Donald E. Franke, Thomas D. Bidner, Manuel A. Persica III, Trent Smith and Joshua D. Domingue
Consumption of beef in the United States last year was about 67 pounds per capita. Many surveys have shown that tenderness of beef is important for consumer satisfaction. One of the primary factors influencing beef tenderness is breed of animal. Genetic differences among individuals within breeds also influence variation in tenderness. While the Brahman breed has contributed significantly to increased maternal and reproductive performance in crossbred beef cattle in the subtropical region of the United States, it also has tougher meat than non-Brahman cattle.
Recent work in the LSU AgCenter suggested that additive genetic influences account for between 20 percent and 30 percent of the variation in meat tenderness of Brahman steers. In addition, the necessity to progeny-test sires to identify those that transmit more favorable genes for tenderness is time consuming.
Enzymes produced by many pairs of genes are involved in the tenderization process. Pairs of genes with the largest effects are called major genes. Research in molecular genetics has identified genetic markers for several major genes that influence tenderness.
A genetic marker is a section of DNA that can be identified by a simple laboratory procedure and is closely linked to a major gene or is located within the molecular structure of the gene. When genetic markers are associated with particular major genes, they can be used to study how the major genes influence traits such as meat tenderness. Research reported here involves the association of known genetic markers for tenderness with shear force of steaks from Brahman steers. Procedure
Over a five-year period, 445 Brahman bull calves that had known parents were purchased at weaning from purebred Brahman breeders in Louisiana and managed as stockers for feed lots. DNA samples were collected from all steers at purchase and stored for future use. Following grazing on ryegrass for 120 days, the steers were transported to south Texas for feeding. Steers were harvested when they reached industry standards of 1,100 pounds to 1,400 pounds with 0.4 inches to 0.6 inches of backfat (for weight and degree of fatness) and processed at a beef processing plant in Corpus Christi, Texas. Loin steaks from each carcass were returned to the School of Animal Sciences Meats Laboratory to measure tenderness after they were aged for seven days or 14 days and then cooked. Six half-inch cores from each cooked steak were tested for tenderness using shear force – a measurement of the force required to cut each core of meat. Shear force averaged 10.1 pounds for seven-day aged steaks and 8.5 pounds for 14-day aged steaks. Steaks that have a shear force of 8.5 pounds or less are considered tender.
Through a cooperative effort with Bovigen LLC, a biotechnology company in Harahan, La., genetic markers associated with genes that produce calpastatin and calpain enzymes were identified from the DNA of each steer. Calpain enzymes are directly involved in the tenderization process. Calpastatin enzymes tie up the enzymes produced by the calpain genes and reduce tenderness. Thus, reducing calpastatin enzymes and increasing calpain enzymes will increase tenderness.
An allele is the form of a gene that contains characteristics from both parents at the same position – or locus – on a chromosome. Because genetic information comes from both parents, the selection of a parent with a genetic marker with both favorable alternatives assures it will be passed on to its progeny.
The calpastatin genetic marker has two forms: T and C. The TT genetic marker genotype for calpastatin is more desirable for meat tenderness than genetic markers TC or CC. Two genetic markers are located in the calpain DNA structure. CAPN-316 has allelic forms C and G whereas CAPN-4751 has allelic forms C and T. Genotype CC at both genetic markers is more favorable for tenderness than other genetic marker combinations. Results
Frequencies of the genetic marker genotypes were determined. The genotypic marker genotypes at the calpastatin locus had frequencies 11 percent for CC, 46 percent for CT and 43 percent for TT, giving genetic marker allele frequencies of 34 percent for C and 66 percent for T. These frequencies are similar to those found in other breeds of cattle and indicate the Brahman has a higher frequency of the favorable marker.
Frequencies for the CAPN-316 genetic marker genotypes were 0 for CC, 6.3 percent for CG and 93.7 percent for GG while the genotypes at the CAPN- 4751 position were 0 for CC, 10 percent for CT and 90 percent for TT. Frequencies of the favorable alleles for tenderness in these cattle were very low, at 3.1 percent and 5 percent for C alleles at the two positions. Low frequencies of the favorable genetic markers at these two positions suggest the genes they are associated with are responsible for a proportion of the higher shear force – or tougher meat – found in these cattle.
The influence of the favorable genes on the reduction of shear force for steaks aged for 7 days and 14 days was measured using statistical analyses. The favorable calpastatin genetic marker, C, was associated with a reduction in shear force of -0.11 pounds after seven-day aging and -0.22 pounds after 14-day aging of steaks. The reduction in shear force for the CAPN-316 favorable genetic marker was -0.46 pounds after seven-day aging and -0.66 pounds after 14-day aging. For CAPN-4751, the reduction in shear force was -0.44 pounds for steaks aged seven days and -0.60 pounds for steaks aged 14 days.
Combining the results from favorable genetic markers from both the CAPN-316 and CAPN-4751 positions, the reduction in shear force for steaks aged 7 days and 14 days would be approximately 2 pounds compared to steers with no favorable genetic marker alleles at these two positions. This lower shear force is very important and is similar to that found in n
on-Brahman breeds of cattle with similar genetic marker composition. Differences in tenderness of fed animals can be predicted before feeding by the combination of favorable genetic markers. While feedlot managers are interested in tenderness of fed animals, cattle breeders who produce feeder calves are more interested in the complement of favorable tenderness genes that sires and dams can transmit to their offspring. Sires and dams with four favorable genetic markers at the calpain positions will transmit two of these favorable markers to each of their progeny.
The total genetic effects of a trait transmitted from parents to offspring are called EPDs – expected progeny differences – which are measures of genetic merit for specific traits, such as tenderness (measured by shear force). EPDs are a measure of what is transmitted to progeny, which is half the breeding value, hence the name “Expected Progeny Difference.” EPDs are expressed from the mean, so we have + EPDs and – EPDs. For shear force, we would want EPDs with a minus value, which would be interpreted as lower shear force, or more tender beef. An EPD is a function of all the individual genes that influence a trait, whereas genetic markers are associated with a single gene or gene pair. Genetic markers tend to be identified when they are linked to a “major gene” for the trait. It is hard to identify a genetic marker when it is linked to a minor gene for a trait because it is not statistically relevant.
Knowledge of genetic markers for tenderness in prospective sires along with EPDs for tenderness will allow cattle producers to use marker-assisted selection to improve genetic potential for tenderness in Brahman cattle. Acknowledgment
This research was funded in part by Federal Hatch and state funds through the Louisiana Agricultural Experiment Station and grants from the Louisiana Brahman Association and the Louisiana Beef Industry Council. Bovigen LLC determined the genetic markers for each steer from individual steer DNA.
(This article was published in the winter 2008 issue of Louisiana Agriculture.)