Farming Magazine - June, 2010


Beef: Reproduction by the Numbers

By Dr. John Comerford

Beef producers often consider reproductive efficiency as the number of cows that produce a calf each year. We often forget, however, the matrix of events and issues that influence reproduction and its affect on profitability in the cow-calf enterprise. The following discussion will highlight some of the “numbers” to be considered to be both reproductively efficient and profitable.

It is not 100 percent

When we consider the potential for a cow getting pregnant in a pasture-mating system, there are three factors that have to be considered:

  • The percentage of cows in heat;
  • The percentage of the heats that are fertile; and
  • The percentage of the cows that are serviced by the bull.

Even though 100 percent of the cows may eventually get pregnant during a breeding season, some pregnancies may be lost because of a failure of one of these three issues. For example, if 95 percent of the herd is in heat in a 21-day period, 95 percent of these heats are fertile and 95 percent of the cows are serviced by the bull when in heat, the result is not a 95 percent conception rate. The conception rate during that 21-day period is 86 percent (.95X.95X.95=.86).

If an AI system is in place, several more factors are involved. These include accurate heat detection, the level of semen quality and the proficiency of the inseminator. Again, the final result is not an average success rate for each factor, but a multiplicative product of the rate for each factor. Table 1 shows the expected percentage of cows in standing heat over a 24-hour period. These data show that if no heat detection is done between 6 p.m. and 6 a.m., 52.9 percent of cows in the herd will not be detected in heat, even though they may have had a fertile heat.

Table 1. Time of day when cows exhibit standing estrus
Time of day Percent of cows in heat
6 a.m. until noon 26 percent
Noon until 6 p.m. 18.1 percent
6 p.m. until midnight 26.9 percent
Midnight until 6 a.m. 29 percent
G.A. Perry, South Dakota State University (unpublished data).

In most cases, nature will take control of the breeding in the cow herd, but sometimes managers put too much pressure on nature and we lose breeding efficiency. For example, we have already shown how 95 percent of cows being serviced by a bull can significantly influence the pregnancy rate in the herd. The major reason successful mating does not occur (with bulls that are known to be fertile and healthy) is because there are too many cows for each bull. Consider these numbers: in a 21-day period during the breeding season with 25 cows, on average there is more than one cow in heat every day. What happens when you depend on an inexperienced yearling bull to breed these cows? More often than not, a pregnancy will be missed, particularly early in the breeding season. The cost of missing the cow for just one heat cycle is about 40 pounds of weaning weight in a uniform weaning program. How many times can we lose $40 to $50 per cow in this enterprise?

The number of cows in the herd exhibiting a fertile estrus is a function of age, health and nutritional status. Obviously, not all cows will have a fertile estrus during a breeding season. The combination of animal age and nutritional status is the major culprit for this failure. Young, growing females that are also being called on the cycle regularly must also have a nutritional status that gives them the opportunity to be fertile as well. The best method of evaluating this issue is a condition score. A fact sheet can be found at that gives excellent illustrations of body condition scoring.

Table 2. 2000-2004 Summary of SPA Analysis for Iowa
Item Value
Percentage of cows pregnant 94.9
Percentage of live calves born 92.2
Percentage of cows weaning a calf 87.5
Average pounds weaned 453
Cow exposed for breeding  
Annual cow cost ($) $345.04
Iowa State University, 2005.

The only result in a breeding program that makes a difference is how many calves we get to sell. The reproductive rate in a herd by definition is the number of calves we get to sell compared to the number of cows we exposed to a bull to produce those calves. The summary of standardized production analysis data from Iowa State University for 2000-2004 is shown in Table 2. It is clear a cow herd will not have 100 percent reproductive efficiency. For example, the average pregnancy rate during the breeding season was 94.9 percent instead of 100 percent for the reasons described above. More importantly, the percentage of calves they got to sell (the weaning percentage) was 87.5 percent, which includes all of the losses associated with getting cows bred and getting those calves to market. The average pounds that were sold for each cow exposed to breeding were 453 pounds, even though the average weaning weight was 518 pounds (453/.875). These results show profitability in the cow-calf enterprise depends, not on pounds weaned, but the number of calves weaned from a breeding program.

Reproductive efficiency in a cow herd is not going to be 100 percent, and failures for any number of many factors affecting calf sales will significantly alter profit potential. Small improvements, however, are possible for many of these issues on the farm, and these changes can add dollars to the enterprise.

Dr. John Comerford is associate professor of dairy and animal science at the Pennsylvania State University.