Heifers Breeding Weight

      Check heifers to be certain they will be heavy enough at breeding

        The period between weaning and breeding is a very critical time in the life of a beef female. At weaning she is between 7 and 10 months old and weighs, in general, 350 to 650 pounds. Some six months later, she is exposed to the bull or to artificial insemination. Hopefully, most of these heifers are bred in the first 21 days and 80 percent or more are pregnant after a 45 day breeding season. Growing programs for weaned replacement heifers must be adequate to allow enough gain from weaning to 13 months of age to allow a high percentage of heifers to being cycling. Since most beef breed replacements will need to gain 240 pounds between weaning and breeding, the heifers must gain at least 1.33 pounds per day. 

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

Angus 550 650 1000
Brangus 600 700 1075
Charolais 700 775 1190
Hereford 600 700 1075
Shorthorn 500 600 925
BritishXBritish 575 675 1040
CharolaisXBritish 675 775 1190
JerseyXBritish 500 600 925
LimousinXBritish 650 775 1190
SimmentalXBritish 625 750 1150
Brahman* 700 750 1150
Santa Gertrudis* 700 750 1150
Brahman x British* 675 750 1150

 *Average puberty age 16 - 20 months 
 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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