Effects of dietary energy and animal frame size on feed intake, body composition and plasma concentrations of insulin and leptin in beef steers

A digestibility trial was conducted with cattle to determine dry matter and nutrient digestibility of the dietary treatments. Steers were placed in metabolism crates for total fecal collection in a 3 x 3 Latin Rectangle design repeated twice. A feedlot trial was conducted to evaluate the effects of the animal frame size and dietary energy concentration on dry matter intake, performance, body composition, and plasma concentrations of insulin and leptin. In addition, the study compared individual observed and predicted dry matter intake by equations of Perry and Fox (1997). Thirty-six beef steers were divided into two frame sizes, large and small, and fed individually with three dietary treatments. The experimental design was a 3 x 2 factorial design. Dietary treatments were 2.4, 2.7, or 3.0 Mcal of ME/kg of DM. The steers were weighed and ultrasound images were taken each 28 days. Daily DMI, ADG, feed efficiency, fat thickness, and longissimus muscle area were measured. Blood samples were also taken to assay for plasma concentration of leptin and insulin. Diets containing higher energy density had greater dry matter and organic matter digestibility than diets with lower energy density. Frame size did not affect significantly (P > .05) DMI, ADG, and feed efficiency. Energy density of diet affected significantly (P < .05) DMI, and steers fed the lower energy density diet had higher intake than those fed the higher energy density diet. The results on performance indicated that cattle fed lower energy density diet consume more feed, have similar gain, and are less efficient than cattle fed higher energy density diet. Fat thickness and longissimus muscle area were greater for steers fed higher energy diets than those fed lower energy diets. Plasma leptin concentration was greater for small frame steers fed the highest energy density diet than steers fed the other treatments. The results indicated that fat thickness and plasma leptin and insulin concentrations are positively and significantly (P < .0001) correlated, and that hormone levels respond to an increase on degree of fatness by regulating feed intake. Perry and Fox (1997) equations were adequate for predicting individual feed intake of steers.