Editor’s note: Balchem Corporation sponsored a post-conference seminar focusing on milk protein production, following the 2015 Penn State Extension Dairy Cattle Nutrition Workshop held recently in Grantville, Pennsylvania. The seminar covered a three-pronged approach to milk protein: feeding, breeding and economics. This article is the first in a series covering the seminar. The second in the series "Breeding for milk protein"

The cow mammary gland cells build milk protein molecules. Some of the potential arises from genetics. Optimizing the animal’s genetic potential, however, requires capturing the mammary gland’s ability to utilize the available amino acid building blocks.

Paying attention to protein fed in relation to milk urea nitrogen (MUN), yield and income over feed cost, plus potentially utilizing amino acid supplements to enhance mammary function, allow the producer to maximize the animal’s genetic potential to create milk protein.

“All we are doing on the nutrition side is trying to keep up with that genetic potential,” Mark Hanigan of the department of dairy science at Virginia Tech said of feeding to maximize milk protein. “All we can do is get it up to where it should be from a genetic standpoint. We can certainly misfeed the cow and drop down.”

There is a direct correlation between metabolizable protein fed and milk protein. Maximizing milk protein means getting more from the metabolizable protein fed, not feeding more crude protein (CP).

The lowest acceptable amount of dietary crude protein, as a percent of dry matter intake, is 15.5 percent, as per the Nutrient Requirements of Dairy Cattle (2001). As CP increases from this point, so does ration cost, decreasing efficiency.

Nitrogen output also increases as CP input increases. Nitrogen conversions in lactating dairy cows are relatively poor compared to other livestock. The N efficiency of dairy cows is approximately 25 percent, Hanigan said. This has an environmental impact as well as an economic one.

Increasing feed efficiency is imperative from both perspectives. Lost nitrogen impacts soil leaching, air quality and even food production per acre, important due to environmental regulations and future food production needs. And as dietary energy is increased, many herds are over on protein, he said.

“Whatever we don’t capture ends up in the manure,” Hanigan said of excess nitrogen. “You can’t make more MUN than nitrogen you put into the cow.”

Nitrogen is lost in the rumen as well as the feces. Of that which is absorbed, only one-third finds its way into milk. The remainder is lost in the urine. The mammary is not very effective in removing N from the blood, which depends both upon the nitrogen concentration in the arterial blood, how much nitrogen is taken out each pass through the system and the mammary gland’s affinity for the nitrogen.

“The mammary has a lot of ability to sort of buffer and fix any problems you might impose upon it,” Hanigan explained. “Affinity is the mammary’s ability to manipulate what it is doing.”

Amino acids

Amino acids are key to increasing mammary efficiency. There are some amino acid supplements available for dairy cows. These work to deliver essential amino acids to the bloodstream. Essential amino acids are those the cow cannot produce itself by rearranging the chemical configurations of others. Some essential amino acids are phenylalanine, threonine, tryptophane, methionine, histidine, leucine and lysine.

“If we don’t have the protein right, we’re not going to have the amino acids right,” Hanigan said.

Protein synthesis is regulated by hormones, which include the amino acids and energy. If you know how much milk protein you want, you can achieve that level by the addition of one, or perhaps many, amino acids. Functional and cost-effective dietary management is needed to make feeding available and individual supplemental amino acids viable.

While methionine and lysine are the two most limiting amino acids in dairy cow nutrition, and methionine is essential for milk protein production, a single-limiting amino acid model does not correlate with the data generated from industry research. Instead, models that account for a variety of essential amino acids are more accurate in predicting milk protein results.

While the addition of a single most limiting amino acid to the ration may or may not give much of a response, adding other amino acids to the mix can often increase that response.

Making the mammary more efficient involves a multi-limiting amino acid approach, Hanigan said. The ribosomes need all of the amino acids to make milk protein. The single-limiting nutrient theory “does not really apply” to maximizing milk protein.

“Every one of them is significant,” he said of the amino acids. “It’s our job to figure out how to feed these (amino acids) so we can get a repeatable and predictable response. We can feed individual amino acids; it’s just a cost issue.”

Managing MUN

Genetic variation that will impact milk protein levels, feed efficiency and dietary salts also plays a role, Hanigan said. To optimize MUN for your herd, feed to NRC 2001 or equivalent rations. Feed for two weeks, measuring MUN. This will establish a baseline for the herd.

Next, reduce rumen-undegradable protein (RUP) 0.25 percent units by reducing CP and keeping energy and rumen-digestible protein (RDP) constant. Feed this diet for three weeks, monitoring MUN and milk yield.

It is expected that MUN will decrease by 0.5 mg per dl and that actual milk loss will be less than predicted by NRC 2001. After calculating income over feed cost, the RUP can again be reduced as previously. The next step is to reduce RDP, using the method above, to 0.5 percent of dry matter while holding RUP constant.

Keeping the MUN at maximum income over feed cost, without losing production, is the target, Hanigan explained.

While amino acids are undoubtedly being lost in the system, the cost of supplements, and a lack of a mathematical model to describe exactly what is happening with nitrogen and milk protein as amino acid levels are increased, make capturing mammary efficiency difficult.

Ongoing development of products which deliver amino acids to the cow’s blood, and guidelines for managing them, will optimize milk protein while minimizing the impact of poor nitrogen conversion in the ruminant animal.  PD

Tamara Scully, a freelance writer based in northwestern New Jersey, specializes in agricultural and food system topics.