Nitrogen Status of Winter Pasture
Nitrogen status of winter pasture is always difficult to assess. By taking soil samples during the fallow period, soil nitrate nitrogen levels can be appraised with a great degree of confidence. Due to the instability of soil nitrogen, even the residual levels identified by soil test often change before plants utilize the applied or mineralized nitrogen. Rainfall, volume of plant residue, and soil temperature influence microbial activity that fix, denitrify, or mineralize soil nitrogen. Soil nitrogen status is continually changing.
Small grain forage protein levels during the fall phase will be in the 20% range, requiring about 80 pounds of actual nitrogen to produce 1 ton of forage. Data accumulated from fertility and variety tests indicate that rye and some wheats have the potential to produce 1.5 tons of forage by March 1.
To supply adequate nitrogen for those yield levels, there must be 120 pounds per acre of actual nitrogen applied or mineralized during the fall production period. Where there have been high nitrogen applications in a grazing system for several years, there should be increased nitrogen reserves in the nitrogen pool. The mineralized nitrogen from the pool can be estimated most accurately with the preplant nitrate-nitrogen soil test.
If there is adequate rainfall to leach nitrogen in porous soils, or if there are flooded conditions over an extended period of time in heavy soils, the nitrogen will be leached or denitrified resulting in a loss of mineralized nitrate-nitrogen. When the loss occurs, additional nitrogen must be applied or the yield potential of the crop will be limited to the nitrogen available.
The nitrogen rate can be estimated by determining the amount of growth that has already been utilized, subtract that from the yield goal, then apply 80 pounds of actual nitrogen per ton of difference. On the other hand, if there is little rainfall resulting in low yields and nitrogen use during the fall phase, a portion of the nitrogen applied in the fall will be carried over into the spring. It is possible to test the soil in this situation and verify the levels that exist before the spring topdressing rate is decided.
When soil samples are taken to assess the nitrate nitrogen levels, they should be taken in 6 inch increments. The increments should be taken to 18 inches deep since small grain plants will develop roots to this depth during their growth cycle. The levels at each zone should be added together for the assessment of total nitrogen available for plant growth. The total should be subtracted from the nitrogen required for the yield goal to determine a nitrogen requirement.
Usually to produce 3 tons of small grain forage, about 200 pounds of actual nitrogen is required. To achieve a yield goal, nitrogen has to be accounted for throughout the growth period. Nitrogen lost must be replaced or the yield potential will be reduced proportionately. If the yield potential is decreased by stand or shortage of rainfall during the fall, the spring topdressing should be reduced accordingly. This is a thought process that I use when assessing the nitrogen status and yield potential of small grain winter pasture. Good Luck!