Relationship of Animal Performance and Dry Matter Intake to Chemical Constituents of Grazed Forage2

Abstract

Midland bermudagrass (Cynodon dactylon) was fertilized with three levels of N (67, 202, 326 kg N/ha) in tests to determine the effects of N fertilization level upon animal performance and forage intake. Sixty Angus × Hereford cows were mated to Angus bulls and randomly assigned to 12 pastures on the basis of calving date. Dry matter (DM) intake and animal weight changes were determined. Additional cows and calves were used as “put and take” animals to regulate the forage quantity available. Forage samples, from which forage yield and chemical composition were determined, were collected monthly under cages (CC). Twelve esophageal-fistulated cow-calf pairs were used to sample the forage being selected for quality analyses. Hand-clipped samples were collected during the same periods of CC sampling to allow a comparison of animal-selected vs available forage. Chemical composition of all samples was determined. Increasing levels of N fertilization had no effect (P>.05) on quantity of forage consumed by cows or calves. Overall mean dry matter (DM) intake values were 12.3 g/Wtkg.75 for cows in May and 97.7 g/Wtkg.75 in July and 42.3 g/Wtkg.75 for calves in July. Forage intake was positively correlated with in vitro dry matter digestibility (IVDMD) and negatively correlated with lignin content (P<.05). Forage intake was regressed on composition crude protein (CP), acid detergent fiber (ADF), neutral detergent fiber (NDF), cellulose, lignin, and residual ash), IVDMD, digestible energy (DE), and gross energy (GE) of esophageal samples. Coefficients of determination (r2) ranged from .003 to .444 for single variables and from .19 to .87 for various combinations of variables. These results suggest that a single prediction equation for intake of bermudagrass of differing qualities is impractical. A number of equations should more accurately predict intake as the quality of forage changes. Increasing levels of N fertilizer increased (P<.05) the CP of forage selected by calves but otherwise had no effect. Calves tended to select forage with higher levels of CP and lower levels of ADF and cellulose than did cows in both trials.