S1045: Genetic Considerations for Beef Cattle Production in Challenging Environments
Statement of Issues and Justification
Statement of Issues:This proposal targets the national science roadmap for agriculture priority #7 to develop new and more competitive animal production practices and products. Beef cattle are important to most state's economies, and this project addresses several breeding and genetic aspects that are important to current and developing U.S. beef cattle production and management strategies, particularly in the southern region. Many traits such as health and hair coat characteristics have not been thoroughly quantified for degree of genetic influence or their impact on reproduction and growth.
Cattle diseases, such as Infectious Bovine Keratoconjunctivitis and Bovine Respiratory Disease Complex, and heavy infestations of specific external pests, such as ticks, have the potential to greatly reduce beef cattle productivity, especially in the Southern Region of the U.S. Producers not only experience reduced beef cattle performance but also have to bear the expense of treating cattle for these maladies, which combined can have a considerable negative impact on producer income.
Breed comparisons and even family comparisons are not static over time as these populations change due to selection strategies, and, conclusions from breed comparisons at a single point in time (i.e. 25 years ago) may not adequately explain current situations. Current performance assessments for breeds are needed to enhance producer breeding decisions for traits related to cow reproduction and calf survival in the Southern region of the United States. Additionally, there is limited research that characterizes the additive genetic control and prediction of breeding values for these important cow productivity traits, particularly in this region. Few research entities in the Southern region have enough cows and other resources to effectively conduct such research independently. A collaborative effort across locations in multiple states would be an appropriate way to address these issues. Historically, researchers in this area had long term (10 yr or more) projects that they published; today, funding organizations are unwilling to support such research. Current information using modern cattle is needed so that cow-calf producers in the Southern region can make informed breeding decisions to profitably produce cattle under their environmental challenges. Tropically-adapted breeds such as the Brahman (and to a lesser extent other adapted breeds such as those of Criollo or Sanga origin), are used widely in this region, however their offspring have a reputation for poor performance in stocker and feedlot operations on the Great Plains. As a result, many cow-calf producers in this region are attempting to use non-adapted cattle, such as British or other Bos taurus breeds.
As more emphasis is placed on genomics, many locations that have historically conducted cattle breeding research have reduced their populations and herd size. In the past ten years, new technologies have become available to identify individual genes throughout the bovine genome and independently assess their relationship to economically relevant traits (ERT) in the beef cattle industry. In order to locate these genes, DNA markers (also known as genetic markers) have been developed to aid in locating genes that can positively or negatively affect the phenotype of a specific trait. Finding genes with major effects is important in all traits especially those that are hard to measure and ones that are easily influenced by the environment, and incorporation of this information in selection schemes could enhance beef cattle improvement programs. However, large populations of cattle of known genetic background are needed to fully characterize new and emerging genetic markers.
In most areas of the world, cattle destined for slaughter are fattened on grass. In the southern U.S. this requires cattle that are well-adapted to the ambient conditions (high temperature and humidity) and it is usually expected that only Bos indicus or Bos indicus crosses can be sufficiently adapted to such conditions to grow rapidly and efficiently. Since both a light coat color and a short hair length contribute significantly to increased heat tolerance, it is possible that the combination of short hair and lighter coloration will result in an animal with high growth potential under grazing conditions in the southern U.S. without Bos indicus influence.
Justification:
It has been documented that beef cattle performance is impacted by diseases, such as Infectious Bovine Keratoconjunctivitis and Bovine Respiratory Disease Complex, and specific external parasites, such as ticks. However, it is less well documented as to what extent resistance to these maladies is influenced by beef cattle genetics. Because external parasites tend to develop resistance to pharmaceuticals used for their control, any genetic means of reducing the susceptibility of cattle to infestation would be of value to the cattle industry in tropical and sub-tropical areas. If genetic variation for resistance to these maladies exists, this information could be utilized by producers to identify genetic types that express resistance to these maladies with a subsequent increase in herd productivity being achieved at a lower cost.
The 13 states in the Southern region (AL, AR, FL, GA, KY, LA, MS, OK, NC, SC, TN, TX, VA) account for 14 million beef cows (42.3% of the nation's beef cow inventory) and 406,200 producers (48.9% of the nation's cow-calf producers) (USDA, 2002). A variety of breeds and crosses are utilized in this region. It is important to provide current characterization of breeds that have potential to improve productivity in regions that have substantial environmental challenges. Direct selection using estimated or predicted breeding values may represent another strategy for improvement of traits related to cow reproductive success and for calf survival. Most research facilities do not have enough cows to adequately evaluate their own results from analyses of traits related to cow reproduction and calf survival. Combining data from multiple locations will provide adequate numbers of records for analyses of such traits. Although the Southern United States has abundant quantity of forage for much of each year, the region can be considered stressful to cattle for many reasons, especially high heat conditions. The predominant forages range from low quality bahiagrass along the Gulf Coast to fescue in the upper South. Summer conditions are often similarly stressful throughout the South, as the toxic endophyte present in fescue amplifies heat stress conditions. Analyses of traits using data from across this stressful region would permit accumulation of substantial numbers necessary for appropriate assessment and would expand application of results throughout this critical production area.
Improving beef cattle performance through genetic selection traditionally involved observing physical characteristics of an animal, choosing the best individual, and using them as parents to produce the next generation. Progress with this type of selection was generally slow because the true genetic value was unclear until performance of the individuals progeny could be measured. This progeny performance information, along with performance of the individual and its relatives, were combined using advanced statistical analysis to produce what is known today as an expected progeny difference (EPD), a measure of an individuals genetic worth. For a variety of traits within a specific breed of cattle, EPD evaluate traits from a collective standpoint where all gene effects that control expression of a phenotype are added together to create a genetic value for each individual. Development of DNA marker and SNP (single nucleotide polymorphisms) technologies has allowed for the addition of a new selection tool, which creates the opportunity to evaluate individuals based on their genotypes from a DNA sample. This information gives producers the ability to make selection decisions much earlier in the growth and development period to identify economically efficient animals. Numerous cattle populations of known genetic background need to be evaluated for genetic markers to understand their utility across the entire industry.
Genetic tests for DNA markers that are associated with simply inherited traits such as coat color, polled condition, several genetic defects, and complex traits like marbling, tenderness, and other carcass traits are being marketed commercially by several U.S. companies. These companies market their tests by claiming a relationship of certain markers to particular traits and the amount of phenotypic variation observed when the marker is present or not. Validation of current markers as well as the effects of newly discovered genes on phenotypic traits will continually need to be investigated. The question for each newly discovered gene is: Does it have an association to one or many traits and, if so, to what degree? These markers also allow for the evaluation of traits that are hard to measure on live animals. For example, tenderness is a very important attribute influencing consumer satisfaction, but it is difficult to measure in that animals must be harvested for an accurate assessment. Similarly, reproductive efficiency and longevity greatly impact productivity. These traits are usually difficult to measure because of the many environmental factors that can mask the underlying genetic variation. Once major genes are identified, independent populations of animals with suitable phenotypes and pedigree information are needed for genetic characterization of these markers to determine how best to apply this new information.
Over the past two decades black has become the preferred color of feedlot and slaughter cattle in the USA, including the Southern Region. As a result, not only has the influence of Angus cattle increased, but the black gene has been incorporated, through upgrading and selection, into a number of previously red breeds such as the Simmental, Limousin, Gelbvieh, etc. This is in spite of the fact that black colored cattle will absorb more solar radiation than red or other lighter-colored cattle. Studies by Mader et al. (2002) and Davis et al. (2003) both showed rather dramatic (up to 0.5ยบ) lower body temperatures while under heat stress for white (dilute-colored Charolais crossbred) as compared to black feedlot steers. This advantage is comparable to the effect of the Slick hair gene for heat tolerance reported by Olson et al. (2003). The combined effects on heat tolerance of cattle that are both light colored and slick-haired have never been evaluated.
While there are limited data from feedlot studies showing that steers with white hair are more heat tolerant than those with dark red or black hair, there is no information available on the pasture growth of cattle varying in coloration and hair coat length. Also, if a specific coloration, hair length, or coloration-hair length combination results in increased tick or fly resistance, it could have a major impact on the genetic types utilized in crossbreeding programs for the Southern region and throughout the tropical and sub-tropical areas of the world utilized for cattle production.
The goal of this project is to address component traits with known genetic influence that impact productivity in cow-calf operations in the Southern Region. Several traits related to health and hair coat have not been well characterized for family differences and may be quite important for adaptation, and therefore could influence reproduction and growth. More thorough genetic evaluation of these adaptation-type traits may allow for previously unrecognized variation in beef cattle production systems to be identified. Many of the traits of interest are categorical in nature, and genetic evaluation of these types is complicated, but needs further study. If this project is not conducted, many questions about these traits and their potential contribution to production system variation will remain, which would likely result in reduced production efficiency in the U.S. beef cattle industry.
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