Glycogen metabolites and meat quality in feed-restricted re-fed beef heifers

Comparing the responses of grain fed feedlot cattle under moderate heat load and during subsequent recovery with those of feed restricted thermoneutral counterparts: plasma biochemistry

Responses to heat stress in ruminants reflect the integration of local climatic conditions, environment/production system and the animal's homeostatic and homeorhetic capacities. Thus, the goal of ameliorating heat stress requires experimental settings that, within limits, closely resemble the target production system and cohort. We investigated the blood biochemical changes of two sequential cohorts of twelve 518 ± 23 kg grain fed Black Angus steers. Each cohort consisted of two treatments of 6 head/group: a thermally challenged (TC) treatment and a feed restricted thermoneutral (FRTN) treatment. Both groups were housed in climate controlled rooms for 19 days, with the TC group experiencing three distinct periods: PreChallenge, Challenge and Recovery. PreChallenge and Recovery delivered thermoneutral conditions, while Challenge consisted of 7 days of moderate diurnal heat load. The FRTN group was maintained in thermoneutral conditions at all times. Both groups were then relocated to outdoor pens for a further 40 days to detect any enduring change to metabolism as a consequence of the treatments. We compared blood biochemical responses of the treatments and inferred likely metabolic changes. Relative to the FRTN group, the TC animals experienced limited supply of triglycerides, cholesterol and glutamine during moderate heat load, suggesting constraints to energy metabolism. Lower blood urea during Recovery and in outdoor pens implied a requirement to capture N rather than allow its excretion. Altered liver enzyme profiles indicated a higher level of hepatic stress in the TC group. By the completion of feedlot finishing, the groups were not separable on most measures.

Can potential for dark cutting be predicted by phenotype? Relationship between sex, carcass characteristics, and the incidence of dark cutting beef

Recent research indicated that reduced carcass muscling and weight are associated with beef dark cutting. Existing data from a single farm (44 heifers, 136 steers) from three normal beef quality grades (Canada AAA, AA, A) and the dark cutting grade (Canada B4) (n = 35, 106, 28, and 11, respectively) were used to identify relationships between sex, live animal, and carcass characteristics and the incidence of dark cutting. Categorical modelling showed a trend (P = 0.106) for heifer carcasses to have a greater probability [11.36 ± 4.78% standard error of the mean (SEM)] of dark cutting than carcasses from steers (4.4 ± 1.76% SEM). Mixed model analysis of variance indicated dark cutting heifers weighed less than cattle from normal grades at weaning (P = 0.0229) and slaughter (P = 0.0295). Logistic regression revealed that the probability of each carcass grade occurring was influenced by dry matter intake (DMI) (P = 0.0034), and the probability of dark cutting was lowest (P = 0.0286) in cattle with carcasses greater than 300 kg. Results suggested that cattle at risk of dark cutting may be identified by weight, average daily gain (ADG), and feed intake.