Heifer Bulls

       If you have even a passing interest in the topic of heifer bulls, then you should take a look at the following information. This enlightening article presents some of the latest news on the subject of heifer bulls.

     So far, we've uncovered some interesting facts about heifer bulls. You may decide that the following information is even more interesting.

Selection of Heifer Bulls
    
The economics of cow-calf production emphasize the importance of replacement heifers giving birth to their first calf by the time they are two years old. Since this female is bred shortly after reaching puberty, her first calf will be born before she is fully developed and mature. Therefore, this first-calf heifer warrants special management and breeding considerations to minimize the frequency of calving difficulty (dystocia).

     Dystocia must be minimized because it is expensive. Of obvious concern is the loss of females as well as calves through calving problems. However, increased calving intervals due to delayed rebreeding and reduced weaning weights also contribute to economic losses of calving difficulty.

     Calving difficulty has been shown to be a problem primarily in two-year-old first-calf heifers. Although dystocia is not uncommon in older females, it occurs at a much lower frequency. Calving difficulty in first-calf heifers has been shown to be three to four times higher than in three-year-olds. Incidence of calving difficulty is twice as high for three-year-olds than four-year-olds.

     Considerable time and effort has gone into research projects to gain an understanding of the causes of dystocia and determine the extent to which various factors are involved. Results of these studies have shown there are several factors involved. However, no more than 50% of variation in calving difficulty can be explained by traits that can be defined or measured. Therefore, it is impossible to predict with 100% accuracy which heifers will have calving difficulty, even if we can quantify the various measurable factors that influence dystocia. Several of the most important factors contributing to differences in calving difficulty include:

     Calf birth weight Dam breed Cow pelvic area Calf shape Cow age Cow nutrition Calf sex Season/temperature Gestation length Cow body condition Cow weight Calf presentation Sire breed Heterosis

     Of the factors listed above, calf birth weight is most highly related to calving difficulty. As calf birth weight increases, the percentage of cows requiring calving assistance also increases. It is likely that some of the factors listed above manifest their effects through their relationship with calf birth weight (calf sex and gestation length). Birth weight is a trait that is easily measured, and is relatively high in heritability (.30-.40) suggesting that selection for lower birth weights is attainable as a means to reduce calving difficulty.

     Birth weight, like any other trait, is controlled by two basic components: genetics and environment. Although environmental factors such as nutrition and season of the year play important roles in determining calf birth weight, research has shown that management practices to control calf birth weight through these factors have little appreciable impact on calving difficulty. Therefore, control of birth weight is most effective through genetic selection.

     It is important to note that although significant differences exist between breeds for birth weight, there is as much variation within a particular breed as there is across breeds for these traits. Put another way, there are bulls within any breed that sire low birth weight calves (as well as those that sire heavy calves at birth). Identification of genetics for birth weight within your breed is most critical.

     Generally, bulls that are below breed average for birth weight should be selected for use on heifers. The threshold birth weight value for any given breed that will result in acceptable birth weights and calving ease will vary from one operation to the next. Several factors must be considered when defining this value, including labor availability at calving time, heifer size and weight, and heifer breed.

     Birth weight has a positive relationship with growth rate. Therefore, most sires that have below average birth weight will also have below average weaning weight and yearling weight. However, there are sires available that are below average for birth weight and excel breed average for growth traits. This is an important consideration, as acceptable growth rate does not need to be sacrificed at the expense of getting a live calf born. Producers should consider setting maximum standards for birth weight along with minimum standards for growth when selecting bulls.

     To Summarize. As noted earlier, we are only able to account for 50% of the variation in calving difficulty. Therefore, complete elimination of dystocia in first-calf heifers is unlikely. However, tools are available that will significantly reduce calving problems. The major selection criteria for bulls to use on heifers should be birth weight. The use of proven sires through AI will further enhance calving ease. Finally, genetics are available that will produce acceptable birth weight and calving ease, while maintaining above average growth potential. Following these selection criteria will improve the profitability of the cowherd through reducing economic losses attributable to calving difficulties.

     There's a lot to understand about heifer bulls. We were able to provide you with some of the facts above, but there is still plenty more to write about in subsequent articles.

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