Rick Grant, William H. Miner Agricultural Research Institute, Chazy, NY
Producing more milk fat and protein is a sure-fire way to enhance herd profitability. And, the best way to boost milk fat and protein is to enhance rumen fermentation with a particular focus on fiber digestion. Over the past 18 months or so, a group of scientists led by Dr. Dave Barbano at Cornell University, scientists from Miner Institute, in collaboration with the St. Albans Coop have been studying the link between on-farm management, nutrition, and milk composition with a focus on bulk tank milk sampling.
De novo Fatty Acids and Rumen Function
Cornell research has shown that the percentage of milk de novo fatty acids is positively correlated with the percentage of fat and true protein in the milk. In fact, regression analysis indicates that de novo fatty acids explain nearly one-half of the variation in milk fat percentage and even more – about 68% – of the variation in milk true protein. De novo fatty acids are the short chain fatty acids (C4 to C14) that comprise about 20-30% of total milk fatty acids. There are also preformed fatty acids which are long chain (C18:0, C18:1, and C18:3) and make up about 35-40% of total fatty acids, and finally the mixed group of fatty acids (C16) which comprise about 35% of milk fatty acids. On-going research is attempting to determine the optimal relationship among these three fatty acid groups that results in the highest milk component output.
The de novo fatty acids are important for two reasons: 1) milk fat and protein are two key drivers of dairy profitability with the pounds of milk components per day positively related to net milk income over feed costs, and 2) de novo fatty acids reflect rumen functioning – especially fiber fermentation that produces acetate and butyrate which are the building blocks of short-chain de novo fatty acids. Additionally, rumen conditions that enhance microbial fermentation should stimulate microbial protein production and thereby increase milk protein content. So, the relative proportion of de novo fatty acids in milk fat should reflect how well the cow is being fed and managed for optimal rumen fermentation. Higher de novo fatty acids in the milk reflect healthier rumen conditions.
Rumen pH and Milk Fatty Acids
We have known for years that nutritional factors may influence milk fat composition such as fat percentage in the diet and its composition, fermentable carbohydrates, and forage particle size. The role of management has been much less clear. Many factors influence rumen fiber digestion and microbial protein production. The intrinsic digestibility of the forage fiber is a function of plant genetics, maturity at harvest, and growing environment which all combine to determine the amount of lignin and the degree of its cross-linking with cell wall carbohydrates. Rumen pH has a large impact on the actual fiber fermentation that occurs and nutritional factors such as too much rumen fermentable starch will lower pH and decrease both the rate and extent of fiber digestion. Additionally, poor feeding management can influence rumen pH and subsequent fiber digestion and microbial protein production if it results in slug feeding and larger drops and greater variability in rumen pH.
What has become clearer recently is that feeding or management practices that degrade the rumen environment – notably anything that reduces pH – results in a shift in rumen biohydrogenation to the so-called alternate pathway and trans-10, cis-12 conjugated linoleic acid (CLA) accumulates. Small amounts of this CLA isomer have a powerful milk fat depressing effect. Recent work from Tom Jenkins at Clemson University shows that even subtle changes in rumen pH can influence milk fat. As little as a 0.10 reduction in rumen pH can translate into a 0.10 decrease in milk fat associated with the shift in CLA isomer production. So, a key question becomes: how can you prevent reductions in rumen pH, or stated another way, what management practices enhance rumen conditions as reflected by de novo fatty acid content of the milk? We set out to answer that question using cooperating farms from the St. Albans Coop in Vermont.
Farm Management, Nutrition, and Milk De Novo Fatty Acids
In 2014 and 2015 we visited over 70 farms in northern New York and Vermont that were members of the St. Albans Coop and assessed their management and feeding practices and then compared this information with their bulk-tank milk composition. Herds were categorized as either high or low de novo herds. In the first year, we evaluated Holstein, Jersey, and some mixed breed farms, but in the second year we focused on Holstein herds that had at least 50 cows and averaged at least 55 lb/d. For year two, the average milk production was 67 lb/d for high de novo herds and 58 lb/d for the low de novo farms. Fat and protein percentage was 4.33 and 3.41 for high de novo and 4.14 and 3.22 for low de novo farms. In 2015, the milk fatty acid profile of the high de novo farms was 25.8% de novo, 39.2% mixed, and 34.7% preformed (as % of total milk fatty acids). In contrast, the low de novo farms had 23.8% de novo, 37.6% mixed, and 38.1% preformed.
We identified four major factors that were consistently associated with greater de novo milk fatty acid content and higher milk fat and true protein content:
Don’t overstock. Herds with higher de novo synthesis were 10x more likely to have feed bunk space of at least 18 in/cow and 5x more likely to have stall stocking density ≤110%. The relationship between stocking density and de novo fatty acid content in milk makes sense given that overstocking increases feeding rate and aggression at the feed bunk, depresses rumination, and increases risk for lower rumen pH. In fact, recent work from the Institute shows that overstocking has a greater impact on rumen pH than dietary peNDF. In a study we have just finished at the Institute, it appears that overstocking combined with limited access to feed (i.e. bare bunk disease) is associated with lower rumen pH and depressed de novo milk fatty acids.
Feed more frequently. High de novo free-stall farms were 5x more likely to feed twice a day which presumably resulted in better rumen conditions for microbial fermentation. For tie stall farms, we saw that high de novo farms were 11x more likely to feed at least 5x per day (many were component-fed herds). Overall, we see that greater frequency of feeding enhances milk components and the de novo fatty acids which reflect rumen pH.
Feed fat properly and meet fiber requirements. From a nutritional standpoint, low de novo herds fed diets with higher dietary fat content. In fact, these low de novo herds were 3.5x more likely to have TMR containing greater than 3.5% ether extract. They were also 8x more likely to be feeding rumen inert fat. We still need to learn more about the composition of the dietary fat and how it contributes to lower de novo fatty acid synthesis, but clearly feeding more fat is a risk factor for reducing de novo milk fatty acids. Feeding too much unsaturated fat may depress rumen fiber fermentation, and feeding too much rumen inert fat may simply drive up the preformed fatty acid content even when rumen conditions are not optimized for higher de novo fatty acid content in milk.
We also found that dietary peNDF was important for enhancing de novo synthesis. In fact, high de novo herds were 10x more likely to feed TMR with ≥21% peNDF (% of DM). This makes sense given the long-known relationship between dietary effective fiber, rumen pH, and microbial fiber fermentation.
The de novo fatty acid content of milk is positively related to milk fat and true protein output on commercial dairy farms. This initial evaluation found that the top-4 factors that boost de novo fatty acids are: 1) lower stocking density, 2) greater feeding frequency, 3) don’t over feed or mis-feed dietary fat, and 4) ensure adequate peNDF in the diet. Stay tuned for more research on this topic.
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