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Title: Effect of frame size and time-on-pasture on steer performance, longissimus muscle fatty acid composition and tenderness in a forage-finishing system

Author
item DUCKETT, SUSAN - Clemson University
item C., FERNANDEZ ROSSO - Clemson University
item G, VOLPI LAGRECA - Clemson University
item M., MILLER - Clemson University
item Neel, James
item RON, LEWIS - Virginia Polytechnic Institution & State University
item WILLIAM, SWECKER, JR - Virginia-Maryland Regional College Of Veterinary Medicine (VMRCVM)
item FONTENOT, JOSEPH - Virginia Polytechnic Institution & State University

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/12/2014
Publication Date: 9/2/2014
Citation: Duckett, S.K., C., F., G, V., M., M.C., Neel, J.P., Ron, L.M., William, S.S., Fontenot, J.P. 2014. Effect of frame size and time-on-pasture on steer performance, longissimus muscle fatty acid composition and tenderness in a forage-finishing system. Journal of Animal Science. 92:4767-4774.

Interpretive Summary: Grass-fed beef is a marketing option for Southeastern US beef producers where forage can be grown almost year-round. One challenge for grass-fed beef production systems is the determination of a slaughter endpoint and its impact on beef quality. In a traditional feedlot finishing systems, slaughter endpoints are typically related to external fat thickness, live weights, and estimated percentage of cattle grading Choice. Comparisons among grain- and grass-fed finishing systems show that grass-fed cattle are typically lighter in weight, have reduced backfat thickness, and lower muscular fat content. Therefore, the use of these indicators for slaughter endpoints may not be practical in a grass-finishing system. Little research has addressed time-on-pasture in a forage-finishing system or assessed relationships between carcass traits and their impact on tenderness for determination of slaughter endpoints. Previous research compared grass- and grain-fed beef when slaughtered to similar animal age endpoints and found no differences in tenderness of ribeye steaks. We hypothesized that time-on-pasture, which directly impacts animal age at slaughter, is a key determinant for producing tender beef in a grass-fed beef production system. We found increased time-on-pasture resulted in heavier animals and carcasses, larger ribeyes, and higher quality grades. Steaks from pasture finished cattle were tender in animals up to 18.3 months of age, while marbling score and USDA quality grade were not correlated with toughness. Lower total animal gain during pasture finishing was associated with toughness, but final 28 or 56 day average daily gain was not correlated with toughness. These results show that time on pasture/animal age is an important determinant in beef tenderness of grass-fed beef and that tender grass-fed beef should be slaughtered at about 18.3 months of animal age or before.

Technical Abstract: Angus-cross steers (n = 96; BW = 309 + 34 kg; 13.5 mo of age) were used to determine the effects of frame size (medium, MED or small, SM) and time-on-pasture (TOP) on meat composition and palatability in a two-year study. Finishing steers grazed mixed pastures (bluegrass/white clover; April start) and were slaughtered after 89, 146, and 201 d TOP. At 24 h postmortem, carcass traits were collected and a rib from each carcass obtained for proximate and fatty acid composition, Warner-Bratzler shear force (WBSF), sarcomere length and postmortem proteolysis. In yr 1, postmortem aging treatments included 14 and 28 d; whereas in yr 2, postmortem aging treatments included 2, 4, 7, 14, and 28 d. Increasing frame size of the finishing steers produced greater (P < 0.05) ADG by 0.10 kg, BW by 24 kg, HCW by 14 kg and ribeye size by 2.65 cm2. All other carcass, meat composition and tenderness measures did not differ (P > 0.05) due to frame size or two-way interaction with TOP. Increasing TOP resulted in quadratic increases (P < 0.01) in BW and HCW. Ribeye area, fat thickness, KPH, marbling scores, quality grades, and yield grades increased (P < 0.001) linearly as TOP increased. Time-on-pasture linearly increased (P = 0.001) palmitic (C16:0) acid, oleic (C18:1 cis-9) acid, SFA, and MUFA in the LM. Both n-6 PUFA, linoleic (C18:2) and arachidonic (C20:4) acids, decreased linearly (P = 0.001) with increasing TOP. Increasing TOP linearly reduced (P = 0.01) concentrations of all n-3 fatty acids in the LM. These changes resulted in a linear reduction (P = 0.01) in n-6 to n-3 fatty acid ratio with advancing TOP; however, the magnitude of the difference was small (1.46 vs. 1.37). At 14 d of postmortem aging, WBSF was lowest (P < 0.001) for 89 d TOP and greatest (P < 0.05) for the 201 d TOP. After 28 d of postmortem aging, WBSF values for 89 and 146 d TOP did not differ (P > 0.05) compared to their 14 d postmortem aging WBSF values. However in steaks from 201 d TOP, additional postmortem aging to 28 d reduced (P < 0.001) WBSF. Sarcomere length was longer (P < 0.05) for steaks from steers finished at 89 d TOP than 146 or 201 d TOP regardless of postmortem aging time. In pasture-based beef finishing systems, increasing TOP increases animal age, HCW, fat thickness and marbling score; however, tenderness of ribeye steaks decreased with advanced TOP such that longer postmortem aging times were required to achieve similar tenderness levels.