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  or lasalocid 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. 

Table 1. Puberty Weight (lb) of Heifers by Breed (assumes small to moderate frame). 
                           Weight                          Weight                         Estimated 
                            at 50%                           at 90%                         mature 
                            cycling                           cycling                           weight

     If heifers are undersized, 6 weeks before the breeding season, there still may be time to change the diet and make up lost ground.  Kansas State scientists studied the reproductive performance of heifers grown at a slow rate of gain for most of the winter and then fed to gain over 3 pounds per day for the last 60 days before breeding.

As long as the heifers "caught up" in weight by the start of the breeding season, reproductive performance was similar to those that gained at a steady rate all along.  One word of caution: Make any ration change gradually to reduce the risk of digestive disorder.

     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.