Bull Selection for Heifers
Livestock Update, February 2000
Scott Greiner, Extension Animal Scientist, Beef, Virginia Tech
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.
Factors Contributing to Dystocia
Considerable time and effort have 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 are 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|
|Gestation length||Cow body condition|
|Cow weight||Calf presentation|
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.
Selection for Reduced Birth Weights
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.
A summary of research conducted from 1970 to 1990 at the U.S. Meat Animal Research Center in Clay Center, NE is presented in the following table. This data represents the effect of breed of sire on calving ease and survivability traits when the bulls were mated to crossbred cows. Significant differences between breeds for the various traits are evident. Breeds that sire calves that are heavy at birth also tend to cause more calving difficulty resulting in a lower percentage of unassisted births. Other research studies confirm that heavy birth weights are the primary cause of calving difficulty. Calf survival to weaning tended to be higher in breeds that required less assistance at birth.
It is important to note that although significant differences exist between breeds exist 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 breed is most critical.
Expected progeny differences (EPDs) are the most valuable tools available to producers for identifying genetically superior sires for any trait. Only bulls that have acceptable birth weight EPDs should be considered for use on first-calf heifers since birth weight is the most significant factor affecting dystocia.
|Birth Weight EPD|
EPDs are most useful to directly compare individuals for a trait of interest. In the above example, assume that the two bulls were each mated to the same set of cows. The difference in the birth weight EPD value between Bull A and Bull B is 4 pounds (5 - 1 = 4). Therefore, Bull A would be expected to sire calves that are 4 pounds heavier at birth on the average than calves sired by Bull B. It is important to recognize that EPDs predict the expected difference in performance, not the actual performance. In other words, the EPDs for Bulls A and B suggest there will be 4 pounds difference in birth weight in their progeny when we mate them comparably. EPDs do not predict what the actual birth weight of the calves will be.
Generally, bulls that are below breed average for birth weight EPD should be selected for use on heifers. The threshold birth weight EPD value for any given breed that will result in acceptable birth weights/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/weight, and heifer breed.
Birth weight has a positive relationship with growth rate. Therefore, most sires that have below average birth weight EPDs will also have below average weaning weight and yearling weight EPDs. 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 EPD along with minimum standards for growth EPDs when selecting bulls. Breed average EPDs for bulls born in 1997 are presented in the following table.
Some breed associations publish calving ease (CE) EPDs. This EPD predicts the ease with which a bull's calves are born to first-calf heifers. Calving ease EPDs are reported as deviations in percentage of unassisted births. For example, if Bull A had a calving ease EPD of 100 and Bull B 105 we would expect heifers bred to Bull B to have a 5% advantage in unassisted births. Some breeds also report maternal calving ease EPDs. As maternal implies, this EPD reflects the ability of a bull's daughters to calve as first-calf heifers. When comparing bulls, a larger maternal calving ease EPD indicates that a bull's daughters will calve with a higher percentage of unassisted births. Calving ease EPDs should be used in concert with birth weight EPDs as another tool to use for avoiding dystocia problems in the cowherd. The front of breed sire summaries should be consulted to interpret the meaning of calving ease EPDs for each breed.
Along with EPDs for the various traits, accuracy values associated with each EPD are also available. These accuracy values range from 0 to 1.0. The accuracy value is a measure of the certainty we have in the EPD. High accuracy values (>.80) indicate that the EPD is less likely to change when more performance records are included in the EPD determination for the animal. High accuracies are indicative of animals that have a large number of progeny records. The accuracy level also gives us an indication of how much the EPD may change. Consider the following two bulls:
|Birth Weight EPD (Accuracy)|
|Bull A||+2.0 (.90)|
|Bull B||+2.0 (.25)|
Bull A and Bull B have equivalent birth weight EPDs. However, Bull A has an accuracy of .90 vs. .25 for Bull B. If these were Angus bulls, we expect the "true" birth weight EPD for Bull A to be between +0.8 and +3.2 (there is a 67% probability that the "true" EPD will fall in this range). For Bull B, his "true" EPD may be from -0.6 to +4.4. Therefore, we are more confident in the published EPD of Bull A. There is a chance that the EPD for Bull B is considerably higher than what is reported. An accuracy of .90 would be typical of a well-proven bull, whereas an accuracy of .25 would be common for a young bull.
Implications of accuracy relate to associated risk. The opportunity to use high accuracy, proven sires is a major advantage of using AI on heifers. The use of high accuracy, low birth weight EPD bulls will decrease the incidence of dystocia. Additionally, there are many AI sires across all breeds that have acceptable birth weight EPDs and still are high-growth EPD bulls. Does this mean that young, low accuracy bulls should not be used? No. Research has clearly demonstrated that EPDs are much more effective than individual performance measures (individual's own birth weight) for predicting future offspring performance. Even on young yearling bulls, EPDs are the most effective selection tool available.
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 EPD. The use of proven sires through AI will further enhance calving ease. Finally, genetics are available that will produce acceptable birth weight/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.