What is the Value of Corn Residue to Grazing Cattle?

The United States produces over 370,840,000,000 kg of corn grain and concomitantly more than 303,410,000,000 kg DM of non-grain corn residues (i.e., leaves, husks and stalk) from grain production annually. Although there is an abundance of available corn residue, only 12% of land planted to corn is grazed after harvest (Schmer, 2017), and based on current estimates of nutrient composition (NASEM, 2016), while 30% of available corn residues could maintain the entire United States cow herd. Grazing cattle often select diets with greater nutrient density and digestibility in comparison to the overall biomass available; however, most estimates of nutrient density in corn residues are based on analyses of mechanically harvested residues (i.e., the overall biomass available for grazing). More accurate measurements of selection of botanical parts by grazing cattle and subsequent nutrient intake can allow for improved estimates of performance of cattle grazing corn residues and for development of management strategies that can optimize forage utilization. The first experiment used 6 ruminally cannulated cows to evaluate predictions of diet selection based on chemical components and post-sampling processing techniques in diet samples collected through ruminal evacuation. Predictions of diet composition were improved by increasing differences in concentration of chemical components between cornstalk and leaf and husk (LH) residues up to a coefficient of variation of 22.6 ± 5.4%. Acid detergent insoluble ash, acid detergent lignin, and near infrared reflectance spectroscopy provided the most accurate estimates of composition of the diet. A second experiment was conducted with 6 ruminally cannulated cows to estimate the caloric value and digestibility of corn husk, leaf and stalk. Cattle were fed to near to their maintenance energy requirements with either corn leaves, husk or stalk and corn steep liquor. Net energy available for maintenance value from corn leaves, husks and stalks were 1.80 Mcal/kg DM, 1.15 Mcal/kg DM and 0.83 Mcal/kg DM, respectively. Differences in energy available for maintenance were largely described by differences in methane emissions; cattle fed leaf residue had 116 and 66% less energy losses from methane than cattle fed husk and stalk, respectively. Clearly, selection of different botanical parts by cattle grazing corn residues can change nutrient and energy intake and diet digestibility. Estimates of nutrient composition of corn residues based on total harvestable biomass are unlikely to accurately reflect diets selected by grazing cows. Development of more accurate estimates of diet selection among grazing cattle and feed values of each botanical part in corn residue will allow more accurate estimates of cattle performance. Further, an improved understanding of the feed value of each botanical part in corn residue to cattle may allow for a better evaluation of different management strategies that influence intake of different botanical parts in corn residues by grazing cattle