Effect of Wagyu genetics on marbling, backfat and circulating hormones in cattle

Backfat depths, marbling scores, and concentrations of plasma insulin, cort isol, triiodothyronine (T3). and thyroxine (T4) were determined in 394 calv es with and without Wagyu genetics. Hormone con centrations were determined three to five times for hormones between weaning and slaughter. Calves wer e weighed and backfat was ultrasonically determined at 28-d intervals, and assigned for slaughter when ultrasonic backfat depths approached 12 mm. Hei fers weighed less (P < 0.001) and had more backfat (P < 0.05) and more marb ling (P < 0.05) than steers at slaughter. Breed influenced slaughter weight (P < 0.001) and average backfat depth (P < 0.001), with Wagyu/Angus and Wa gyu/Hereford crosses and Angus purebreds generally having greater backfat d epths and lower slaughter weights than other crosses and purebreds. Marblin g scores were not affected by breed (P > 0.1), although Wagyu crosses had m ore marbling (P < 0.05) compared with purebreds when variability in marblin g due to differences in backfat depth was removed. Plasma cortisol, T3 and T4 were affected by calf breed, although there were no clear patterns for g roups of breeds or breed crosses. There was a slight trend (P < 0.2) toward s higher insulin levels and lower cortisol, T3 and T4 levels in some groups of calves with more backfat. Higher mean levels (P < 0.05) of T3 and T4 we re found in all but one group of animals with more marbling. Thus, thyroid hormones may have a role to play in enhancing marbling in cattle.