Two experiments were conducted to measure efficiency of energy use in limit-fed cows. In Exp. 1, 32 pregnant, crossbred cows were used to examine the effects of dietary energy concentration and intake level on energy utilization and digestion. In a 2 × 2 factorial treatment arrangement, cows received diets formulated at either 1.54 Mcal NE_m/kg high energy (H) or 1.08 Mcal NE_m/kg low energy (L); amounts of each diet were fed at amounts to achieve either 80% (80) or 120% (120) of maintenance energy requirements. Fecal grab samples were collected on days 14, 28, 42, and 56 for determination of energy digestion and metabolizable energy (ME) intake. Acid detergent insoluble ash and bomb calorimetry were used to estimate fecal energy production. Cow body weight and 12th rib fat thickness were used to estimate body energy, using 8 different methods, at the beginning and end of a 56-d feeding period. Energy retention (RE) was calculated as the difference in body energy on days 0 and 56. Heat energy (HE) was calculated as the difference in ME intake and RE. Energy digestion increased (P = 0.04) with intake restriction. Cows consuming H tended to have greater (P = 0.08) empty body weight (EBW) gain than cows consuming L, but no difference was observed (P = 0.12) between cows fed 120 compared with cows fed 80. Estimates of HE were greater for L than H (P < 0.01) and greater for 120 than 80 (P < 0.01), such that estimated fasting heat production of H (57.2 kcal/kg EBW^0.75) was lower than that of L (73.3 kcal/kg EBW^0.75). In Exp. 2, 16 ruminally cannulated, crossbred steers were used to examine the effects of dietary energy concentration and intake level on energy digestion. Treatment arrangement and laboratory methods were replicated from Exp. 1. Following a 14-d adaptation period, fecal samples were collected, such that samples were represented in 2-h intervals post-feeding across 24 h. Diet × intake interactions were observed for nutrient digestibility. Energy digestibility was greater in steers fed H than in steers fed L (P < 0.01); however, digestibility of each nutrient increased by approximately 10% in steers fed H80 vs. those fed H120 (P ≤ 0.03); nutrient digestibility was similar among levels of intake in steers fed L (P = 0.54). These results suggest that intake restriction may increase diet utilization and that the magnitude of change may be related to diet energy density.

Lauren N Bierschwale

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