PS11-8. Relationships Between Visceral Organ Mass and Feedlot Performance of Holstein Steers, SimAngus x Holstein Steers and Heifers, and SimAngus Steers.

Abstract Visceral organ mass contributes substantially to maintenance energy expenditure and, as a result, influences growth performance and feed efficiency in feedlot cattle. This study evaluated visceral organ weights expressed as a percentage of empty body weight (EBW) and the relationships between those percentages and performance traits. Feedlot performance was compared among Holstein (HO) steers (n = 42), SimmentalxAngusxHO (SMANxHO) heifers (n = 38) and steers (n = 39), and SMAN steers (n = 42). Cattle were randomly allocated by body weight (BW) to seven pens per cattle type and fed for 189 (SMANxHO heifers), 232 (SMANxHO steers), 272 (SMAN steers), and 335 d (HO steers). Body weight, average daily gain (ADG), and gain to feed (G:F) were determined every 28 d, and dry matter intake (DMI) was determined every 14 d. A subset of 28 cattle (7 per type) were scheduled for harvest at a similar empty body fat (32.5%) as predicted by ultrasound. Empty BW was calculated as shrunk BWx0.891. The HO and SMAN steers had the greatest EBW, followed by SMANxHO steers, and lastly heifers (P < 0.01). Gastrointestinal tract (GIT) and non-GIT organ weights will herein be expressed as a percentage of EBW. Esophagus, rumen, reticulum, cecum, and pancreas were similar among cattle types (P > 0.08). Across cattle types, GIT weight was negatively correlated with EBW (R=-0.71;P<0.01). The GIT weight was greatest for SMANxHO heifers, intermediate for HO and SMANxHO steers, and least for SMAN steers (P = 0.05). Within GIT components, SMAN steers had lesser omasum, abomasum, and small intestine weights compared with the other cattle types (P ≤ 0.05), while having similar abomasum weights to SMANxHO steers. Large intestine weight was greatest for SMANxHO heifers (P < 0.01), followed by SMANxHO steers, and lastly HO and SMAN steers. Non-GIT organ weights were greater for SMANxHO cattle than HO steers and SMAN steers (P < 0.01). Liver weight was greater for HO steers and SMANxHO cattle compared with SMAN steers (P < 0.02). Total GIT and non-GIT weights were greater for SMANxHO cattle compared with SMAN steers (P < 0.01). For HO steers, GIT and liver weights were negatively correlated with ADG (R=-0.66) and DMI (R=-1.00;P<0.05), which may reflect increased metabolic maintenance costs associated with greater GIT and non-GIT organs. The GIT weights for SMANxHO cattle were positively correlated with G:F (R = 1.00;P<0.05), suggesting that greater digestive capacity may support improved nutrient utilization in beef x dairy cattle. For SMAN steers, GIT (R = 0.94) and liver (R = 1.00) weights were positively correlated with ADG (P < 0.05). The SMANxHO cattle exhibited greater relative GIT and non-GIT organ weights but also demonstrated superior G:F. Collectively, these results indicate differences among beef, beef x dairy, and dairy cattle in GIT and non-GIT organs contribute to variation in feedlot growth and feed efficiency. For image description, please refer to the figure legend and surrounding text.