An upper limit for caloric density of finishing diets.

This review assessed the relationships between dietary energy density and animal performance in an effort to evaluate a possible upper limit for energy density in finishing diets for cattle. Data were combined from 49 experiments (69 trials; 243 treatment observations) in which the dietary ME concentration (Mcal/kg of DM) was varied by level of concentrate, grain source, grain processing, and level of supplemental fat. Dietary concentrations of ME were determined using 1) NRC values of ME from diet ingredients; or 2) values derived from the literature, in which ingredient ME had been calculated from animal performance. Procedures for pooling data from multiple studies were used. The dependent variable was fit to a model that included a random slope and intercept clustered by trial. Trial-adjusted dependent variables (animal performance and carcass characteristics) were regressed on the independent variable (dietary ME concentration). Models were fit to cubic equations, and then reduced from cubic to quadratic to linear equations when the cubic and quadratic terms were not significant at P > 0.10. When NRC values were used, the relationship of DMI (% of BW) to dietary ME was linear (DMI decreased as ME increased; R2 = 0.631). However, the slope of ME intake (Mcal/kg of BW(0.75)) vs. dietary ME content did not differ (P > 0.25) from zero, supporting the concept that ruminants on high-grain diets (2.7 to 3.3 Mcal of ME/kg of DM) eat to maintain constant energy intake. Quadratic relationships were observed (P < 0.05) when ADG and G:F vs. dietary ME concentration were analyzed. Gain:feed was maximized with 3.46 (NRC) to 3.65 (calculated) Mcal/kg of ME from the total diet, 2.99 (NRC) to 3.40 (calculated) Mcal/kg of ME from grain, and 0.43 (NRC) to 0.53 (calculated) Mcal/kg of ME from supplemental fat. Most relationships of carcass traits to dietary ME were not significant (P > 0.10). Increased 12th-rib fat at greater ME and increasing KPH suggests greater fat deposition with increasing caloric density. Assuming that NRC ME values for ingredients commonly used in finishing diets are correct, the upper caloric limit for maximizing ADG and G:F was 3.16 and 3.45 Mcal/kg of DM, respectively. Reaching the upper caloric limit for G:F would require most grains to be processed or fed in high-moisture form. Whether maximizing G:F results in the most desirable carcass composition and yield of retail cuts should be determined.