Duration of grazing
To ensure adequate residue remains on the field after grazing, we can use animal weight and grain yield to determine the amount of grazing available. Cattle will readily remove approximately 15% of the residue (leaves and husk) but can be forced to remove more if desired. The goal should be to leave at least ½ of the total amount of residue on the field.
If an irrigated corn yield is 180 bu/acre, a rule of thumb is to divide by 3.5 to get grazing days for a 1200-pound cow. In this case, 180 bu/acre corn residue should provide approximately 51 days of grazing (180/3.5 = 51) for a 1200 lb cow. The harvest index (grain production/total biomass) is similar for corn and grain sorghum (1.6%). So, an 85 bu/acre dryland sorghum divided by 3.5 would provide approximately 24 days of grazing (85/3.5 = 24). A lactating cow or a heavier cow will consume more dry matter, and the grazing days need to be adjusted downward. A spreadsheet is available to calculate the stocking rate based on animal body weight and grain yield.
Selective grazing
Cattle will selectively graze the crop residue, eating the highest quality portions first, grain, then leaves and husks. Depending on the stocking rate, amount of grain available, and nutrient demands of the cows, no energy or protein supplementation may be needed early in the grazing period for dry cows with a body condition score of 5 or more and grazing as described above. Weathering and trampling will decrease quality over time, and this loss is greater with moisture and high humidity.
Soil compaction considerations
Cattle will cause soil compaction in paths leading to and around a water source. These compacted areas will only be surface compaction in the top two inches of soil. These compacted areas can be remedied by shallow tillage below the compaction layer. Results on soil compaction from grazing have shown mixed results. A study near Bushland, TX, found surface compaction in a no-till system reduced crop yield after several years of grazing. While grazing studies from Nebraska found no increase in compaction and increased crop yield. Studies from western Kansas found compaction to only occur in the top two inches when grazing occurred on wet soils and shallow tillage removed any compaction. Compaction will be less on frozen, dry, sandy soils. It is best to remove cattle from the field to a nearby perennial pasture if the field is wet and not frozen. Also, the producer should be open to using shallow tillage should compaction occur.
Nutrient removal from grazing
Another common concern about grazing residue is nutrient removal. Nutrient removal will vary by the type of animal, with a growing calf requiring more nitrogen than a mature dry cow. Drycows will typically be used to graze residue, which will remove between 1 and 2 lbs of N per acre (depending on crop yield) and few other nutrients. Crop residue is low in phosphorus (P); thus, producers will likely supply a free-choice mineral, resulting in an increase in the amount of P and calcium left in the field. Wind will blow leaves and husks off fields, but manure remains in place. Therefore, with proper grazing management, grazing can benefit livestock, soils, and economic profit.
Nutrients in Residues (continued from previous page)