From the December issue of Drovers Cow/Calf.

Beef producers often take a “one size fits all” approach to the cows in the cow herd. This singular approach to how a cow’s genetics affects cow reproduction can have economic ramifications.

It should be obvious that not all cows have the same genetic makeup, which goes on to affect the herd’s overall productivity.

Simply put, genetics drives bodyweight and bodyweight drives the intake of forages and feedstuffs. Data from the National Research Council shows us heavier cows have greater dry-matter intake potential to consume feed; likewise lighter cows consume less. Through dry-matter intake cows consume the required energy, protein, fats, vitamins and minerals required for maintenance and production. The consumption of nutrients drives the ability of cows to effectively reproduce.

So why are differences in dry-matter intake so important for the cow herd? The cow herd’s feed requirements amount to 50 to 75 percent of the annual maintenance costs for the herd.

That 1,000-pound cow

Cow-calf producers who don’t routinely collect bodyweight data on their cow herd often underestimate the actual bodyweight of cows in the herd. It seems a pervasive assessment that most cows weigh, or at least the herd average is, 1,000 pounds. Likely, a more correct assessment of the herd cow bodyweight would reveal a greater proportion of the cow herd with bodyweight greater than 1,000 pounds. The increase in cow bodyweight over the years is likely an effect of cow-calf producers placing greater emphasis on the genetics of calf weaning weight, yearling weight, and the necessary increase in cow milk production required to support desired calf growth performance. The desire for larger calves likely conspires to increase actual cow bodyweight over time. An assessment of cow bodyweight at weaning of three cow herds in Table 1 demonstrates the fallacy of assuming the herd average cow bodyweight is 1,000 pounds.

None of the three herds’ average cow bodyweight is 1,000 pounds. One herd’s average cow bodyweight is 1,053 pounds, but the other two herds have average cow bodyweight over 1,200 pounds. Therefore, if total cow herd nutrition decisions were made on the basis of 1,000-pound cows, those decisions would be wrong and would negatively impact cow reproduction.

Small-calf complaints

There are objections associated with a smaller cow. One issue is the potential for lighter weaning weights for the calves produced from lighter body-weight cows. True enough, if weaning weight as a percent of cow bodyweight remains constant between heavy and lighter body-weight cows, the total calf weaning weight can’t be compensated by realistic increases in stocking density (number of cows in the herd). However, in actuality, cow bodyweight and calf weaning weight do not track in parallel.

In examining the data from the three cow herds there is a trend that as cow bodyweight increases the calf weaning weight as a percent of cow bodyweight decreases. This trend was consistent across the three herds even though the herds have different breed composition, sires, sire types and overall breeding programs. The heaviest cows never come close to weaning 50 percent of their bodyweight — which is a general industry bench mark — whereas, the lightest cows wean calves close to or over 50 percent of the cow’s bodyweight.

In these examples, the two herds A and B have an average cow bodyweight of 1,224 pounds, and the average cow from those two herds weaned an average of 48.5 percent of cow bodyweight. In order for the 1,000-pound cows to wean the same total amount of calf weight, the 15 percent increase in cow herd number (stocking rate) would have to be coupled with a 3 percent increase in weaning weight — meaning 53 percent of the cow bodyweight.

Certainly, a 3 percent increase in calf weaning weight is achievable; in fact, the C herd with a mean calf weaning weight of 55 percent as a percent of cow bodyweight surpasses that benchmark.

Later puberty

Cow size or bodyweight also has some important effects on cow herd productivity. Starting at the developing heifer, projected mature bodyweight affects the rate of maturation associated with reproduction of developing heifers. As mature bodyweight increases, age at puberty increases, and this effect is greater for late-maturing breed types compared with early-maturing breed types.

Likewise, as bodyweight increases the percent of heifers cycling and conception rate decreases, and again the effect is greater in late-maturing than early-maturing breed types. Florida-based research by Vargas et al. (1999) supports the concept as Brahman cow frame size (and bodyweight) increased from small to medium to large, age at puberty increased from 633 to 672 days of age.

Lower calving rates

The Vargas data also is a great demonstration of the effect of cow size on cow productive traits across first, second and third or greater parity. As cow frame size increased and cows aged, calving rate decreased. Calving rate difference specifically led to differences in survival rate during the first parity. Large cows had a 48 percent survival rate compared to an 81 percent survival rate for small cows. Calving date within the calving season was similar among cow size; however, the change in calving date from first to third parity was two times larger for large cows compared with small cows. Weaning rates during the first and second parity were greater for small and medium-sized cows compared to large cows that had weaning rates of less than 50 percent. Weaning weights and pre-weaning average daily gain of calves was greater for calves from large cows compared to small and medium cows. Despite smaller calves, cows of small and medium size produced more pounds of calf weaned relative to the total number of cows exposed for breeding during the first and second parity.

Cow mature bodyweight has important implications for many of the production parameters associated with the overall cow herd. Heifer development, cow reproduction and calf performance can be affected by cow bodyweight. A key component to efficient calf production is the appropriate cow size. Cows with moderate size (bodyweight), with good maternal traits and genetics for calf growth, are the cows to target and retain in the cow herd.

Indeed, if you can’t measure it, you can’t manage it, and cow bodyweight certainly falls in that important category.

Matt Hersom is associate professor and beef cattle specialist in the Department of Animal Sciences at the University of Florida.