Can potential for dark cutting be predicted by phenotype? Ⅱ: Relationships between farm, phenotype, carcass characteristics and the incidence of dark cutting beef in heifers

摘要

Reduced subcutaneous fat depth, marbling and live weight in heifers have been associated with increased frequency of dark cutting. A data set of cattle (n = 845) was used to extract data from three farms for a balanced analysis of variance examining the effect of farm and Canada carcass grade on heifer live and carcass characteristics. A data set of 72 heifers (Canada AAA, AA, A, and B4, n = 9,21,21 and 21, respectively) from the original data set (n = 845) was created by selecting carcasses from the four grades matched as closely as possible within slaughter lot by date of birth to all available Canada B4 carcasses to establish a balanced design between heifer live and carcass measurements and the dark cutting Canada B4 grade. Data were from heifers on study from 2003 to 2011 on three farms, designated A, B and C, which contributed n = 18,47, and 7 heifers, respectively. Heifer data from Farm A were presented originally in Mahmood et al. (2014a) and were included in testing the new hypothesis that heifers producing a dark (Canada B4) carcass can be identified from live animal measurements. Analysis of variance of the heifer data (n = 72) indicated that farm was a significant source of variation (P ＜ 0.05) for average daily gain (ADG), age at feeding, age at slaughter, ultrasound fat depth (uFD), ultrasound rib eye area (uREA), ultrasound marbling score (uMS), grade rib eye area (gREA) and grade marbling score (gMS) and reflected different genetics and live weight endpoints between farms. Grade was unrelated (P ＞ 0.05) to ADG, feed conversion, and residual feed intake, although Farm C heifers that cut dark had the fewest days on feed (P ＜ 0.05). Heifers that cut dark had uFD, uMS, and grade subcutaneous fat depth (gFD) similar to heifers that produced Canada A and AA carcasses, and gMS similar to Canada AA carcasses. Binomial and multinomial logistic regression of the data sub-set indicated that heifers were most likely to cut dark at uFD between 2.5 and 7.5 mm but at uFD greater than 7.5 mm a heifer carcass was most likely to grade Canada AAA (＞45%). Results indicated that feeding to Canada AAA specifications may reduce the incidence of dark cutting in heifers.