Beef Improvement - Texas

NON-TECHNICAL SUMMARY: While beef production continues to be the largest agricultural enterprise in the U.S. (20.5% of total farm cash receipts; ERS, 2008), rising feed costs, global competition, and societal concerns about food safety, energy policy and the environment have created new economic challenges for the industry. Productivity of the U.S. cattle industry has improved substantially over the last 50 years, with beef production per total inventory increasing from 137 pounds in 1955 to over 250 pounds in recent years. Most of these productivity gains were realized through use of grain-fed beef production systems, adoption of nutrition, reproductive and pharmaceutical (e.g., ionophores) based technologies, and the application of crossbreeding systems and selection programs that focused on output traits. It is remarkable that these beef productivity gains were achieved in the absence of direct selection to improve feed efficiency. In fact, there is little evidence that genetic merit for feed efficiency or maintenance energy requirements in beef cattle has improved in the past 50 years. A number of factors have limited genetic progress in feed efficiency of beef cattle including focus on output traits, emphasis on population vs individual animal variation, lack of consistent selection goals and the high costs of acquiring feed intake data. The lack of an appropriate trait for use in selection programs has also curtailed genetic progress in feed efficiency. Traditional ratio-based efficiency traits like feed conversion ratio (FCR; feed:gain) are confounded by variation due to maturity patterns, and are strongly correlated in a negative manner with production traits (e.g., growth). Thus, selection to reduce post-weaning FCR will increase growth and mature size of replacement females, resulting in concomitant increases in the costs of feed required to maintain the breeding herd. Recent developments have provided opportunities to improve the genetic merit in cattle for feed efficiency: (1) development of an alternative efficiency trait (residual feed intake; RFI) that facilitates selection for efficient cattle independent of level of production, (2) advances in RFID-based technologies to measure feed intake and other physiological traits (e.g., heart rate, temperature, feeding behavior) to quantify inter-animal variation in biologic processes that impact feed efficiency and (3) advances in animal genomic technologies. Measuring RFI phenotypes in cattle remains expensive despite the use of new RFID technologies. Thus, discovery of gene markers and biomarkers (e.g., serum IGF-I) that are predictive of RFI will greatly enhance our ability to more accurately and cost-effectively identify cattle with favorable RFI phenotypes. Knowledge gained from the proposed studies, in combination with the development of educational programs to facilitate producer adoption of these technologies, will significantly reduce feed input costs and environmental impacts of beef production systems, as well as improve the competitive position of U.S. beef producers. <P>

APPROACH: Two sets of studies are proposed. The first study will consist of four experimental phases involving genetically identified Brahman heifers and bull calves. During the Pre-weaning Phase, growth traits will be recorded and immune function measurements obtained from Brahman calves, including offspring of animals that have been previously evaluated. Temperament indicator traits will be measured at weaning and on d 0 and 70 of the RFI-measurement periods. In Phase I, post-weaning feed intake, growth and ultrasound carcass measurements will be obtained to determine RFI. The heifers will be ranked by RFI, and the low and high RFI phenotypes identified (n = 8 per RFI phenotype). Phase II will employ methodologies to measure digestibility, energy expenditure and activity in order to examine the relationships between RFI and these biological processes. Also, in vitro proliferation of lymphocytes and immunoglobulin production will be determined to examine the relationships between RFI and immune function. For Phase III, previously RFI-phenotyped bulls and cows will be used in 70-d grazing trials to determine the correlation between RFI determined in a Calan-gate system on a growing ration and RFI determined when older and while grazing. Blood samples will be collected at selected times during all phases of this study to measure serum concentrations of immunoglobulins, catecholamines, cortisol, BUN, glucose, NEFA and IGF-I. Tissue samples from all Brahman and Bonsmara cattle are being collected and stored at -80C for future genomic evaluation. The second study will be conducted in two Phases using two sets of genetically-identified animals (60 purebred Bonsmara heifers and 60 Angus-sired crossbred heifers). For each set of animals, RFI will be measured during Phase I following a similar protocol as described above, except that feed intake and feeding behavior traits will be measured. Digestibility, heat production, activity, and immune function assays will be measured in low and high RFI phenotype heifers. Blood samples will be collected at selected times during both phases to measure serum concentrations of catecholamines, cortisol, IGF-I and metabolites (BUN, glucose, NEFA). For both studies, growth and composition traits of the Brahman bulls and Brahman and Bonsmara heifers will be measured from birth through approximately 18 mo of age. Ultrasound measurements of rib fat thickness, intramuscular fat and ribeye area will be obtained from at weaning and 56 d after weaning and on days 0 and 70 of the RFI measurement period. Nitrogen and phosphorus concentrations in the excreta of efficient and inefficient animals will be determined. Differences in ruminal fermentation activity will be examined, and VFA ratios, and methane production activity determined. Energy expenditure measurements will be obtained from calves with divergent RFI phenotypes. Liver biopsy samples will be obtained to examine the association of mitochondrial function and RFI. Puberty will be determined through weekly blood sampling for serum P4 determination in both Brahman and Bonsmara heifers.