Understanding Supplements in Dairy Cow Diets

Dietary supplements can be divided into two categories: those that provide nutrients and those that do not. Supplements that provide nutrients are included in the diet to help meet the animal’s requirement for those nutrients when the base diet contains a limited amount. Nutritional supplements include vitamins, minerals, amino acids, and energy/fat products. Supplements that do not provide nutrients are broadly referred to as feed additives. The most recent NASEM publication detailing the Nutrient Requirements of Dairy Cattle published in 2021 defines feed additives as “optional diet ingredients that are not nutrients but can affect digestion, metabolism, and production.” Additives typically include enzymes, ionophores, yeast products, prebiotics, probiotics, buffers, and binders.

“... when making a change, it usually takes several weeks before any positive effects become noticeable, so be careful not to pass judgement too quickly.“

Besides the standard Vitamin A, D, and E mixes included in most dairy rations, various B vitamin supplements are commercially available. Because these nutrients can be broken down in the rumen, all are fed in a rumen-protected form to ensure their availability to the cow.
Niacin is a B vitamin that helps reduce fat utilization in early lactation and can reduce heat stress. Biotin is also a B vitamin that can help improve hoof health, glucose metabolism, and milk production.

Although not a vitamin per se, choline is another nutrient closely related to B vitamins that is often supplemented, particularly to close-up and fresh cows. Choline is required for fat transport throughout the body and has been shown in numerous studies to have positive effects on immune function in dairy cows. It also has shown potential to reduce the occurrence of fresh cow diseases, including retained placenta, displaced abomasum, and possibly fatty liver.

Most dairy rations include mineral supplements to ensure basic mineral requirements are met. However, some additional mineral options exist such as cationic salts, anionic salts, and zinc methionine.

Cationic salts are supplements that contain higher levels of minerals, like potassium and sodium, to promote a positive dietary cation-anion difference. These salts are particularly useful in helping cows maintain their electrolyte balance during periods of heat stress. They also are important for regulating the metabolic acid-base status of cows during lactation, which can enhance milk production.

In contrast, anionic salts contain higher levels of minerals, like chloride or sulfur, to reduce the dietary cation-anion difference. These salts are frequently included in close-up or dry cow diets to help minimize milk fever after calving. They are NOT recommended for lactating diets.

Another mineral supplement that is frequently included in rations is zinc-methionine, an organic form of zinc that can promote immune function, reduce somatic cell count, and improve hoof health. This supplement can be particularly useful in herds experiencing issues with hoof health or high somatic cell counts.

Ration balancing in the dairy industry has become quite sophisticated over the last two decades. In the past, over feeding protein was the method utilized to ensure amino acid requirements were met. With recent advances, we are now able to reduce the protein content of the diet and balance the diet for individual amino acids instead. This is a great leap in terms of economic and environmental sustainability in the dairy industry. While most amino acid requirements can be met by feeding common feedstuffs, meeting nutritional requirements for the two most limiting amino acids for dairy cows fed corn-based diets (i.e., lysine and methionine) can still be challenging.

That said, rumen-protected lysine and methionine products exist and are included in many dairy rations. While supplementing both amino acids have shown positive changes in milk and milk composition, rumen-protected methionine has been more extensively researched than rumen-protected lysine. Methionine can be helpful in improving milk yield, feed intake, and fat production. There is also evidence that methionine can improve the immune function of fresh cows. It is important to keep in mind though that the magnitude of response that is observed when providing these amino acids in the diet will depend on how deficient the diet was to begin with. Supplementing methionine to cows who are not very methionine deficient will not have as great an effect as will supplementing methionine to cows who are very methionine deficient.

One of the most commonly used additives in cattle diets are ionophores, with Monensin (Rumensin™) being utilized widely in the dry and lactating cows. Monensin is the only ionophore approved for use in adult dairy cattle in the United States. It modifies the bacterial populations in the rumen, which alters fermentation and ultimately improves feed efficiency.

Yeast products and probiotics are both considered “direct-fed microbials” because they contain living organisms. While the mechanism of action is not fully understood, these products are suspected to alter digestion by changing the bacterial population in the rumen. While production responses are variable, there is evidence that these products can increase milk production, milk component production, and feed efficiency.

Enzymes can also be included in the diet to enhance fiber or starch digestion. Such enzymes can increase feed intake, milk yield, and milk fat yield. However, responses to enzymes in the diet are highly variable probably due differences in feed intake, ration ingredients, and stage of lactation.

There are many dietary supplements that can be incorporated into dairy rations and just a few of the most common have been discussed here. When choosing a supplement, it is important to consider the research supporting its use and whether results have shown consistent, repeatable responses. If you are considering adding or removing a supplement to the ration, be sure to weigh the cost of the supplement against the anticipated return. It’s always a good idea to consult with your nutritionist and ask questions before making the change. Keep in mind that when making a change, it usually takes several weeks before any positive effects become noticeable, so be careful not to pass judgement too quickly.