357 Late-Breaking: An evaluation of computer-based 3-dimensional images and weight based calculations of maintenance energy requirements to measures of fasting heat production in growing beef cattle.

Abstract Maintenance energy requirements are important to feed intake, growth and feed efficiency in cattle. Current estimates of maintenance energy requirements in cattle are generally expressed as a constant or metabolic rate multiplied by a ‘metabolic body size’ which is derived from measures of body mass (BW). This approach does not allow for determination of individual differences in efficiency between cattle with the same BW, and individual differences in shape and body composition (i.e., density) create inherent errors in estimates of maintenance energy requirements from metabolic body size. The use of computer imaging (CI) and artificial intelligence can facilitate rapid and accurate non-destructive measures of surface area and volume of cattle to determine maintenance energy requirements instead of metabolic body size. Twelve crossbred steers were used to evaluate maintenance energy requirements via fasting heat production (FHP) at 5 different BW (247 ± 19, 302 ± 31, 396 ± 45, 429 ± 44, and 540 ± 50 kg shrunk BW) to estimates of maintenance energy from measures of surface area (SA) collected via CI or metabolic body size. FHP was measured using indirect calorimetry after 5 and 6 d of fasting, respectively. Measures of shrunk BW and SA were collected within 24 h of completing indirect calorimetry. At slaughter, hides were collected to compare SA from CI to measures from hides. Measures of SA from CI (r2 = 0.97) and metabolic body size (r2 = 0.98) were closely related to FHP. However, estimates of metabolic body size yielded estimates of maintenance energy requirements that were 22.4% greater (P < 0.01) than FHP. Correspondingly, measures of FHP per SA of body were less (1,106 ± 23.8 kcal/m2) compared to maintenance energy estimates from metabolic body size per SA (1,354 ± 18.2 kcal/m2). Bias in maintenance estimates from metabolic body size could be attributed to the exponent used to calculate metabolic body size. An assumed metabolic rate of 77 kcals resulted in a calculated exponent of 0.716 ± 0.022, which was less than the 0.75 exponent commonly used to calculate metabolic body size. Limits in the metabolic body size concept were also clearly observed because the calculated exponent increased with increased BW (linear, P < 0.01). At slaughter, measures of SA from CI were closely related to measures from hides when regressed through the origin (r2 = 0.99), but measures of SA were 14.5% greater than measures from hides, which could indicate a more complete measure from CI. These data indicate that CI can be used to estimate maintenance energy requirements of beef cattle and that current weight-based models overestimate maintenance energy requirements when compared to FHP.