Development from Weaning to
Breeding It should be emphasized that replacement heifers need to be fed separately from the rest of the herd. Because of their size and age, as well as higher nutritional demands, they simply cannot compete with the rest of the cow herd, nor can they be expected to efficiently utilize poorer quality forages and still breed as yearlings. If there are wide ranges between the smaller and larger heifers, they should be divided into two feeding groups to reach their desired weight by breeding time. The days between initial weighing and beginning of the breeding season are then calculated. The average daily gain necessary to reach the desired breeding weight is determined, and the heifers are fed to attain that average daily gain. The addition of approved levels of ionophores such as monensin (Rumensin; Elanco) or lasalocid (Bovatec; Alpharma) to the ration will improve the average daily gain and enhance onset of puberty. Onset of puberty is affected by age, weight, breed, and adverse environmental stresses such as temperature and parasitism. Of these factors, weight is the one that most producers can readily influence. Researchers and ranchers have observed that high percentages of heifers will not reach puberty until they have reached a minimum weight. These weights usually represent about 65% of the potential mature size. Therefore, the first target weight to consider is that at the beginning of the breeding season. If the heifers weigh about 55% of the mature size producers can expect only 50% of them to be cycling at the beginning of the breeding season. However, about 90% of most beef breed heifers will be cycling when they weight 65% of their mature weight. Many ranchers have not recently weighed the adult cows in their herd to know what average mature weight to expect. Therefore most commercial ranchers would underestimate the mature size and underestimate the target weights for the heifers. Recent data from the American Angus Association (with records of over 20,000 cows) indicates that average mature size in the seedstock portion of their bred is about 1,200 pounds. Heifers from 1,200 pound mothers will need to weigh about 780 pounds by the start of the first breeding season. If the mature size of the herd is 1,100 pounds then the heifers can be about 715 pounds when breeding begins. And only heifers with potential mature size of 1,000 pounds can be expected to cycle at 650 pounds. These weights will not be exact since there is considerable variation within breeds, but the data show that large cattle must be fed for greater growth rates than smaller cattle.
Individual rather than group weights need to be considered when developing replacements. Simply because a group of heifers has reached a desired average weight at 15 months of age does not mean that all will reach puberty. If the group averages 700 lbs, some probably will weigh 600 and others 800. Those that weigh 600 will not breed well, while those weighing 800 have been fed more than was required. Replacement heifers should be sorted by size and fed to reach the desired weight, thereby giving the most fed to the heifers that need it. Age is also an important factor, especially in Brahman cattle. Many of these heifers will not reach puberty until they are 16 to 20 months of age. The same rule of thumb concerning 65% of the mature weight still applies for Brahman cattle, but the additional days of age also are important.
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