Effects of supplemental rumen-protected conjugated linoleic acid or linoleic acid on feedlot performance, carcass quality, and leptin concentrations in beef cattle.

Thirty-six Angus x Hereford heifers (365 kg) were used to determine effects of dietary lipid supplementation from two sources during the final 32 or 60 d of feeding on serum and adipose tissue leptin concentrations, animal performance, and carcass characteristics. Following an initial feeding period of 56 d, heifers were fed one of three diets in a 3 x 2 factorial arrangement: 1) basal diet, 2) basal diet plus 4% (DM basis) corn oil, or 3) basal diet plus 2% (DM basis) rumen-protected conjugated linoleic acid (a mixture of Ca-salts of palm oil fatty acids with 31% conjugated linoleic acid). Jugular blood samples were collected at 28-d intervals (d 28 to 118) and serum subsequently harvested for leptin quantification via RIA. Real-time ultrasound measurements were collected at 28-d intervals across time on feed. At slaughter, samples were obtained from various adipose depots. Data were analyzed with dietary treatment, length of supplementation, adipose depot (when appropriate), and all two- and three-way (when appropriate) interactions in the repeated measures model. Measures of feedlot performance, including ADG, DMI, and gain:feed did not differ (P > 0.23) with dietary treatment or supplementation length. Heifers supplemented with corn oil tended (P < 0.07) to have higher marbling scores following 32 d of treatment than those supplemented with rumen-protected conjugated linoleic acid, with controls intermediate. Quality grade and hot carcass weight did not differ (P > 0.15) with treatment or length of supplementation. Leptin concentrations were higher (P < 0.05) from d 57 to 118 on feed than the initial period (d 0 to 56) of dietary adaptation when all animals received the basal diet. Circulating leptin concentrations were not affected by dietary treatment. However, leptin concentrations in adipose tissues were greater (P < 0.05) for heifers supplemented with corn oil than either control or rumen-protected conjugated linoleic acid diets, which did not differ. Compared with adipose tissues from rumen-protected conjugated linoleic acid-supplemented animals, tissues from heifers fed corn oil contained 68% greater leptin concentration. Correlations between performance, carcass traits, and serum leptin concentrations were low. Serum leptin concentrations across time on feed were not associated with carcass and performance data, including ADG, DMI, and gain:feed. Based on these data, concentrations of leptin are not related to indices of feedlot performance and carcass quality in beef cattle.