Vitamins like minerals are essential nutrients for cattle. As a group, they are involved in all aspects of the animal’s metabolism including growth, reproduction and health. There are two general classes of vitamins. These include the water-soluble and fat-soluble vitamins. The water-soluble ones include the B vitamins and vitamin C. Fat-soluble vitamins include A, D, E and K.
As indicated, all are essential, however, with beef cattle we generally concern ourselves only with fat-soluble vitamins. In most situations rumen bacteria supply the animal with an adequate source of B vitamins as well as vitamin K. While there are some exceptions to this statement (i.e. the role of thiamine in treatment and prevention of polio in cattle), this article will focus on the fat-soluble vitamins.
In terms of function, vitamin A is important for vision, reproduction and immune function, while vitamin D plays a critical role in calcium and phosphorus metabolism and bone growth. Vitamin E is an important antioxidant that interacts with selenium to provide protection to cells and tissues and is involved with immune function. As their group name implies, they are soluble in fat and are stored by the body in adipose tissue and the liver. This is important, as it means that these vitamins do not need to be supplied daily to the animal. This contrasts with water-soluble vitamins where a daily supply (from the rumen microbes or the diet) is required.
Common questions regarding supplementation of fat-soluble vitamins focus on the need, timing and method of supplementation. To help address these questions, let’s first look at why we need to supplement.
It is obvious from the partial list of metabolic functions given above, that these vitamins are critical to the normal functioning of an animal. We also need to recognize that the level of vitamin A and E in common feed sources varies widely with time of the year, as well as the type and form of the feed. Generally speaking, fresh pasture and harvested green, leafy forages have relatively high levels of vitamin A and E. However, drought, extensive storage and/or processing can result in marked reductions in concentration. Cereal grains are poor sources of vitamin A. With respect to vitamin D, while forages are an important source of a precursor for this vitamin, cattle are able to synthesize vitamin D if exposed to adequate sunlight.
The result of this variation in forage vitamin content is that the need to supplement varies seasonally. Cattle grazing fresh green pasture in the spring and summer generally do not require supplementation. Further, since the body stores these vitamins, they typically have a two- to three-month supply when coming off pasture. However, during the winter, when cattle are fed conserved forages and daylight is short, the need for supplementation increases for all three vitamins, particularly as calving approaches.
Vitamin requirements are given as international units (IU) per kilogram (kg) of feed dry matter (DM). For example, according to the National Research Council (NRC 2000 update), vitamin A and D requirements for growing cattle are 2,200 and 270 IU per kg of DM, respectively. Respective values for pregnant and lactating cows are 2,800 and 3,900 IU per kg of DM. Values for vitamin E are not as clear. NRC (2000 update) recommends between 15 and 60 IU per kg of DM for growing calves and up to 100 IU per day of added vitamin E to finishing diets. Higher levels of vitamin E (i.e. 100 IU per kg DM) in receiving rations may help to improve immune function in stressed calves.
More from the Canadian Cattlemen website: Vitamin E, selenium requirements an open book
With respect to method of supplementation, there are two basic approaches. A two- to three-month supply of A, D and/or E can be injected intramuscularly as per label/veterinary instructions. While this is a tried-and-true method of supplementation, it is generally not recommended as intramuscular injections result in carcass tissue damage. The preferred method is via the feed typically as part of a mineral program or through a specific vitamin supplement.
To illustrate the former, consider a pregnant beef cow consuming 12 kilograms of DM. This cow requires approximately 33,000 IU of vitamin A and 3,300 IU of vitamin D, daily. Consider that the commercial mineral available to this cow contains 450,000 and 45,000 IU of vitamin A and D per kg, respectively. At a consumption rate of 75 grams (2-1/2 ounces), the cow receives 33,750 IU of vitamin A and 3,300 IU of vitamin D daily, matching her requirement. As with calves, the vitamin E requirement for breeding cows is not well defined, but has been suggested to range from 200 to 300 IU per kg for pregnant cows (Source Alberta Agriculture and Rural Development). To achieve an intake of 200 IU per day, the mineral fed to this cow would need to contain approximately 2,700 IU of vitamin E per kg.
The bottom line is that calving season is approaching and now is the time to review your feeding program. As with mineral feeding, adequate vitamin nutrition is key to a successful calving and rebreeding season.