As long as cows have grain to select in a cornstalk field, the diet is probably above 8-9.5% crude protein (CP) and as high as 70% total digestible nutrients (TDN). This exceeds the protein and energy needs of a 1,200-lb. cow in mid-gestation, says Rick Rasby, University of Nebraska animal science professor.
The average TDN content of a cornstalk field with about 1 bu. of ear-drop/acre is about 66% TDN with a protein content likely between 7-8% the first 25 days the residue is grazed. As days of crop residue grazing increase, energy content decreases.
Between days 25 and 45 for cows grazing cornstalk residue, TDN of the diet will average 54%, and CP about 5%. For a 1,200-lb. cow in mid-gestation and average body condition, the energy is adequate, but CP is about 0.38 lb. deficient.
This protein deficiency can be made up by feeding 2.5 lbs. of alfalfa (18% CP, 58% TDN, 88% dry matter)/head/day; or 1.5 lbs. of a 32% range cube (90% dry matter)/head/day.
As grazing days on the same residue field increase, nutrient quality decreases. This is also the same time spring-calving cows move closer to calving and nutrient needs increase.
At recommended stocking rates, after 50-60 days of grazing in the same cornstalk field, it's best to move to a fresh field. Nutrient quality of a stalk field between 50-60 days of continuous grazing will average about 48% TDN and 4.9% CP. This leaves cows in late gestation about 0.67 lb. deficient of protein and 1.9 lbs. deficient in energy daily. All calculations are based on a stocking rate determined using grain yield and that 50% of the remaining residue is available to be grazed.
Rick Rasby, University of Nebraska animal science professor