Effect of growth path and breed on the calpain system in steers finished in a feedlot

This experiment examined the effect of feeding a concentrate diet ad libitu m for two periods of time in two breeds of contrasting mature size cattle, on the activity of the components of the calpain system and the association between components of the calpain system and tenderness as determined by s hear force in the longissimus muscle. Nine-month-old Angus and Simmental st eers were either fed ad libitum on a feedlot from 9 months of age (F9) or o n pasture until 25 months of age (F25) and then fed on the same ad libitum ration as the F9 steers. Groups within a breed were killed at mean liveweig hts of c. 800 kg for the Angus and 950 kg for the Simmental. Steers were sl aughtered in a commercial abattoir and muscle samples collected from the lo ngissimus muscle at the 13th rib within 60 min of slaughter. The activities of the components of the calpain system were determined 2 and 24 h post-sl aughter. The length of time on the feedlot diet had the largest effect on s hear force of the longissimus muscle and the activity of mu-calpain. The st eers growing fastest prior to slaughter (F25) had the highest mu-calpain ac tivities two h post-slaughter and produced the most tender meat, even thoug h they were slightly older at slaughter. The best component of the calpain system for predicting shear force over all the treatments was the activity of calpastatin 24 h post-slaughter. This association between shear force 21 days post-slaughter and the activity of calpastatin 24 h post-slaughter wa s weak and varied between the treatments. The strongest associations occurr ed in the F9 Angus steers who also had the toughest meat and the slowest gr owth rates prior to slaughter.