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In a beef-on-dairy system, one copy of the F94L myostatin allele caused…
Beef from dairy herds accounts for over 20% of U.S. beef production; however, one of the greatest disadvantages of producing beef from dairy cattle is lack of muscularity resulting in poor carcass conformation, ill-shaped ribeyes, and poor carcass cutout yields. Using beef sires on dairy cows is an efficient and effective method of reducing these carcass conformation/yield deficiencies; however, it is critical for the beef industry to identify the correct sires for these dairy cow matings.
Myostatin is a protein that inhibits muscle growth. There are several known mutations in the gene for myostatin that result in varying degrees of increased muscle growth through hyperplasia, an increase in number of muscle fibers. Some of these mutations result in extreme muscle growth sometimes termed “double muscling”. However, these mutations have been shown to increase dystocia, and calving ease is one of the top considerations when U.S. dairy producers are selecting beef bulls. The F94L myostatin gene has been shown in increase muscularity in beef carcasses without the negative dystocia effects of the more extreme myostatin mutations.
The objective of this study was to determine the effect of the F94L myostatin gene when utilized in a beef-on-dairy mating system on muscle fiber type distribution and size, carcass traits (including ribeye shape) and USDA grades, carcass cutout yields for boxed beef and retail, strip loin steak shape and dimensionality, and tenderness and other sensory traits of various beef muscles.
In Phase I, carcasses (n=58) from steers resulting from the mating of two Limousin/Angus sires heterozygous for the F94L myostatin mutation to Jersey/Holstein dams were utilized. As indicated by DNA analysis, 30 carcasses were from steers with one copy of the F94L allele and 28 carcasses were from steers with zero copies of the F94L allele.
Within one hour postmortem, longissimus and semitendinosus muscle samples were excised from the left side of each carcass and fixed for muscle fiber type analysis. Immunohistochemical analysis determined the number and average cross-sectional area of each fiber type. Carcass grade data were collected at 48 hours postmortem, as well as ribeye shape (ribeye length, ribeye width at the 25/50/75% of length) and lean and fat instrument color readings.
The right side of each carcass was fabricated first into beef subprimals to determine boxed beef yield, and subsequently into retail cuts to determine retail yield. Lean trimmings for each carcass separately were course ground, mixed, and analyzed for chemical fat content to adjust each carcass’s trimmings to a standard 15% fat.
Strip loins were cut into 1-inch-thick steaks, and individual steaks from the entire strip loin were imaged at a fixed height above the steak on a gridded background. Each digital image was processed using an image analysis software to determine total muscle area, muscle length, and muscle widths at 25/50/75/87.5% of length. Samples of longissimus (strip loin), psoas major (tenderloin), gluteus medius (top sirloin), semitendinosus (eye of round), serratus ventralis (chuck flap), triceps brachii (boneless arm roast), and biceps femoris (bottom round) muscles from each carcass were vacuum packaged, aged until 10 days postmortem, and frozen for sensory and shear force analysis.
In Phase II, carcasses from 58 other steers, sired by three bulls heterozygous for the F94L allele were identified in a commercial packing plant for additional carcass data and shear force testing. Following carcass data collection, longissimus muscle samples from the 13th rib location were excised and transported to Texas Tech University for shear force and sensory panel testing. Muscle tissue samples from these 58 steers were DNA tested for F94L genotype and breed composition. Data from traits that were evaluated in both Phase I and Phase II were pooled and analyzed as one experimental design.
Calving data were obtained from the cooperating dairy producer and feedlot performance was obtained from the cooperating feedlot. The F94L allele did not affect gestation length, birth weight, or percent unassisted births. This is very important because dairy producers cite calving ease as a high priority when selecting beef sires to mate with dairy cows. Other myostatin mutations are known to significantly increase dystocia; therefore, the lack of increased dystocia from the F94L gene is good news for dairy producers. The F94L allele did not affect average daily gain in the feedlot, live weight at harvest, hot carcass weight, or dressing percentage (P > 0.05).
In a beef-on-dairy system, one copy of the F94L myostatin allele caused increased muscling, resulting in larger, more symmetrical ribeyes, more desirable yield grades, higher boxed beef and retail yields, and less angular strip steaks, all of which address inherent deficiencies in dairy and dairy-cross carcasses. These improvements were realized with no negative effects on calving ease or live performance and minimal effects on steak palatability. The F94L did cause a significant and meaningful reduction in marbling score; therefore, marbling ability should be paramount in sire selection if F94L sire are utilized. Using a beef sire homozygous for F94L myostatin in a beef-on-dairy system would ensure that all resulting progeny have exactly one copy of the F94L allele, meaning that this genetic tool could be rapidly implemented in the beef-on-dairy industry segment.