The US beef industry is emphasizing the production of lean beef but with adequate marbling. These two goals seem to be contradictory to each other. Beef cattle are fed to a level of external fat that should ensure an adequate amount of marbling for grading choice. However, the National Beef Quality Audit determined that $190 per animal was being lost to the industry due to excessive external fat. Many US breed associations have established carcass EPD's for use by breeders in sire selection. The American Angus Association has probably the most information and numbers on marbling EPD's. A group of researchers from the Animal Science Department at the University of Nebraska utilized the marbling EPD's from Angus bulls to evaluate the effect of selection for marbling on the performance and carcass characteristics of progeny. Their full report is in two articles in the Journal of Animal Science 74 (5) p. 1009-1035.
Six Angus bulls with high (>.4) and six Angus bulls with low (<-.16) EPD for marbling were bred to MARC II composite cows in a two year study. At weaning, the steers and heifers were separated, placed on growing diets until they entered the finishing phase. Both steers and heifers were slaughtered at two mean fat thickness end points estimated visually to be .25 and .5 inch backfat. The study was done on a total of 256 animals.
The JAS articles contain many tables detailing the many aspects of the data. This newsletter article summarizes some key points pertinent to the question of what effect does selection for marbling have on other traits. Also, because differences between steers and heifers were either non-significant or could be explained by age differences, the data is combined in the following table.
The major difference observed in the calves was the lower weaning weights from the calves sired by the HIGH marbling bulls. Selection for the test bulls was based on marbling EPD's only. The differences in the calves' weaning weight are probably due to the higher growth EPD of the bulls in the LOW marbling group.
Daily gain, dry matter intake, feed efficiency and final weights during the finishing phase were not statistically different for progeny sired by the HIGH or LOW marbling EPD sires. It is in this area of performance that the data is unclear. Previous researchers (before marbling EPD's were available) had shown that selection for growth increased frame size and delayed maturity; both of which led to a decrease in the marbling score. The data from this research, as well as from other recent articles, indicate that cattle can be selected for growth or increased retail product and reduced external fat without sacrificing marbling traits. If that is indeed true, the trick will then be to find those sires with high marbling scores and growth EPD's.
| Marbling EPD | |||
|---|---|---|---|
| Item | High | Low | |
| Number of animals | 125 | 128 | |
| Sucking phase | |||
| Birth weight, lbs | 90 | 89 | |
| Calving difficulty | 1.45 | 1.40 | |
| Weaning weight, lbs | 488 | 517 | |
| Finishing phase | |||
| Initial weight, lbs | 659 | 688 | |
| Final weight, lbs | 1079 | 1158 | |
| Daily gain, lbs | 3.06 | 3.04 | |
| Feed intake, lbs/day | 21.2 | 21.7 | |
| Carcass | |||
| % choice | 74.5 | 47.0 | |
| Yield grade | 2.69 | 2.67 | |
| % YG 1 & 2 | 70.9 | 77.1 | |
| % YG 3 & 4 (no YG5) | 29.1 | 22.9 | |
The biggest differences observed in this trial were in the number of cattle grading choice. As shown in the table, more than 70% of the progeny from the HIGH marbling bulls graded choice as compared to less than 50% from the LOW group. This difference was even greater for the cattle slaughtered at .25 in backfat where 52% of the progeny from the HIGH marbling sires graded choice as compared 17% from the LOW group. Cattle that were fed to .5 in backfat were 96 and 78% choice for the HIGH and LOW groups, respectively. Although backfat was slightly greater in cattle from the HIGH marbling progeny, the differences were not statistically different indicating that it is possible to use available genetics to select for cattle with increased marbling at the same fat thickness.
This experiment indicates that Angus sires can be selected to produce progeny that have an increased ability to marble without increasing backfat. This would enable cattle to grade choice with less time on feed and potentially reduce the amount of external fat produced and subsequently trimmed. The selection of sires based on any single trait will influence other traits of economic importance. As individual producers, knowledge of what the consumer and industry wants from our cattle will direct the traits we seek in our sires. This paper shows that it is possible to have more marbling without increasing backfat.