Several of the cooperators whom I have been in contact with during the past few months have indicated an interest in retaining and developing replacement heifers this year. If one considers the current position in the cattle cycle and the current and potential value of quality replacement females, this decision may be logical. There are several factors to consider to ensure that this venture is successful. An effective replacement heifer development program requires more time, labor, and feed resources than does the mature cow herd. However, if managed efficiently, heifer development can be cost effective, profitable, and relatively simple. The goal for a heifer development program should be to get the largest number of heifers bred in the shortest time and, ultimately, to get a high percentage of these first-calf heifers bred back for their second calf.
To become bred, yearling replacement heifers must reach puberty before the actual breeding season. Research has repeatedly shown that neither age nor actual body weight is a reliable predictor of reproductive ability: threshold values are a more effective indicator. Body weight relative to mature weight can effectively indicate puberty or first estrus in heifers. If 90 percent of a group of heifers are expected to be in heat during the breeding season, they should weigh a minimum of 65 percent of their expected mature weight at the beginning of the breeding season, which means the smallest heifer in the group, not the average of the group, should weigh 65 percent of her expected mature size at the start of the breeding season. Research has shown that the probability of breeding heifers that have reached or exceeded their minimum target weights is 80 to 90 percent. However, the conception rate of heifers lighter than their minimum target weight can be significantly lower.
Many commercial ranchers underestimate the mature size of their cow herd and consequently underestimate target weights for the heifers. This mistake is very easy to make, so keep it in mind as you are setting a target weight for your heifers.
Most heifers of most Bos taurus breeds are expected to reach puberty by 12 to 14 months of age. Bos indicus heifers typically reach puberty at 14 to 18 months of age. Once again, these are thresholds, not absolutes. The actual minimum target weight for yearling heifers at the beginning of the breeding season differs widely among and within breeds and crossbreeding combinations. Within a breed or crossbreeding combination, there are very large differences in mature cow size from herd to herd because of herd differences in nutrient availability and genetic selection for growth. However, in nearly all cases the minimum target weight can be expressed as 65 percent of the anticipated mature weight. To maximize the conception rate of heifers, we should use a combination of age thresholds and 65 percent of mature weight.
Yearling heifers should be bred to start calving 15 to 30 days ahead of the mature cow herd and for no more than a 60-day calving season. Calving first-calf, two-year-old heifers early in the calving season gives them more time to recover from calving and ultimately a better chance of breeding back for their second calf. Keep in mind that the most nutritionally demanding time in a cow's lifetime is the first 30 to 120 days after she calves as a two-year-old, at which time there are several nutritional stresses, including lactation demands, continued growth, and tooth loss. On top of that, we are expecting these heifers to cycle and rebreed 60 to 90 days after calving. Therefore, these heifers must receive adequate supplements. Remember that the actual pounds of protein and energy are about the same for heifers and cows, but physiologically a heifer can't consume as much feed as a cow; therefore, you should supply a higher-quality diet to virgin and first-calf heifers.
People often ask how fast heifers should gain weight. Researchers at Kansas State University found that the reproductive performance of heifers that gained 0.55 pound per day until two months before breeding and then were grown at 2.5 pounds per day was equal to that of heifers grown at 1.31 pounds per day from November to May. Oklahoma State University workers have shown that heifers wintered at 0.6 pound of gain per day and then fed a complete ration that resulted in 1.9 pounds of gain per day reached puberty 20 to 30 days sooner than their counterparts that were fed to gain at more uniform rates. This information is particularly applicable this year because of the very restricted growth of winter pasture in most of our service area. It also reinforces the potential for accelerated heifer growth on spring small-grain pasture. Providing heifers with access to small-grain pasture for graze-out would allow them to graze high-quality pasture at an accelerated rate of gain until breeding.
You shouldn't ignore the advantage to using an ionophore in the diet of growing heifers. The feed additives monensin (Rumensin) and lasalocid (Bovatec) are ionophores that are approved for growing heifers. Research has shown that using either of these compounds in the diet of replacement heifers can increase daily gains by 0.10 to 0.20 pound per day and shorten the time to first estrus by 1 to 2 weeks.