Why is vitamin A important?
Vitamin A is considered by many to be the most important vitamin regarding the need for supplementation. Vitamin A is necessary for proper bone formation, growth, energy metabolism (glucose synthesis) and skin and hoof tissue maintenance, as well as vision. The vision function is associated with visual purple in the eye when animals are trying to adapt from light to dark.
• night blindness
• reproductive failures
• skeletal deformation
• skin lesions
The best source of this vitamin is beta-carotene, a pigment in green plants that animals convert to vitamin A. If cattle are grazing green grass, they will get plenty of vitamin A. During winter months or drought,vitamin A deficiencies are common because dormant plants don’t contain the levels of beta-carotene needed compared to the green forage levels in the growing months.
How much vitamin A do cattle need?
Where other nutrient requirements are expressed as a percent of the diet or as parts per million (ppm), vitamin requirements are generally expressed in international units (IU). For example, one milligram of beta-carotene is equivalent to 400 IU’s of vitamin A. The primary sources of vitamin A for the biological functions mentioned previously are 1) green, leafy forages, 2) liver stores and 3) supplemental sources such as commercial concentrate feed supplements and commercial mineral supplements. The liver stores vitamin A, however, it is thought that those stores can last only two to four months if a severe dietary deficiency exists. Table 1 shows vitamin A requirements for different classes of beef cattle.
|Table 1. Vitamin A requirements for beef cattle.|
|*Daily requirements are calculated based on 500-pound growing or stressed calves and 1,200-pound cows.|
|Growing Steers and Heifers*||Stressed Steers and Heifers*||Gestating Cows*||Lactating Cows*|
|Vitamin A, IU/lb of feed DM||1000||2250||1300||1800|
|Vitamin A, IU per day||12500||15000||34000||54000|
How much vitamin A is present in typical feeds?
Feeds vary substantially in beta-carotene concentration and therefore, in vitamin A equivalents. Lush, green, immature forages have the greatest concentration of beta-carotene. However, those same plants may contain near-zero beta-carotene during the winter when they are mature, dormant and brown. Bright colored, early harvested hay contains relatively high amounts of beta-carotene. Harvested forage beta-carotene concentration declines over time. While the rate of degradation is extremely variable, a conservative rule of thumb is that the beta-carotene concentration (and therefore vitamin A potential) declines by about one-third to one-half each year in storage. Feed grains and other concentrate feeds are generally low in beta-carotene. Table 2 shows a summary of rounded vitamin A equivalent values from one experiment where 85 feed and forage samples were analyzed for beta-carotene concentration.
|Table 2. Approximate vitamin A equivalents in various feed sources.|
|Source: Adapted from Pickworth et al., 2012 available at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3849816/pdf/nihms527534.pdf|
|Feed Source||Vitamin A Equivalents, IU/lb feed dry matter|
|Fresh grass pasture||18000|
|Dried Distiller’s Grains||200|
|Wet Distiller’s Grains||350|
Considerations for supplementing vitamin A
Generally speaking, vitamin A has been inexpensive to include in mineral mixes and commercial feed products for many years. As a result, vitamin A has probably been overfed, and in many cases, drastically overfed. The appropriate amount of vitamin A to use in the cattle ration depends upon the level of carotene in the forage, liver stores and length of feeding period, among other factors. One consideration is that most commercial feed and mineral products have historically included 100 percent or more of the vitamin A requirement for most classes of cattle. This could change with the vitamin A shortage, so be sure talk to your feed or mineral supplier to determine what current level is being included in the supplement product.
Another consideration is not to double up on vitamin A. For example, if a free-choice mineral supplement is put out, and a commercial concentrate supplement of some sort is fed, there may be more than twice the amount the animals need ingested. If this is occurring during the spring of the year, perhaps as much as five times the amount required is consumed.
As shown above, high-quality hay may contain 50 to 100 percent of the Vitamin A needed. Consequently, a supplement product containing 50 percent of the animal’s requirement would be appropriate. If your hay quality is marginal, 100 percent supplementation is suggested. Obviously, cattle grazing wheat pasture and spring/early summer abundant immature growing forage should not require vitamin A supplementation.
Examples and amount of vitamin A provided based on feeding rate and concentration
Table 3 provides examples of the amount of vitamin A supplied per day, based on forage intake and forage quality. Tables 4, 5 and 6 provide guidelines regarding the amount of vitamin A supplied, given the concentration of vitamin A in the mineral or concentrate product and given the amount of mineral or feed being provided. These tables can be used to estimate the vitamin A balance of your current program. They can also be used to determine the approximate concentration of vitamin A that might be appropriate in the feed or mineral product you are considering purchasing. For example, assume gestating beef cows are being wintered with one-half high-quality hay and one-half low-quality hay similar to the forages described in Table 3. The requirement from Table 1 is approximately 34,000 IU per day. Subtracting the amount provided from forage in Table 3 (18,200) from the daily requirement, a supplemental need of 15,800 IU is determined. This value can be compared to the amount supplied by different products in Tables 4, 5 and 6. From there, one can determine the approximate vitamin A concentration and daily supplement consumption for this situation without drastically overfeeding vitamin A.
Besides standard mineral packages, vitamin A can be supplied by injectable products and liquid drench forms. The issue with injections is they need to be repeated approximately every 28 days to maintain safe liver stores. The critical time for injection in pregnant cows is two months prior to calving.
If you don’t feel comfortable with nutrition topics, consult resources like the local county Extension educator, livestock nutritionist or veterinarian for help.
|Table 3. Approximate vitamin A equivalents supplied by class of cattle and forage quality.|
|*Forage consumption is estimated to be 12.5, 26 and 30 pounds DM per day for calves, gestating and lactating cows, respectively. High-quality hay estimated to contain 1,400 IU/lb DM.|
|Approximate Vit A equivalents supplied by forage, IU per day*|
|Forage source||Growing||Cows, Gest||Cows, Lact|
|Green, early-harvested new crop hay*||17,500||36,400||42,000|
|Low quality, late-harvested, 2-yr-old hay||0||0||0|
|50% high quality, 50% low quality hay||8,750||18,200||21,000|
|Table 4. Approximate vitamin A delivered by mineral supplements.|
|Mineral Intake, ounces/day|
|IU/lb Mineral Mix||2||3||4||5|
|Table 5. Approximate vitamin A delivered by concentrate supplements.|
|Feeding Rate, pounds/day|
|Table 6. Approximate vitamin A delivered by complete feeds.|
|Feeding Rate, pounds/day|
Area Extension Livestock Specialist
Beef Cattle Specialist