The following answer is provided by Laurie Winkelman and Tom Overton of Cornell University. It is excerpted from a presentation they made at the recent Cornell Nutrition Conference.
Protein is the most valuable milk constituent in multiple-component pricing systems, receiving the largest dollar per unit price of all the milk components. At the end of 2009, milk protein was valued at $2.88/pound, compared to $1.55/pound for milk fat. Long-term projections are for 2 percent annual increases in demand for dairy products (FAO) because of increasing worldwide demand for milk protein and whey components. The financial incentive for milk with higher protein content as well as the growing consumer demand for milk protein highlights the need to gain a greater understanding of nitrogen efficiency within the cow and how milk protein is synthesized within the mammary gland. The efficiency of converting dietary nitrogen into milk protein output is relatively poor in the lactating animal, between 25 to 30 percent (Bequette et al., 1998). This low level of efficiency also represents an area of opportunity for the dairy industry. Improving nitrogen efficiency within the cow will help the industry to avoid costly nitrogen loss to the environment.
The Dairy NRC (2001) summarized available information regarding dietary influences on milk protein content and yield, with primary focus on modulation of milk protein through amino acid supplementation. In general, the opportunity to increase milk protein content and yield through known dietary strategies appears to be less than 5 percent per day. However, if we gain further understanding of the process of milk protein synthesis and what regulates that process, we might be able to reach higher levels of milk protein production in dairy cows.
Insulin and milk protein
Starting in the 1990s, a series of studies conducted by Dale Bauman’s group at Cornell demonstrated that chronic elevation of circulating insulin concentrations through the use of hyperinsulinemic-euglycemic clamps can result in larger increases in milk protein content and yield than those described above. McGuire et al. (1995) reported a 0.07 kg/d increase in milk protein yield in post-peak lactating cows subjected to the hyperinsulinemic-euglycemic clamp with insulin concentrations elevated five times above baseline. Griinari et al. (1997a) observed increases in milk protein yield and concentration under the hyperinsulinemic-euglycemic clamp with and without abomasal casein infusion. In a study by Mackle et al. (1999), use of the hyperinsulinemiceuglycemic clamp increased milk protein concentration by 11 percent and total milk protein yield by 25 percent when cows were abomasally-infused with casein (500 g/d) and branch-chained amino acids (88 g/d). These changes in milk protein output were observed 4 d after the start of the hyperinsulinemic-euglycemic clamp. In cows treated in early lactation with recombinant bovine somatotropin (rbST) as well as insulin and glucose, during insulin infusion, milk protein yield was increased by 0.05 kg/d compared to saline-infused controls (Leonard and Block, 1997). In this study, glucose was infused alone in a separate treatment at a rate of 50 g/hour, and milk protein yield was decreased by glucose infusion by 0.05 kg/d compared to salineinfused controls. All of the studies referenced above indicate that insulin influences mammary gland protein synthesis...
Among the long-acting insulin analogues commercially available for human use is an analogue called insulin glargine. Insulin glargine is a recombinant, human insulin.
In an effort to avoid the confounding effect of severe hypoglycemia induced by insulin, we chose to use insulin glargine in lactating cow studies to investigate the role of insulin in milk protein production. With its long duration of action, insulin glargine was an ideal choice to study the effect of insulin-action in lactating cows without intensive use of venous catheters, infusion pumps, and exogenously-supplied glucose.
Insulin glargine (Lantus) was used in a dose response study to determine the response of lactating dairy cows to this insulin analogue. (To read about the study more specifically, click on the longer version below.)
The studies described here are the first to use insulin glargine in high-producing, lactating cows to elevate insulin activity in the cow without severe hypoglycemia. Though plasma glucose was significantly reduced in these experiments, which served as our proxy for insulin-like activity, the fact that overall milk yield and dry matter intake were not reduced during treatment are interesting results. The increase in milk protein yield in the 10-day study reported here support the observations from Dale Bauman's group at Cornell (Griinari et al., 1997a; Mackle et al., 1999; Mcguire et al., 1995) that insulin does indeed influence milk protein production in lactating cows.
Implications and conclusions
Continuing to fine-tune dairy cow nutrition management to improve protein efficiency within the cow has large implications for both the environment and the bottom-line of dairy farms. As we continue to improve conversion efficiencies of feed nutrients and nitrogen into milk protein, less nitrogen will be wasted and excreted into the environment. As margins have tightened on farms due to high feed prices and low milk prices, improving nitrogen efficiency is increasingly important to keep farms profitable.
Based on the research reported here, it appears that there is more efficiency to be gained in milk protein production within the cow. With no change in total milk volume, cows treated with insulin glargine had improved use of amino acids and greater milk fat and protein output than control cows. As we continue to learn more about the role of insulin in milk component synthesis, there may be opportunity to fine tune rations to alter circulating insulin concentrations to boost milk protein content without compromising total milk yield or animal health and body condition.
Further research is being carried out to determine the mechanism of insulin glargine action in the mammary gland. The effect of insulin glargine on global protein synthesis within the gland will be examined by looking at protein expression from mammary biopsies collected at the end of the 10-day study reported here. We hypothesize that insulin glargine is acting through mammalian target of rapamycin (mTOR) signaling pathways to improve milk protein synthesis (Menzies et al., 2009; Rius et al., 2010; Toerien et al., 2010).