Manipulating Milk Protein Percentage and Production in Multiparous Lactating Dairy Cows
The protein content of milk has become much more important in recent years. This reflects its higher value to dairy farmers due to the continued high consumption of cheese, as well as the perception of consumers that milk fat, and fats in general, are unhealthful while milk protein is healthful. Regardless of the reasons, dairy producers are paying much more attention to the protein production of their cows, both in pounds per day and as a % of milk, since both can influence the economic value of the milk.
There are a number of factors that affect production of milk protein by dairy cows. These largely reflect inherent characteristics of the cows and the diets that they are fed. Unfortunately, not all of these factors are easily manipulated on-farm in the short term, but those that can be manipulated can be utilized under commercial conditions to impact milk protein percent and/or production. The purpose of this article is to highlight characteristics of cows and their diets, including feed additives, that impact milk protein % and milk protein production.
If there is a concern about low milk protein percent of yield in a group of cows, then a number of characteristics of the cows in the group being assessed should be used to create reasonable expectations of milk protein percent and/or production.
Variation in Genetic Merit for Protein Production Among Cows
There is a range of milk protein production potential within any group of cows that tends to cluster most cows around the herd average value. However, there are cows with higher and lower values. For example, in a group of 350 commercial multiparous Holstein cows that were 62 to 92 DIM, producing from 96 to 137 lbs of milk/day, and fed a similar diet, average milk protein % was 2.82. However, the range among individual cows was 2.41 to 3.30%. These higher and lower values indicate cows that are expressing their individual genetic predisposition to produce protein at either a higher, or lower, % of total milk.
Parity of the Cows
Milk protein % declines steadily from lactation 2, but production of milk protein increases from 2nd to 3rd lactation due to increased milk production, before beginning a slow decline due to the lower milk protein %. If the average lactation number of a group of cows changes, you can expect the highest milk protein production when lactation 3 and 4 cows dominate the group, with declines when cows with higher and lower lactation numbers increase.
Stage of Lactation of the Cows
A normally fed group of Holstein cows can be expected to have a substantial difference in milk protein % with stage of lactation. For Holsteins, a milk protein % of up to 3.5% in the 1st week of lactation will typically decline to 2.6 to 2.8% by ± 60 DIM, and then slowly increase to 3.5 to 3.6% by the end of lactation. As the average DIM of a group of cows increases, except in very early lactation, you can expect a steady increase in the milk protein %, and a steady decline in milk protein production.
Season of the Year
We have seen annualized plots of the pattern of milk protein % over the year. Typically, these express protein as % of milk with a minimum in July and maximum in November, and a difference of ~0.25 % units. These plots are often national and include all dairy farms. Dairy farmers can expect the highest milk protein % in late fall/early winter with lowest values in summer, but the change in milk protein production over the year is trivial due to similar changes in milk yield over the year.
Milk Production of the Cows
Although often overlooked, let’s be clear that the most effective way to increase milk protein production is simply to milk cows that produce more milk, because the relationship between milk production and milk protein production is very high. Often this reality can get lost in the desire to increase milk protein production (or %) by manipulating dietary nutrient levels, and/or by using feed additives.
The dry period, especially the close-up dry period, is critical to preparing cows for a successful calving and subsequent lactation. While many strategies, interventions, compounds and products are marketed or recommended by various groups and persons to facilitate success, few have been examined using large groups of cows, and even fewer have evaluated their impacts on milk protein production in the subsequent lactation.
The exception is a study conducted by University of California-Davis research using 81 primiparous and 171 multiparous dairy cows on a commercial California dairy farm. In this study, feeding a close-up dry diet with 14.4% CP, versus a control with 11.7% CP, for 9 to 12 days pre-partum increased subsequent lactation milk and milk protein production. Reasons for benefits of higher levels of CP in rations fed to dry cows in the late close-up period are not well defined. It is known that dairy cows have relatively small reserves of body protein that can be mobilized in the late closeup dry period to support rapid increases in the protein needed to support colostrum production. As a result, cows go into a negative protein balance immediately pre-partum, which carries into early lactation and results in lower body reserves of protein to support milk production. Most proposed theories for benefits of late dry period CP supplementation revolve around a scenario in which short term feeding of higher CP diets in the late close-up period acts to spare these relatively small protein reserves so that in early lactation they can be utilized to support milk production in the critically important first ~14 days of lactation prior to cows attaining full DM intake.
There are dietary factors that impact milk protein values which can be relatively easily manipulated during lactation. However, they may have collateral effects that should be considered before a decision is made to utilize them in practice.
Balance of fermentable starch and degradable protein
The rumen needs fermentable starch to support microbial growth, which in turns stimulates microbial protein production that may be an important part of milk protein when it passes to the small intestine. But too much fermentable starch may be harmful to fiber digestion due to decreasing rumen pH. It may not be a delicate balance, but it is a balance nonetheless to ensure the rumen bugs are fed well enough so they can do their job, but not harm fiber digestion and fiber digesting bugs.
Dietary Fat Level
While the level of fat in the diet can be manipulated relatively easily on commercial dairy farms, decisions to increase or decrease dietary fat levels are generally made relative to the perceived energy needs of the cows, and perhaps the desired level of milk fat production, rather than to increase milk protein % or production. Nevertheless, it is clear from research completed on commercial dairy farms in California (30 studies with 43 treatment combinations) that increased diet fat is associated with reductions in the milk protein % by up to 0.2 % units, but only above 5.5% fat in the DM of the TMR.
In contrast, milk protein production increases as milk protein % declines, at least as dietary fat rises above 5.5% of DM. It seems that as more fat is added to the diet, it has a greater impact on milk fat production, as well as lactose (the major regulator of milk volume), than it does on milk protein.
It might seem intuitive that the most effective method to increase production of milk protein would be to simply feed more protein in the diet. However, based upon a very large body of research studies going back many decades, as well as research in California, it is clear that increasing the CP content of the diet has a very small effect on milk protein production.
A difficulty in assessing the impact of CP in the diet on milk protein production is a functional inability to predict the separation of diet protein which is degraded in the rumen by rumen microbes from that which escapes the rumen undegraded to be absorbed from the intestine. There is little value to protein in the diet that degrades in the rumen and is lost as ammonia, at least on milk protein synthesis. It is the combination of digestible microbial protein and digestible dietary escape protein that determines the quantity of absorbable protein delivered to the intestinal absorptive site, and it is the quantity of absorbable protein that may limit, or enhance, milk protein synthesis.
While a great deal of research effort has been devoted to predicting quantitative delivery of intestinally absorbable protein in dairy cows, and ration formulation software is available to estimate it, the reality is that proportional escape of dietary protein from the rumen, as well as microbial protein flow from the rumen, remains informed guesswork because no analytical methods are available to measure them. Thus model estimates of ‘metabolizable protein’ (i.e., absorbable protein digested from the small intestine) have huge predictive errors.
Finally, the poor relationship of the amount of protein consumed and that amount of protein in milk is also due to the metabolic ability of the cow to use protein absorbed from the intestine to support production of milk protein, as well as the ability to convert absorbed protein and its constituent amino acids to metabolic compounds that can be used to provide energy to animal tissues. Exactly what the absorbed protein is used to support by the metabolism of the cow depends on its biological need.
The dairy industry is rife with feed additives with a wide range of claims. However, these products often lack supporting research results from independent groups, thereby making assessment of their real impacts and economic value very difficult to determine. There are some feed additives that do increase milk protein production, and they are outlined below.
Ruminally Protected Methionine (RP Met)
In a California data set that evaluated diet RP Met addition there were 8 direct comparisons of a Met supplemented diet versus a non-Met supplemented control diet. Overall, there was increased output of milk (0.6%), milk fat (2.7%) and milk protein (3.8%) with the RP Met supplemented diet. This is similar to findings of a published review of some years ago where benefits (in 54 direct comparisons) were 0.3%, 2.6% and 4.4% respectively. It seems clear that Met addition tends to reprioritize net energy use because the protein and fat production response is bigger than that of milk response itself, and the protein response is higher than that of fat.
Saccharomyces Cerevisiae Yeasts
In a review of a few years ago, using 19 direct comparisons from 3 products, the average increase in milk (3.0%), milk fat (2.7%) and milk protein (2.1%) to yeast feeding were all positive. But, in contrast to RP Met, these yeasts did not re-prioritize energy to protein and fat, but rather re-prioritized it to lactose because the milk production increase was more than of fat or protein.
It is important to keep in mind that the most effective way to increase milk protein production is to milk cows with higher milk production levels because, overall, there is a very strong correlation between milk production and milk protein production. In addition, characteristics of the group of cows being assessed (such as breed, lactation number, genetic merit for milk protein production and stage of lactation) need to be considered in order to evaluate whether the actual milk protein production of the cows is within the expected biological range.
New feed formulation software packages are a tool that a nutrition professional can use to identify conditions where action can be taken to enhance, or reduce, the extent of a situationally expected change in milk protein production. There are numerous feed additives marketed with claims that they will increase production of milk protein. While supportive third-party evidence is often thin, there are a few additives that have a demonstrated capacity to increase milk protein production.
Adapted from P.H. Robinson – Cooperative Extension Specialist – University of California, Davis