Figuring out protein nutrition in ruminants can be tricky. The protein fed in the diets of dairy cows and other ruminants is not directly absorbed and metabolized like it is in monogastrics such as pigs and chickens. The protein – more specifically, the nitrogen and amino acids in the protein – fed to ruminants is first used to feed the microbes in the rumen. About 50 percent of the protein that a cow metabolizes comes from the dead rumen microbes that have finished their fermentation obligations and go on to be absorbed in the small intestine. This microbial protein is considered to be the best protein in terms of quality and amino acid profile.
If not complicated enough, the other half of the protein that a cow needs does indeed come from the feedstuffs in the diet. However, the nutritional value and efficacy of that protein in those feeds is highly dependent upon the amino acid (AA) profiles in those feedstuffs and how well those AA survive the journey through the rumen and make it to the small intestine for absorption. Rumen microbes get first dibs on protein from feedstuffs, break them apart, take what they want and leave the rest. The resulting profile of AA coming from feedstuffs absorbed in the small intestine can often be deficient in essential AA.
Years ago, most dairy farms had small herds and the cows derived most of their nutrition from forages – either hay or pasture. High grain and commodity byproducts were not in common use and balancing rations was in its infancy. It was understood that immature forages had more protein and was preferable over mature forages. An understanding of how much and how rapidly protein degraded in the rumen wasn’t fully understood until the 1960s.
There has been a paradigm shift in how we view protein in dairy cow diets. We no longer can speak of crude protein (CP) levels in a ration because CP is merely a measure of nitrogen content and not of specific AA. Neither do the concepts of rumen degradable protein (RDP) and rumen undegradable protein (RUP) fully address the complexity of balancing amino acids. Many feedstuffs and byproducts do not have the proper profile of essential AA required by high-producing cows.
Much of the research into establishing AA requirements has focused on the AA profiles in cow’s milk and body tissue and comparing it with the AA profile of microbial protein in the rumen. The more microbial protein that can be developed in a rumen, the better the protein status of a cow will be, lowering the need for protein byproducts that offer questionable levels of rumen bypass amino acids.
Delivering accurate and consistent levels of metabolizable AA by way of feedstuffs has been a challenge due to the variation of amounts and bioavailability in feedstuffs. Diets that consist of high percentages of corn and corn byproducts are recognized to be deficient in lysine. Diets that consist of high percentages of a single feedstuff can be highly deficient in many of the essential AA. Diets should be formulated to include a variety of feedstuffs in order to avoid limiting one or more of the essential amino acids.
Rumen-protected methionine has been available for many years and has a credible track record for good results and return-on-investment in increasing milk protein levels in milk. Rumen-protected lysine has been available for a shorter period of time and, though the initial results are encouraging, more time is needed to evaluate its efficacy. A number of rumen-protected methionine and lysine products are on the market today.
The dairy industry once again finds itself in a situation of low milk prices and milk-over-feed margins. Dairy farmers continue to seek ways to lower feed costs without losing milk production or lower milk components. The dairy industry struggles to do its best in the prevailing economic environment, competing for scarcer resources along with having to address the issue of nutrient (nitrogen) management and economic stewardship. A complete understanding of AA metabolism and requirements in the dairy cow has been at the center of the quest to improve feeding efficiencies and reduce environmental nitrogen pollution.
Too much protein in a dairy cow diet costs money. Not feeding enough protein or the correct type of protein in a dairy cow diet costs money, too. Improving our knowledge of protein nutrition in dairy cows will accomplish at least three things:
- The intrinsic and virtuous goals of more completely understanding the biology of cows’ metabolisms so that we can improve the value of the milk produced along with feeding efficiencies – and ultimately the profitability – of our cows and our dairy businesses. Decreasing the cost of protein nutrition in dairy diets will mean money in the pockets of dairy farmers.
- Environmental concerns and costs associated with nutrient management and nitrogen pollution. Improving protein efficiency in dairy cows means reducing nitrogen being released into waterways, aquifers and the atmosphere.
- Incorporation of rumen-protected technology for amino acids will reduce the need for the use of animal-sourced byproducts as a protein source.
Balancing for amino acids in dairy cow diets is complex and will not produce desired results if other areas of dairy management are not up to par. Addressing critical management items such as days-in-milk, cow comfort, feedbunk management, vitamins, minerals and water quality as well as dry matter intakes, moisture levels of silages, and feeding high quality forages will more than likely increase milk production per cow long before addressing specific amino acid needs. Holstein herds that can’t manage a 60-pound milk average have other things to fix first.
Most dairy farms continue to find it necessary to improve feeding efficiencies and increase the value of the milk produced by increasing milk components to stay in business. Protein nutrition and amino acid metabolism will continue to be an area of intense focus in the fine-tuning of dairy cow diets.
Read more: Optimizing the Rumen