important to ensure a low utilisation of amino acids for gluconeogenesis. Finally, the status of the animal's protein reserves during early lactation is more important than formerly appreciated