Birth weight of beef calves may be one of the single most important factors in profitability of the beef herd. Birth weight accounts for over 70 percent of the variability in calf survival to weaning, yet many producers do not manage birth weight in their cows. There are two major ways to manage birth weight.
The effect of cattle breed on birth weight regulation is relatively small except when individual animal birth weight prediction tools, such as EPDs, are used. Consider data from the 2009 American Angus Association Sire Summary. Angus cattle are generally accepted as having lower birth weights compared to many other breeds. However, an examination of over 5 million progeny records indicates birth weights can vary from 70 pounds to 94 pounds when bred to average cows based on the genetic predictions of individual Angus bulls. Most other breeds would exhibit similar variation within the breed, leading to the concept there is usually as much variation in traits like birth weight within a breed as there is across breeds.
There are sire and dam breed effects that influence birth weight. Results in Table 1 show how one breed of sire may differentially influence birth weight in progeny from three other dam breeds. Therefore, it may not be reasonable to assume a certain breed of sire will influence birth weight in the expected way.
The most accurate method to control birth weight genetically is by using genetic prediction tools available on individual animals. In a study from Arnold et al (1990), it was shown the predicted difference in progeny birth weight using the EPD for birth weight of the sires was 6.6 pounds, and the actual birth weight difference in the progeny was 8.4 pounds. Further, this study also indicated selection for genetically antagonistic traits like birth weight and yearling weight could be done successfully using genetic prediction tools. Cattle can have genetics for low birth weight and still have superior growth potential.
|PHOTO BY KIMBERLY STOCKWELL-MORRISON.
A significant step was recently made in evaluating the genetic potential of cattle using across-breed EPDs. Historically, EPDs were calculated separately by breed and there was no connection between them. Table 2 shows the results of these EPDs for some popular cattle breeds. Breeds are compared equally for their EPD for birth weight by adding or subtracting the adjustment factor from the table to their individual EPD. For example, an Angus bull with a birth weight EPD of 2.9 can be compared to a Simmental bull with a birth weight EPD of 0.8. The EPD for the Angus bull is 2.9 (2.9 + (adjustment factor = 0) = 2.9) and the equivalent EPD for the Simmental bull is 6.3 (0.8 + (adjustment factor = 5.5) = 6.3.
There is a popular action used by producers to reduce birth weight by restricting feed intake prior to calving. There are mixed results when using pre-calving feed restriction. The report from Bellows et al (1978) indicated feeding in excess of National Research Council (NRC) requirements increased birth weight by 4.2 pounds, but there was no difference in calving difficulty or weaning weight of calves. Another study from Anthony et al (1986) indicated feeding the same amount of energy, but 20 percent less protein, would not change birth weight or calving difficulty, leading to the assumption that changes in feed energy—and not protein—are more closely related to changes in birth weight. Corah et al (1975) reported feeding 65 percent of pre-calving NRC energy requirements resulted in 4.4 pounds lower birth weight, 7 percent lower survival rate and 29 pounds lower weaning weight in progeny of restricted cows. These results were found again by Freely et al (2000), where cows that were reduced from a condition score of 5.8 to 4.8 from feed restriction had calves with 10-pound lower birth weights. In summary, nature may draw a hard line in the sand about how much one should restrict feed intake in late-gestation cows. Cows that get below a condition score of 5 will certainly not breed back as quickly, and could have restricted growth in their calves.
Most mature cows can deliver a live calf over a wide range of birth weights. Therefore, more attention to birth weight should be paid to younger cows, where feed restriction in late gestation can seriously affect reproductive efficiency in the subsequent breeding season. In most cases, using cow condition score will be the most effective tool to monitor the level of pre-calving nutrition that is needed. Keeping the cows of any age in a 5 to 6 condition score will assure both adequate nutrition for reproduction without overfeeding and also indicate the baseline of condition the cows should be carrying.
Dr. John Comerford is associate professor of dairy and animal science at the Pennsylvania State University.