Post-harvest residue in corn and grain sorghum fields can be used to provide a substantial number of days of grazing. Crop residues are an inexpensive feed source, because most costs are charged against the row crop enterprise. In a five-year study at Iowa State University, 113 grazing days were obtained when cornfields were grazed after corn harvest with a stocking rate of 1.9 aces per cow. When grassed waterways, terraces and field borders are present and are properly managed and used, this option becomes even more attractive. Iowa State University data indicates for each acre of corn stalks grazed, approximately one-half ton of hay will be saved.
Crop residues usually represent about half of the pre-harvest plant dry matter. For example, a field producing 120 bushels of corn grain (about 7,200 pounds) will contain three to four tons of roughage dry matter per acre. Depending on stocking rate and grazing method, cows grazing corn stalks or grain sorghum stubble will consume 25 to 30 percent of the available residue in 30 to 100 days, still leaving enough material to prevent soil erosion.
The optimal grazing allowance on corn crop residue fields will depend on the weight gains necessary to obtain a desired body condition. With low supplementation, cows can maintain body weight with as little as one-half acre of corn crop residues per cow per month, but may need as much as two acres per cow per month if weight gain is desired.
Livestock first select the portions of crop residues with the highest digestibility and protein concentration, so supplement needs beyond trace mineral salt and vitamin A are likely to be minimal for the first month of grazing. Providing simultaneous access to stockpiled grass or late summer pasture growth may supply protein and energy, and thereby reduce needs for supplementation. As winter progresses and crop residue quality decreases because of grazing selection and weathering, supplementation of protein and phosphorus may become necessary.
As with stockpiled forage, strip grazing crop residues allows more efficient use, resulting in more grazing days and helps ensure a high-quality diet over a longer period of time by reducing selective grazing. A caution associated with grazing corn crop residue: livestock may overload on excessive amounts of grain left in the field, putting them at risk of founder (or acidosis), a serious digestive problem. Strip grazing reduces the likelihood of this disorder.
Before grazing crop residue fields it is important to check the labels of any pesticides used on the crop to see if they are cleared for grazing crop residues. It is also advisable to make certain no poisonous plants are present in fence rows or other areas adjacent to fields to be grazed. Forage produced in fence rows and waterways within row crop fields is of most value if mowed, fertilized and managed as stockpiled forage. Research conducted at several universities has shown no difference in the performance of cattle grazing Bt corn crop residue and those grazing non-Bt corn crop residues.
Grazing of crop residues is not an option for every livestock producer and, like any other practice, requires some management. However, producers who have access to summer row crop residues can use this technique to great economic advantage by using it to reduce supplemental feed costs.
Don Ball is an Extension Forage Crop Agronomist with Auburn University.