Vitamin A is essential for many biological processes. Cattle cannot manufacture vitamin A themselves, so it must come from the diet. Vitamin A is found at higher levels in fresh green forage, and at much lower levels in weathered forage and grain. It can be stored in the liver and fat when the diet contains more vitamin A than the animal needs. The stored vitamin A can then be released later when dietary levels are too low. This allows animals to deal with seasonal fluctuations in dietary vitamin A levels. However, feeding a vitamin A-deficient diet for a prolonged period of time can eventually exhaust the liver stores and result in a deficiency. Vitamin A deficiency has been seen in Canadian feedlot calves that were raised on dry pasture, then fed straw and grain for nine months.
The most unique symptom of vitamin A deficiency is night blindness, but ill health, reduced feed intake, poor hair coat, joint swelling, and drooling may also occur.
Because vitamin A is important and cheap, feedlot nutritionists may err on the side of caution and supplement more than the National Research Council recommends. However, vitamin A inhibits some enzymes involved in fat cell development, and may negatively affect carcass grade. Several research studies in Japan, Australia, China and the U.S. have examined the effects of removing supplemental vitamin A from corn-or sorghum-based feedlot diets. None of these studies have reported adverse health effects or differences in performance or yield grades, but several reported that removing supplemental vitamin A from the finishing diet increased carcass marbling.
It is difficult to predict how removing supplemental vitamin A would affect the health, performance and carcass quality of cattle fed barley-based feedlot diets in Western Canada. For one thing, the grazing season is shorter in Canada than in the U.S., so vitamin A stores in the liver may be different at the start of the finishing period. Vitamin A levels are also thought to be lower in barley than in corn.
Dr. Darryl Gibb recently completed an experiment funded by Alberta Beef Producers that examined whether removing supplemental vitamin A from feedlot diets will improve carcass marbling score.
What they did:120 newly weaned 600-lb. heifer calves were divided into 12 pens of 10 head each. All cattle were fed barley silage/barley grain-based feedlot diets. Sixty heifers (six pens) were given a standard level of supplemental vitamin A (1,650 IU/lb. of diet dry matter). The other 60 heifers were fed exactly the same diet, except with no supplemental vitamin A. Heifers were backgrounded for 58 days on 40 per cent barley grain, then finished for 160 days on 86 barley grain rations. Feed and blood samples were collected at the start, midpoint and end of the trial to assess vitamin A levels in the diet and serum. Feed intake, growth rate, feed effi-ciency, and animal health were monitored throughout the trial. Carcass measurements were collected when the cattle went to slaughter.
What they learned
Vitamin A levels:Some vitamin A was present in the barley grain, but vitamin A was 38 times more concentrated in the barley silage. This means that even the unsupplemented cattle were getting some vitamin A, although the supplemented cattle were getting about 10 times as much. Serum vitamin A levels were the same in both groups of animals at the start of the trial. Vitamin A levels were 10 per cent higher in the supplemented group after 113 days on feed, and nearly 40 per cent higher at the end of the trial.
Feedlot performance:Growth rates during the backgrounding or finishing periods were the same for both groups. The unsupplemented heifers ate about a third of a pound less per head per day than supplemented heifers during the trial, but feed efficiency was similar in both groups. No signs of vitamin A deficiency were observed, and diet did not affect animal health treatment rates for foot rot or BRD.
Carcass measurements:Diet did not affect dressing percentage, carcass weight, yield grade, backfat depth or rib-eye area. Removing vitamin A did increase the marbling score: 42 per cent of the heifers on the supplemented diet graded average Choice or higher and 58 per cent of heifers on the unsupplemented diet graded average Choice or higher. Proportionally, nearly 40 per cent more cattle on the unsupplemented diet graded average Choice or higher compared to those on the supplemented diet.
What it means:Removing supplemental vitamin A from a barley-based feedlot diet improved carcass marbling scores without impacting animal health or performance. However, these cattle were not vitamin A deficient to begin with, and were fed a silage-based backgrounding diet containing natural vitamin A. Removing supplemental vitamin A could have negative implications for calves moving directly from dry pasture to a finishing diet. Feedlot operators are advised to work closely with both their nutritionist and veterinarian before implementing this strategy. Larger trials would help to confirm whether the performance, health and carcass results hold up in commercial feedlots.