Impact of the drought on 2013 fertilizer rates
To predict the impact of harvesting forage one needs to compare nutrient status of the plant when harvested for forage compared to what it would have been if it had made a full grain crop.
Table 3 compares the nutrient harvest index of the grain crop with nutrients accumulated as the percentage of total nutrient uptake at early reproductive growth stage (R1 for corn and R3/4 for soybean).
When the values are equal there is no difference between the amount of nutrient removed in the grain and the amount of nutrient removed in the harvested forage. For example, when you harvest corn at R1 you remove the same amount of nitrogen as if you had harvested the full crop as grain.
When the harvest index value is greater than early reproductive content than fewer nutrients are removed by forage harvest; there will be drought nutrient credit for the crop. For example, when you harvest whole corn plants at R1 your remove less phosphorus than you would have with the full grain crop.
When the harvest index value is less than early reproductive content than more nutrients are removed by forage harvest; harvesting the forage will remove more nutrients than the grain crop. For example, when you harvest whole corn plants at R1 your remove more potassium than you would have with the full grain crop.
In soybean the harvest index is always greater than the percentage of total nutrient uptake at R3/4 so there is always a potential drought nutrient credit if the crop is harvested as forage at this stage of growth.
A soybean field had phosphate and potash applied based on a 50 B/A yield goal. The field was harvested as green chop with limited pod set.
Drought phosphate credit: [1-(40/80)] X 50 B/A X 0.8 lbs phosphate/B = 20 lbs phosphate/A
Drought potash credit: [1-(37/58)] X 50 B/A X 1.25 lbs potash/B = 40 lbs potash/A
Note that less nitrogen was removed by the soybean plant than with grain harvest; but no nitrogen fertilizer was applied to the soybean crop. So it is not appropriate to take a drought credit for nitrogen. We do recommend taking the rotation nitrogen credit for soybean of 30 lbs N/A despite the drought if corn is grown following soybean.
Corn crops in particular have been harvested as forage at various points during the growing season. Some have harvested the crop soon after silking (near R1).
Others have waited until the crop dropped in moisture content to a point it could be ensiled or baled. There are others who are realizing that there is little grain in a mature field and are baling it as a source of roughage to be mixed with other higher quality feeds such as distiller’s grains.