Energy utilization and organ mass of Targhee sheep selected for rate and efficiency of gain and receiving high and low planes of nutrition.

Two experiments were conducted to examine changes in usage of energy and visceral organ mass in Targhee lambs selected on the basis of improved rate and efficiency of growth. In Exp. 1, Targhee ram lambs from a breeding line selected for improved growth rate and feed efficiency and a control line that was maintained without selection for 20 yr were provided with either a high (ad libitum) or low (maintenance) plane of nutrition. Rams were slaughtered and weights of the visceral organs were recorded. Analysis of variance, using a factorial model with BW as a covariant, was used to examine effects of lines of breeding, plane of nutrition, interaction between line of breeding and plane of nutrition, and days on feed. Weights of liver, kidneys, rumen, abomasum, and small and large intestines from lambs receiving a high plane of nutrition were, respectively, 39, 25, 12, 28, 40, and 31% greater than weights of those tissues in lambs receiving a low plane of nutrition. Ruminal weights were 13% greater for rams from the select line of breeding than for those from the control line. In Exp. 2, seven Targhee ewe lambs from the select line and eight from the control line were examined for differences in heat production and energy usage by indirect open circuit respiration calorimetry using a completely randomized design. Fasting heat production of lambs from the select line was 7.8% greater than that of lambs from the control line. Partial efficiencies of ME used for maintenance and tissue accretion were not different between lines of breeding.(ABSTRACT TRUNCATED AT 250 WORDS)