Matching the number of cattle to the grass and feed resources on the ranch is a constant challenge for any cow-calf producer. Also, producers strive to maintain cow numbers to match their marketing plans for the long-term changes in the cattle cycle. Therefore, it is a constant struggle to evaluate the number of replacement heifers that must be developed or purchased to bring into the herd each year.
As a starting place in the effort to answer this question, it is important to look at the “average” cow herd to understand how many cows are in each age category. The Dickinson, North Dakota Research and Extension Center reported on the average number of cows in their research herd by age group for a span of 20 years. The following graph depicts the average percent of cows in this herd by age group.
The report indicates that the typical herd will, on the average, introduce 17% new first-calf heifers each year. Stated another way, if 100 cows are exposed to bulls or artificial insemination each year, 17 of them will be having their first baby. Therefore, this gives us a starting point in choosing how many heifers we need to save each year.
Next, we must predict the percentage of heifers that enter a breeding season that will become pregnant. The prediction is made primarily upon the nutritional growing program that the heifers receive between weaning and breeding. If heifers are grown slowly and weigh 50% to 55% of their mature weight at the start of breeding, then about half of the heifers will be cycling early in the breeding season.
In this 100-cow-herd scenario, about 30 heifers need to be kept and exposed to artificial insemination or the bull to assure the target number of pregnant heifers is met. This allows for natural selection pressure on early puberty and reproductive soundness if the breeding season is short (30 to 45 days). More pasture space and breeding costs will be needed because of the larger number of heifers kept.
Growing the heifers at a higher rate of gain would be necessary to reach 60% to 65% of the mature weight at breeding. Utilizing a growing program such as wheat pasture (for spring calving heifers) would allow the heifers to gain 1.5 to 2 pounds per day, and about 90% or more of the heifers should be cycling early in the breeding season.
Even in the best scenarios, a few heifers will be difficult or impossible to breed. Most extension specialists and researchers write about the need to always expose at least 10% more heifers than you need, even when they are grown rapidly and all weigh at least 65% of the expected mature weight. Therefore, in the example of a 100-cow herd, if the heifers are fed to reach over 60% of the mature weight at breeding, we expect to keep back 19 or 20 heifers to go into the growing program and breeding season. Fewer heifers are started in this growing program, but higher feed costs per heifer will be necessary to reach the higher rate of gain.
Like so many decisions in the beef industry, there is more than one answer to important questions.