Agronomic Decisions

(Joe Lauer) I’m Joe Lauer, and I’m with the University of Wisconsin. I’m a professor in corn agronomy for the state of Wisconsin. And we do research on agronomic decision making for corn farmers. This project that I’m going to talk about today basically looking at the response of corn to variable rate. One of the real changes that have been going on with corn has been an increase in plant population. Since the 1980s we’ve moved in Wisconsin and many of the Corn Belt states from a typical average of around 20,000 to 22,000 plants per acre to 30.000 to 32,000 plants per acre. And this has really been occurring over the last generation of farmers, the last 20 to 30 years. Along with that, so farmers are asking the question, what is the maximum yield? Where does the maximum yield occur? What plant population? And where is the economic optimum plant populations that occur? And then that follows naturally into, if there are optimums out there, does that need to change as you move throughout a corn field? In other words, should some parts of that field be planted at lower populations than another part of the field? So, we took three fields in Wisconsin. And these fields were chosen because they had long field histories where we had yield maps on them. We broke those fields down into subfields and looked at the overall yield over a 13-year period using the yield maps, as well as look at that standard deviation of the variance of those subfields. And then we classified those into high yield, part of the field with low variation, in other words every year they’re high yielding; and low yielding, low variation where every year they’re low yielding; and then you’ve got the in between where you’ve got, it could be either high or low but they have a lot of variation associated with those. Within each subfield then, we put out four plant populations 22,000, 30,000, 38,000 and 46,000 plants to the acre, to be able to draw a line to identify where that optimum population was for that subfield. So, we have three management zones, close to 30 different subfields. And our results were that the overall pattern, regardless of where we were, really didn’t change all that much as far as the pattern. What changed was where that line crossed what we call the intercept on the graph. In other words, if it was high yielding was above the low yielding parts of the field, just exactly as what we thought would happen. So, the point is that it is a very broad shouldered response for corn plant population. And when you divide the field into these different management zones, it really didn’t seem to matter all that much. Now these fields had about 40 to 50 bushels difference in terms, these subfields, had about 40 to 50 bushels difference in terms of their yield performance over the last 13 years and about 40 bushels standard deviation, plus or minus 40 bushels around those means. So in that kind of a situation, we could pick up how they would respond, but the pattern was very similar. So, this was rather discouraging because again, one of the tenants with variable rate technology is that we should be adjusting these plant populations as you go through the field, and we just couldn’t pick that up. So then we asked the question, well can we even predict the pattern out there in the field in terms of yield? And so for this we took another 26 years of data and we looked at the yield pattern within that field and we figured that over 26 years we would have the pattern figured out. In this field there are treatments where we’ve got continuous corn for 30 plus years, conventional till, no till, for close to 30 years and we wanted to see when those patterns evolved. So we had different subfields in that field and we wanted to see when was something significant? In other words, if you added years of yield maps to that subfield, when do we start to pick up differences among different parts of different subfields within the field when we’re in a continuous corn, conventional till type of a system. Well, we could pick ’em up, usually it took anywhere from two to I believe it was about 12 years before we pick up a significant difference of just any pattern that would exist out there. And it depended upon the rotation as well as the tillage.

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