Impact of Cow Size on Efficiency

Linda Benedict  |  3/19/2015 11:41:13 PM

Ryon S. Walker

Eight years ago a beef extension specialist asked a Montana rancher who owned 400 head of Angus cows how much his cows weighed. “They will average 1,250 pounds across the board,” he replied. After weighing his cows at weaning in the fall, he found to his surprise the herd of 400 Angus cows averaged 1,400 pounds – with one cow, who had a 395-pound calf, weighing 1,900 pounds.

Over the past 30 years, U.S. cattle producers have seen a tremendous change in cow body weights. Data from the National Agriculture Statistics Service indicate that between 1975 and 2005, carcass weights of bulls have increased 223 pounds; cows, 146 pounds; steers, 144 pounds; and heifers, 194 pounds. Based on this, it is estimated that cow size has increased 300 pounds during this period, with an estimated average U.S. cow weight of 1,350 pounds.

Beef cow size is determined by evaluating the mature body weight of a cow (5 years of age or older) at a body condition score of 5. Body condition score is a measure of the condition of an animal using a scale of 1 (emaciated) to 9 (obese). Over this same span of 30 years, the cow/calf segment of the beef industry has improved weaning weights significantly. This is in response to pressure from the feedlots to select for more growth and performance in calves. Because there are strong positive genetic correlations between weaning weight and mature cow size and carcass weights and mature cow size, the mature size of cows has increased as a direct response to selecting for growth and performance, particularly in Bos Taurus breeds (Angus, Hereford, etc.). It is unknown, however, how the increase in mature cow size has affected how much that animal eats, how efficient she is at eating and meeting the nutritional needs of her body, and how efficient she is at weaning a calf.

Another concern is the impact of cow size on production efficiency, which refers to how efficient a bigger cow is at weaning a heavier calf compared to her lighter herd mates. Researchers at the Dickenson Research & Extension Center in North Dakota evaluated production efficiency in their herd and reported body weight ranged from 856 to 1,935 pounds, with their bigger cows 300 pounds heavier than their lighter cows. In addition, those cows weighing 300 pounds heavier produced only 1 pound more of calf at weaning than their lighter herd mates.

Estimates of feed required for maintenance of body weight date back 30 years, and this change in cow size may have in turn affected intake and feed efficiency. The “Nutrient Requirements of Beef Cattle” handbook, a national publication, states that daily dry matter intake (daily amount of forage or feed an animal consumes on a dry matter basis) increases 1.5 pounds for every 100-pound increase in mature cow body weight, assuming milk and stage of production are constant (Figure 1). This equates to 548 pounds of additional dry matter intake per year for a cow that weighs 100 pounds more than her herd mate. To understand cow efficiency and how cow size have affected the ability of that cow to raise a calf and maintain her body weight every year, researchers can evaluate how much she eats and her production efficiency.

Dry Matter Intake and Production Efficiency

Researchers at the LSU AgCenter Hill Farm Research Station in Homer conducted a study to determine differences in dry matter intake and efficiency during a 70-day feeding period in beef cows based on their mature body weight. The study was conducted in a group of 38 Angus crossbred beef cows during their lactating (with heifer calves at side) and non-lactating (following weaning) stages of production.

Based on body weight at the beginning of the feeding period, cows were sorted into a light or heavy body weight group for evaluation of ryegrass baleage and hay intake. During both feeding periods, cows in the heavy body weight group averaged 1,377 pounds compared to 1,210 pounds for cows in the light body weight group, a difference of 167 pounds. During lactation, cows in the heavy body weight group consumed 2 pounds more of dry matter per day than cows in the light body weight group; however, the light body weight cows gained 5 pounds more over 70 days (Figure 2).

During non-lactation, cows in the heavy body weight group consumed 2.43 pounds more of dry matter per day than cows in the light body weight group with similar body weight gains. This data would suggest that for every 100- pound increase in body weight, dry matter intake would increase 1.18 pounds during lactation and 1.45 during the non-lactation period. This indicates the difference in intake based on body weight may be less than the nutrient requirements of beef cattle as stated in the handbook. In addition, the increase in cow size may not have affected intake of forages for the larger cows as a group.

AgCenter researchers evaluated production efficiency in the beef herds at the Hill Farm (commercial Angus crossbred) and at the Dean Lee Research and Extension Center (3/4 Angus and 1/4 Brahman) in Alexandria in 2012 by sorting the cows into heavy and light body weight groups, based on body weight at weaning. At the Hill Farm, cows in the heavy body weight group averaged 163 pounds heavier and weaned a 32-pound-heavier calf at weaning than cows in the light body weight group (Figure 3). At Dean Lee, cows in the heavy body weight group averaged 165 pounds heavier and only weaned a 4-pound-heavier calf at weaning than cows in the light body weight group (Figure 4).

In a previous evaluation of 12 herds in northern Minnesota in 2010, four of the 12 herds had an average difference of 173 pounds in body weight between the heavy and light cows within each herd, with their heavy body weight cows weaning only 0.25 pounds more at weaning. The other eight herds had an average difference of 181 pounds in body weight between the heavy and light cows within each herd, with their heavy body weight cows weaning 25 pounds more at weaning.

Increasing cow size has a point of diminishing returns, but to what extent is still questionable. There appear to be differences among commercial herds in weaning production efficiency based on cow size. However, most producers do not know the production efficiency of their herd. If bigger cows wean a heavier calf, does that difference in pounds weaned pay for the difference in forage that the larger dam will consume? In today’s economic environment, arguably the most valuable piece of equipment on a cow/calf operation is a good set of scales. Before deciding whether to keep or sell your bigger cows, producers should measure what they weigh and what they produce.

Ryon S. Walker is an assistant professor and researcher at the Hill Farm Research Station in Homer. He is also the beef coordinator in the Northwest Region and responsible for the Master Cattleman program.

(This article was published in the Winter 2015 issue of Louisiana Agriculture.)

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