Seedbed: What are those partial stands of red clover worth?

Monday, May 4, 2009

Here's a quick way of determining the best single rate of nitrogen to apply to your field so as to give the greatest net return

by KEITH REID

Every year, we get questions about how much nitrogen credit to give for an uneven stand of red clover. It would be easy if half the field had a good stand, and half was poor, so that we could recommend giving full credit to the good half and no credit to the poor half.  Unfortunately, most fields are so patchy that applying N separately to the good and the poor areas is not an option.

The real question, then, comes down to picking a single rate for the entire fieldwhich will avoid spending too much on the areas where the clover is able to provide lots of N, and avoid losing too much yield on the parts of the field that would be short of N if the fertilizer rate were reduced.

To try and answer this question, I went to the results of N response trials on a typical field that did not have any clover, and tried to predict what the impact would be on optimum rates for the whole field if there was clover on part of the field. 

I could calculate what the yield would be at different rates of N, and then assume that the areas with a full stand of red clover would behave as if the full N credit were simply added to the fertilizer applied, and recalculate the yield curve for the field with clover.

The next step is to calculate the value of the yield, and the net value of the crop after the cost of nitrogen is deducted. The maximum net value will occur at the Maximum Economic Rate of Nitrogen (MERN). In the example field I used, the MERN without any clover was 130 kilograms a hectare, and since the N credit for red clover I used was 80 kilograms a hectare, the MERN with clover was 50 kilograms a hectare.  

To look at the impact of variable stands of clover, I calculated the net return to the field where the same rate of nitrogen were applied to the whole field but there were different proportions of red clover. I could then determine the best single rate of N to apply to the whole field to give the greatest net returns. The results are shown in Figure 1.  

It turns out that the amount of credit you should apply to the field is exactly proportional to the percentage of the field with a good stand of red clover.  If there is 75 per cent of the stand, you would apply 75 per cent of the credit (or 60 kg/ha). If there is only 25 per cent of a stand, you should still reduce fertilizer rates by 25 per cent of the full credit (20 kg/ha).  

This relationship seems to hold across a wide range of corn and fertilizer prices.  It shows that, while we don't get as much benefit from a partial stand of clover as we would like, there is still value. BF

Keith Reid is soil fertility specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs based in Stratford.