February 2011 Articles

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Continued from Beef Sire Selection

Continued from Beef Sire Selection, University of Arkansas
Selection Criteria Beef cattle selection should be based on many factors . growth and reproductive performance, fertility, health, disposition, age, frame size, muscling, etc. Single trait selection should be avoided when selecting a herd sire. Overemphasis on one or a few traits may reduce performance for other traits. There are several genetic antagonisms that may result in performance tradeoffs. For example, selection for high growth (high weaning and yearling weights) may simultaneously increase birth weights and calving difficulty. The reverse is also the case: growth sacrifices may be made when selecting for low birth weights. Of course, there are bulls within every breed that have the genetic potential to transmit both high growth and low birth weights to their calves. Other common performance tradeoffs include red meat yield versus red meat quality, fertility/reproduction versus growth rate/lean yield and milk yield versus cow maintenance requirement. A balanced approach to sire selection focusing on multiple economically important traits can go a long way towards herd genetic improvement. Frame Size Changing the frame size of the calf crop can be accomplished through sire selection and selective culling of the cow herd. Inappropriate carcass size and weight ranked second among the “top 10 quality challenges” identified in the 2000 National Beef Quality Audit. Frame size describes the overall skeletal size of cattle and is a useful tool for evaluating the lean-to-fat ratio of an animal. It is an indication of growth and is related to slaughter weights at which cattle should attain a given amount of fat thickness. Large frame steers (frame scores 7, 8 and 9) with the genetic potential to grade Choice are expected to do so at 1,250 pounds or higher, while large frame heifers with the genetic potential to grade Choice are expected to do so at 1,150 pounds or better. Medium frame steers and heifers (frame scores 4, 5 and 6) with the genetic potential to grade Choice are expected to do so at 1,100 and 1,000 pounds, respectively. Small frame steers and heifers (frame scores 1, 2 and 3) with the genetic potential to grade Choice are expected to do so at less than 1,100 and 1,000 pounds, respectively. Frame scores are calculated using hip height measurements. Hip heights can be measured with a hip height stick or pull-down tape measure and converted to frame size scores by calf sex and age using a frame score table. Although actual hip height may increase as an animal matures, most cattle maintain the same frame score throughout their lives. This allows one frame score to be used for an animal, regardless of when that animal’s hip height was evaluated. Larger framed cattle require more forage and feed resources than smaller framed cattle, so matching cattle size to production resources is important. Culling extremes for frame size (large and small) can also be useful in improving herd uniformity. Bull frame size can be strategically matched to cow frame size to produce calves within a targeted frame size range. However, caution should be used when breeding a large frame bull to small frame cows or heifers due to the increased risk of calving difficulty. Some breed associations compute yearling height EPDs that can be used in predicting a sire’s ability to transmit yearling height to his calves. Muscling Thickness or muscling is important in beef cattle because muscle is what is sold in the retail meat case. The degree of muscling impacts yield grades, average daily gains and dressing percentages. Lightly muscled cattle are significantly discounted at sale time. Muscling can be improved through bull
selection. USDA feeder cattle thickness grades used to classify muscling range from 1 to 4, with 1 being the heaviest degree of muscling. There are several good indicators of muscling in beef cattle (Figure 1). Muscling may be evaluated in the quarter or round, stifle, gaskin, twist, shoulder, forearm and across and along the back. Do not just look in one area to determine muscling. An animal may be thick through the quarter but lack adequate muscling in the forearm. The forearm is an excellent place to look for muscling because there is usually less fat cover in this area. Extremely muscled heifers or cows may have fertility problems, so sire selection for muscling should be optimized instead of maximized. In addition, a coarse, bunchy-muscled bull may sire similar calves causing trouble at calving.
Figure 1. Indicators of Muscling in the Beef Animal
Growth Traits Growth traits include weaning and post-weaning (yearling) growth performance. Growth performance
information available on performance-tested bulls may include average daily gain, weight per day of age, adjusted weaning and yearling weights and weight ratios within contemporary groups. A contemporary group is a group of cattle of the same sex and
age that are managed under like conditions. An average weight ratio for a contemporary group is always equal to 100. A calf with a weaning weight ratio of 105 has a weaning weight that is 5 percent above the average of the group. A weaning weight ratio of 90, on the other hand, indicates that the calf ’s weaning
weight is 10 percent below the average of the contemporary
group. Expected progeny differences for weaning and yearling weights are a fairly standard component of national sire evaluations conducted for specific breeds. Carcass Traits Carcass traits are a key consideration in bull selection, particularly when cattle ownership is retained and cattle are sold on value-based pricing grids. Table 1 lists industry targets for beef carcasses
outlined during the strategy workshop of the 2000 National Beef Quality Audit. Carcass traits are moderately to highly heritable, so genetic improvements can be made in a shorter period of time than less heritable reproductive traits. Ultrasound carcass scanning technology allows carcass information to be collected on live animals instead of having to wait until cattle are harvested. Yearling bulls out of potential herd sires may be ultrasound carcass scanned for 12th to 13th rib fat thickness, rump fat thickness, ribeye area and intramuscular fat percentage (marbling). Each of these traits is significant in the determination of red meat yield and quality, and each is at least moderately heritable. Participants in the Arkansas Steer Feedout Program receive carcass information on calves after completion of a finishing phase. This information can be used to evaluate growth and carcass traits in the herd and compare the carcass merit of calves out of different herd sires. Expected progeny differences are also available for many carcass traits, including hot carcass weight, marbling, ribeye area, rib fat thickness and percent retail product. Calving Ease Calving ease is an important consideration in the sire selection process, particularly when first-calf heifers or small-framed cows are to be bred. Labor availability may influence how a “calving ease” or “heifer” bull is valued. Birth weight has often been used as an indicator of calving ease, but there can be a lot of variation in calving ease. Birth weight is just one of many factors that affects calving difficulty in beef cattle. According to the Beef Improvement Federation, other factors affecting calving ease include age of dam, calf sex, pelvic area, gestation length, cow size, shape of calf, breed of sire, breed of dam, uterine environment, hormonal control, geographic region, season of year, environmental temperature, nutrition of dam, condition of dam, implants/feed additives, feeding time and exercise. Birth weight and several other factors are components of calving ease EPDs. Selection based on both calving ease and birth weight EPDs is discouraged since it may put too much selection emphasis on birth weight. Emphasizing calving ease in selection rather than birth weight may make it easier to select for calving ease and growth performance at the same time. Birth weight will still be accounted for in calving ease EPDs. The two types of calving ease EPDs are calving ease direct and calving ease maternal. Calving ease direct EPDs provide information about the expected assistance required at birth for a sire’s calves and predict the ease with which a bull’s calves will be born to first-calf heifers. Calving ease direct indicates the percent more or less of calves sired by a particular bull that are expected to require assistance at calving out of two-year-old heifers. For example, a bull with a calving ease direct EPD of +10 percent compared with a bull within the same breed with a calving ease direct EPD of +2 percent is expected to sire on average 8 percent (10 . 2) more calves that can be born unassisted. Calving ease maternal or daughter’s calving ease EPDs, on the other hand, give an indication of the expected assistance required at calving for calves out of a sire’s two-year-old daughters. In this case, the bull on which the EPD is evaluated would be the grandsire of the calf for which the necessary assistance at birth is being predicted. Calving ease maternal is also referred to as daughter’s calving ease or maternal calving ease and is the ease with which a sire’s daughters calve as first-calf heifers.