Author’s Note: I summarize some of my research to give a perspective on why N recommendations for corn are changing. I thank all the students and companies that helped.
Anyone in the fertilizer business knows that nitrogen (N) usage per acre has been flat for a long time. This is readily seen by examining the per acre N usage in Illinois over the last 46 years. It must have been nice in the early years (1965-85) when each year brought higher N rates, and there were corresponding increases in yield. Fertilizer was relatively cheap, too.
Recommendation systems devised during this period were yield-based, with the idea that more yield required more N. A yield/N coefficient of 1.2 pounds (lb.) of N per bushel (bu) was typically used, multiplied by the yield the grower wanted to produce. In the 1980s, this was changed to “proven yield” based on a field’s 5-year average. This N requirement can be shown by dividing the N rate used by the yield achieved. The fitted trend line shows it hovering right around 1.2 lb. N/bu, but it has been decreasing since 1985, but the N rate per acre (/A) has been a constant 160 lb. On the other hand, grain yield has continued to increase at a rate of 1.7 bu/A per year. The only way that yields can continue to increase at this level is if the plants are using the N more efficiently. Have plant breeders indirectly selected for improved N use?
More of the annual increase in grain yield can be attributed to improved genetics than to better production inputs and crop management. This difference will get even larger in the future with continued advances in biotechnology, particularly drought stress and N use efficiency.
To verify that genetic improvement was responsible for the decreasing N requirement, we compared the N response of an older hybrid (B73 x Mo17) with its modern counterpart (FR1064 x LH185). Indeed, the modern hybrid was more efficient, requiring 27 lb. less N to produce 14 bu more grain. The modern hybrid also has a lower N requirement, 0.9 lb. N/bu compared to 1.2 lb. N/bu for the older hybrid. Although not widely acknowledged, I believe this is the main factor driving new recommendations for fertilizer N on corn.
Modern N Use
All recommendations systems have at their core some type of yield response function to varying rates of N.
The response curve was generated on-farm using commercial variable-rate applicators, and yields obtained using as-applied N maps and yield monitors. It represents 17 on-farm sites in Illinois and Indiana in 2005, and 20 sites over five states in 2006. I believe this curve is as close as possible to the “real world” picture of N use in commercial corn production.
Three important aspects can be gleaned from this curve including: 1) the check plot yield, or the yield without added fertilizer N; 2) the delta-yield, or the magnitude of increase in yield from applying the optimal N rate; and 3) the minimum N level required for maximum yield, based on the inflection point of the yield response function.
Surprising to many farmers are the check plot yields, averaging 112 bu/A produced without any fertilizer N. Although yield increases with added N, each successive increment results in a lesser increase up to the point where more N does not increase yield. Based on this function, 130 lb. of N produced a maximum yield of 173 bu/A. This represents a biological N requirement of 0.75 lb. N/bu, which is considerably less than the 1 to 1.2 lb. of N/bu typically used by these growers. This is why yields have continued to increase even though N rates have remained constant, and why we are seeing new recommendation systems for fertilizer N.
New N Recommendations
The “new” N recommendation system being advocated by Extension takes an economic approach by accounting for the value of the grain and the price of the N. The obvious goal is to maximize the grower’s return to N, hence the name “maximum return to N (MRTN).” An “N Rate Calculator” is used to determine a range in economic N rates based on a number of production factors like previous crop, yield level, and organic matter. This Web-based calculator, http://extension.agron.iastate.edu/soilfertility/nrate.aspx, is maintained by Iowa State University for a number of Midwestern states. I show this concept graphically in Fig. 3 using the response curve from our on-farm N response trials.
Because N isn’t free, the N rate giving the highest MRTN will always be less than the N rate required for maximal yield. Using our on-farm N response curve and current grain and fertilizer prices ($3.30/bu and 0.35/lb. N), a maximum return to N of $160/A was achieved with 110 lb./A of N, which was 20 lbs. less than the amount needed to produce maximal yield. It is also 50 lb. less than the average 160 lb. of N that was used on each acre of Illinois corn.
It all depends on the grain/fertilizer price ratio. For example, when grain prices are high, and the fertilizer price low, the calculator will recommend more N. Depicted graphically, the lost profit from either under- or over-fertilization is readily apparent. There is really no such thing as “insurance N,” extra N in case the environment is supportive of high yields, but only lost profit from excess fertilization above the optimum. It is this extra N that also causes the environmental problems. I believe that if the MRTN system were widely adapted it would probably lower the overall usage of N.
There has been a lot of talk lately about improving N use efficiency (NUE) with biotechnology, and all the major seed companies have some type of research and development program to improve N use. Does this represent future decreases in N sales? Of course it depends on how soon improved NUE hybrids are available, and the type of improvement that they exhibit; both of which remain a mystery.
Most of the information suggest improved hybrids are 5 to 10 years away. The type of improvement is less certain. The potential improvement scenarios that have been reported include: the same yield with 25% less N or 25% more yield with the same level of N. Although sounding similar, these approaches are actually very different, and would have different implications to fertilizer sales.
A hybrid producing the same yield with 25% less N, the crop would have to be improved in its ability to acquire N at low levels, but not able to utilize additional N should it be made available. This approach would clearly lower fertilizer usage, but I don’t see it being widely adopted by U.S. farmers.
A hybrid producing 25% more yield with the same level of N have to be improved in their ability to acquire N at low levels, and be able to utilize this N for additional yield production. This is similar to how modern hybrids use N compared to older ones. In theory, these hybrids should not change N usage, but I’ll bet that in practice they would result in a slight increase. This is because farmers tend to overfertilize to assure they are supplying optimal N level.
Alternatively, my idea for a better NUE hybrid has improved yield at low levels of N, but continues to yield even more with additional N. Physiologically, this scenario makes the most sense to me, and is what I believe will be the outcome of biotechnological improvements in NUE.
These hybrids would be adapted because they would ensure against yield losses due to insufficient N, but also provide even higher yields when adequate N is available. Even though such a hybrid would have a lower N requirement, it would actually require more total N to achieve its maximum yield. This equates to 45 lb./A. If U.S. farmers add this extra N to achieve top yields, then biotech improvements in NUE could actually increase rather than decrease N use.
Rootworm Trait And N
There’s a hint that the biotech industry has already altered corn’s N requirement with the Bt-rootworm trait. I’ve seen big yield advantages conferred by the rootworm trait that are hard to attribute entirely to rootworm control. One of these early studies where we compared N response for the same hybrid with varying biotech trait combinations. Plots had low rootworm pressure and were treated with soil insecticide at planting, so I wonder if the 50-plus yield advantage of the triple-stack version was solely rootworm control. Still, you will notice that this yield boost comes with an almost 100 lb./A increase in the fertilizer rate needed for maximum yield. I think this is probably a little higher than normal, but it does demonstrate that biotech improvements in yield are likely to increase the need for N.
The delta-yield represents the amount of yield increase that occurs from optimal fertilization. For example, the 61-bu yield increase between no fertilizer N (check plot yield) and the N rate that gave maximum yield is the delta-yield.
It makes sense to me that the bigger the delta-yield, the greater the amount of needed N/A. Such a system could also account for future yield increases regardless of whether they are driven by biotech or crop management.