Corn stalk residue would be considered about the same quality of an average quality hay. Due to the improvement in corn genetics and disease resistance, there is not a lot of corn left in the fields after harvest. In addition, the combines seem to be more efficient at getting the corn out of the fields. I would use the following information to determine the number of days that an acre of stalks can carry a cow:
An Animal Unit Month (AUM) is defined as the amount of forage to sustain a 1000 pound cow for a month or 26 lb of forage dry matter per day. So a 1,200 pound cow is 1.2 AUMs or 31 lb of forage dry matter per day. Therefore one AUM is 780 (26 lb/da x 30 days = 780 lb) pounds of forage dry matter basis per month and for a 1,200 cow that would be 936 pounds of forage dry matter per month. Cows are selective grazers, meaning that they will select the corn first, followed by the husks, and finally the cob and stalk. The cob and stalk don't have a lot of nutrient value.
Residue, or leaf and husk, yield is related to grain yield, but hybrids obviously vary in this relationship. With high producing corn (irrigated or with ample rainfall), there will be about 16 pounds of dry leaf and husk per bushel corn yield. Some residue disappears by trampling and other factors (2004 Nebraska Beef Cattle Report, page 13). Estimate 50 percent utilization of the leaf and husk in the stalk field. Therefore, 150 bushels of corn produces 2,400 pounds (16 pounds of dry leaf and husk per bushel of corn, multiplied by 150 bushels an acre, equals 2,400 pounds of husk and leaf per acre) leaf and husk and you can assume 1,200 is going to be consumed by the cow. This is equivalent to about 1.54 AUM (1,200 pounds of husk and leaf divided by 780 pounds of forage dry matter per AUM). One acre would carry a 1,200 lb cow for 38 days or a 600 pound calf for 76 days. Higher grain yields provide more AUM and lower yields less.
