Most dairy producers focus on their cows’ ability to produce milk and milk components, and become pregnant, because profitability of a dairy operation occurs here.
However, this does not align with the biological priorities of a lactating dairy cow. A cow’s biological priorities look like this:
- Self-maintenance
- Milk production
- Milk components and body condition
- Reproduction
When a cow consumes and digests nutrients, the nutrients are used up in a specific order to meet the cow’s highest biological priority first. Thus, if nutrient intake is insufficient in meeting all nutrient requirements, the cow’s highest biological priority will use the limited nutrients first.
When determining the nutrient requirements of a dairy cow for ration-balancing purposes, the cow’s bodyweight, milk production, milk composition and whether the cow is gaining or losing body condition are all considered.
These parameters are great at predicting the energy, protein and mineral requirements for just milk production. Only the newer models are starting to predict the supply of acetate, propionate and amino acids to supply the metabolic pools needed for high performance.
When instructing undergraduate and graduate students, one of the questions I pose to the nutrition classes is, “What is the first productive performance parameter to suffer and the last productive performance parameter to return when changing nutritional programs for lactating dairy cows on a dairy operation?”
The answer is reproduction or reproductive efficiency. The point to consider is: If the cow cannot consume sufficient nutrients to supply the appropriate metabolic pools to take care of itself and the baby calf the cow currently is feeding, then why would the cow direct nutrients to breeding back? This would be detrimental to itself and its calf.
If a cow is not consuming sufficient nutrients while producing milk for its calf, the nutrients the cow does consume will be directed toward the needs with the highest priority, such as maintenance, milk production and milk components. Only when those needs are met will the nutrients be directed toward replenishing body condition or weight gain and, finally, reproduction.
This is not solely an energy and protein nutrient supply issue; it’s also a supply to key metabolic pools. The metabolic pools of interest here are acetate and propionate. These volatile fatty acids are produced by the ruminal microbes during the fermentation of fiber, starch and sugar.
Thus, increasing the supply of digestible fiber (and maybe specific sugars) will increase the amount of acetate produced by the ruminal microbes while the fermentation of starch and sugar typically results in the production of propionic acid or propionate.
Thus, many feed additives on the market can shift microbial fermentation to increase acetate, propionate or both that have implications on the performance of the lactating dairy cow.
A research trial conducted several years ago at the USDA-ARS Energy Metabolism Unit measured the partial efficiency of acetate utilization by dairy cattle. Holstein dairy cows were fed either a high-forage or a high-grain ration and then infused with water or acetic acid for 14 days.
The cows were then placed in a respiration calorimeter (energy chamber) to measure the partial efficiency of acetate utilization by the dairy cow. Acetic acid infusion had no effect on blood pH or acid base balance. Cows fed the high-forage ration demonstrated a lower partial efficiency of acetate utilization while cows fed the high-grain ration demonstrated a much higher partial efficiency of acetate utilization.
The partial efficiency of acetate use was much higher on a high-grain ration than on a high-forage ration. Therefore, as the amount of acetate in the metabolic pool becomes limited on a low-forage ration, the dairy cow must prioritize where the acetate is to be used based on biological priorities.
The cow will prioritize the acetate to be used first as energy to support maintenance, followed by milkfat, body condition replenishment and, finally, reproduction.
In biochemical pathways, acetate is used as an energy source but can also be used as the building block for milkfat and cholesterol synthesis. A dairy cow uses cholesterol for the hormonal synthesis of progesterone, which is responsible for the maintenance of pregnancy.
Two biochemical reactions are needed for a cow to convert cholesterol to progesterone. However, if an insufficient supply of acetate is available to the dairy cow, and what acetate is available is being prioritized to higher biological needs, then the synthesis of progesterone and similar hormones are lacking for the maintenance of pregnancy. This may explain why cows cycle but do not conceive when inseminated.
Acetate is produced by rumen microbes from the fermentation of neutral detergent fiber in the ration from forages and fibrous byproducts.
This mechanism explains the emphasis placed on improving the amount of digestible neutral detergent fiber in the ration, as not only a source of energy but also as a source of acetate to increase the cow’s metabolic supply.
Therefore, it is biologically possible to have high herd performance with high milkfat and excellent reproductive efficiency by focusing on ration forage and fiber digestibility.
In conclusion, a dairy cow has biological priorities for using nutrients and must be kept in mind when troubleshooting performance problems on the dairy. In addition, there is only so much corn and fat that can be fed in the ration as sources of energy to meet the energy requirements of a high-producing dairy cow.
However, these specific energy ingredients do not increase the supply of acetate through metabolism to meet all the requirements of a high-producing dairy cow.
Feeding highly digestible forages and fibrous byproducts (i.e., soyhulls) can increase the acetate supply to the cow. If this acetate is limited, the cow will utilize the available acetate for the highest priorities first, which are maintenance and milk composition, and for reproduction last. This is why reproductive efficiency is the first performance problem on the dairy to appear and the last to respond to ration manipulations. 
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David P. Casper
- Technical Services
- Leader/Dairy Nutritionist
- Furst-McNess Company
- Email David P. Casper
