Comparing energy intake estimates derived from using respiratory gas flux measurements with backward dietary energy partitioning for beef cattle

The objectives of this experiment were to refine the use of open-circuit gas-quantification methodology to estimate energy intake by ruminants and compare 4 different approaches to account for urinary N (UN) excretion. Eight cross-bred beef steers with a beginning body weight (BW) of 244 ± 15.1 kg were used in a 77-day feeding experiment. Steers were assigned to 1 of 3 multiples of the calculated metabolizable energy (ME) requirement at maintenance (10.7 megacalories of ME), which are 1.1 x maintenance, 1.4 x maintenance and 1.8 x maintenance. Diets were long-stemmed wheat (Triticum aestivum L.) hay and supplemental alfalfa (Medicago sativa) pellets. Respiratory gas fluxes were measured using an automated head chamber system (AHCS; GreenFeed, C-Lock, Inc., Rapid City, SD, USA). Urinary N excretion was accounted for with 1 of 4 approaches and included estimating UN excretion using prediction models based on either 1) diet quality and dry matter intake, 2) BW, 3) assuming a constant value centered on the average of the 2 model estimates (60 g/day), and 4) assuming UN excretion to be 0. Pearson correlations between estimated gross energy (GE) (r = 0.95), digestible energy (DE) (r = 0.88), and ME intake rates and measured values were highly positive (r = 0.85) regardless of the approach used to estimate UN excretion. Approaches to estimate UN excretion differed by 0.81 % at the most when calculating ME intake and concordance of correlations coefficients showed good or better agreement between the estimated and measured GE, DE, and ME intakes (rc ≥ 0.45). This experiment confirmed that short-term breath measurements from an AHCS, together with BW change, can be used to estimate GE, DE, and ME intake by cattle.