Cow grazing cornstalks

Editor’s note: The following was written by Mary Drewnoski, University of Nebraska beef systems specialist, and Daren Redfearn, Extension forage crop residue specialist, for the university’s Crop Watch website Nov. 5.

A pregnant cow grazing at suggested stocking rates for 90 days on a 230 bushel per acre corn field will remove:

  • About half a ton of residue.
  • About 2 lbs. of nitrogen (N) per acre.
  • No phosphorus (P) or calcium (Ca). The amount of P available in residue is not sufficient to meet nutritional needs. Therefore, cattle producers usually provide a free-choice mineral supplement that contains both P and Ca. There would actually be about 1/2 lb./acre of both P and Ca added in this situation.
  • Essentially no potassium (K).

Let’s look at a scenario for a field with a grain yield of 230 bu./acre. In this situation, cows can graze for 90 days when stocked at 1.3 acres per cow. In lower-yielding fields, fewer cows or fewer days per acre should be applied because there is less residue for grazing. Thus, the nutrient removal estimates on a per acre basis would be lower (i.e., high yield fields can be stocked heavier and have greater potential removal than lower yielding fields).

Suggested stocking rates

Corn residue is about 10 percent husk and 34 percent leaf, with the remaining residue being stalk and cob. Recommended stocking rates are based on the ability of a pregnant cow to maintain body weight without supplementation of protein or energy. The rates suggest that you can graze a 1,200 lb. cow for 30 days for every 100 bu. of corn grain produced.

This would result in the cow consuming only about 12 percent to 15 percent of the corn residue in the field, and nearly all would be husk with some leaf. Cob and stalk have less energy available. Forcing the cow to consume cobs and stalks will actually result in a loss of body weight.

Given the proportions available, a cow’s diet would be 80 percent husk and 20 percent leaf.

A cow eats about 2 percent of its body weight per day (DM basis) when grazing corn residue. Thus, a 1,200 lb cow would consume about 24 lbs. of DM. Over a 90-day period, she would consume 2,158 lbs. of DM. This is about 1,001 lbs. of organic husk matter removed.

For leaf matter, a total of 1,148 lbs. of OM was removed per cow over the 90-day period.

What is the organic matter removal per unit area of land? Remember that the target stocking rate was one cow per 100 bu. of grain, so a cow could graze on a corn field that yielded 230 bu./acre for 69 days if stocked at one cow per acre.

To get to 90 days, the stocking rate would need to be 1.3 acres per cow. Thus, the 1,148 lbs. of OM removed needs to be divided by 1.3. This would result in 886 lbs. of OM/acre disappearance.

In this example, no grain intake was assumed. Grain is more digestible, but typically less than 1 bu./acre of grain remains after harvest, which would be no more than 4 percent of the total intake. At a glance, that's 886 lbs. The OM/acre seems excessive; however, in this case, only about 14 percent of the total OM was removed.

Not all of the OM from residue remaining on the soil surface will become soil organic matter. Some research suggests that in no-till systems, about 10 percent of crop residue OM will become soil stable OM.

Mineral removal (P, Ca, K and N)

The mineral (N, P, K, etc.) requirements of a beef cow do not equal the amount that she retains in her body. A non-pregnant, mature cow is at maintenance (not gaining or losing weight) and would only require the minerals that are lost through excretion in the urine and manure.

However, it is typical to graze pregnant cows, which do retain some minerals to support calf growth. Using fetal mineral concentrations, the mineral removal rates can be predicted.

It’s estimated that during the third trimester of pregnancy, a cow will retain 208 grams P, 125 g. Ca, and 38 g. K over a 90-day period to support development. Using the above stocking rate assumption, this retention is equivalent to 0.35 lb. P/acre, 0.21 lb. Ca/acre, and 0.06 lb. K/acre.

However, we also need to consider that a typical production practice is to supplement a mineral with both P and Ca to cows grazing corn residue because there is not enough P in the residue to meet requirements (allowing her to recover what she excretes and retain what she needs for the pregnancy).

Usually minerals containing P have an equal amount of Ca to make sure that the ratio in the diet remains in balance. The net result is actually more P and Ca being added than removed.

The amount of N removed is more difficult to estimate. Not all N in plant residues will become plant available and there are many potential avenues for loss of N in the system, including volatilization, denitrification, leaching and runoff.

The amount of N retained by the pregnant cow would be estimated at 292 grams over the 90-day period. In this situation, N loss from volatilization of N that is consumed and then excreted in urine and manure would be about 10 percent and considered one of the major potential sources of loss. She would excrete 106 g. of N per day, resulting in an additional loss of 2.1 lbs. N to volatilization for a total loss of 2.7 lbs. of N.

It should also be noted that all nutrients will be redistributed across the landscape unequally. It is likely that more nutrients will be excreted in areas where cattle spend more time, such as around the water source and in sheltered areas.