As selected corn producers make 400 and even 500 bushels per acre, 300 bushels remains a lofty goal for most, and the inputs to achieve it are not insignificant.

“I think we have a decent handle on how to fertilizer for 300-bushel corn. In fact, Georgia has had recommendations for a long time, and with this resurgence among producers of wanting to go for high yields, we’ve brought those back out,” says Glen Harris, University of Georgia Extension agronomist.

Harris has tested those recommendations, with two years of data now recorded. This past year, he looked at 348 individual plots for corn research, considering variables such as nitrogen source, timing, P and K placement, and others.

“I can tell you that our high-yield growers approach it from a Liebig’s Law of the Minimum strategy. If you provide a maximum amount of inputs you’ll get a maximum yield. But usually something is not provided in an adequate amount, and it’s usually more than one thing. I can see how they come up with this strategy, but I think there can sometimes be some flaws with it. One of the flaws is that it isn’t always economical to provide these maximum inputs, so you have to be careful,” says Harris.

Water is likely the most limited factor in corn production, he says. “You also need air, N, P and K, and micronutrients. Organic matter also is important, as are heat and light. The law of the minimum system makes sense to a point. But there are things that aren’t on there that will limit you in making 300 bushels. Obviously, variety will be a key, along with planting date, and whether or not you use a starter fertilizer. Row patterns, uniform stands, and what you need for your final population all play a role,” says Harris.

Most producers, he says, have heard about the “four R’s” of fertilizer stewardship – right source, right rate, right time and right place, and all are equally important within the issue of corn fertilization.

“Also, we haven’t always sprayed corn with a fungicide, but it has become more common now, especially later in the year. So there are many factors that are not in this law-of-the-minimum strategy that are required for higher corn yields,” he says.

Harris urges growers to keep in mind realistic yield goals. “Obviously, if you fertilize for 300-bushel corn and make only 200 bushels, you’ve lost money. To be fair, you also have to look at it the other way. If you have the potential to make 300-bushel corn, and you fertilize for only 200 bushels, then you’ve also lost money.

“So what’s your realistic yield goal? I was taught you take your last five years, throw out your high and low, and then average the remaining three. The problem with that is how far back do you need to go, with rotations and everything, to get five years of corn?”

Variety choice might have changed dramatically during your last five years of corn production, says Harris.

“Basically, we’re saying if you’ve never made 300 bushels or close, but you’ve made 200, then fertilize for 250 bushels and do some of these other things. Once you’ve made 250, ramp it up and go for 300 bushels. What really scares me is when growers who have never made 100 bushels of corn start fertilizing for 300. You can get caught up in this high-yield strategy very quickly, because it’s exciting.”

UGA corn fertilization recommendations

Current University of Georgia corn fertilization recommendations can be found in the UGA soil test hand book at: http://aesl.ces.uga.edu/publications/soil/STHandbook.pdf for both dryland and irrigated cropland, says Harris.

“When I first got to Georgia, we had one nitrogen recommendation for cotton, and it was 60 pounds. Growers started maker higher yields and we did the research and came up with new recommendations.

“We took the concept we used for cotton and applied it to corn, for N, P and K recommendations for 150, 200, 250 and 300-bushel corn. Remember that we don’t test soil for nitrogen – it’s based on research and yield studies, and we basically recommend 180 pounds of nitrogen for 150 bushels of corn, 240 pounds for 200 bushels, and 300 pounds for 250 bushels, and 360 pounds for 300 bushels.

“If you have high levels of P and K in your soils, and you’re going for only 150 bushels, then you have our lowest P and K recommendation. Your highest P and K recommendation would be if you’re low in both, and you’re shooting for 300, 200 and 280 bushels. That’s the difference.

For irrigated, let’s say your yield goal is 150 bushels. But in the fine print, it says that for every 10-bushel increase, you add 12 pounds of nitrogen, 6 pounds of phosphorus, and 10 pounds of potassium.”

Obviously, recommendations don’t call for maintaining low levels of nutrients, says Harris. The idea, he says, is to keep nutrient levels at medium to high, and then put on fertilizer. This doesn’t take into account timing, source and placement, he says.

In 2013, Harris planted 35,000 plants per acre on about March 20. It had about 60 pounds of N and 60 pounds of P. “Fertilizing for 150-bushel corn, I made 157 bushels. Fertilizing for 200, I made 222. For 250 bushels, I made 254, but I made only 270 bushels at the 300-bushel goal. I’ve talked with a lot of people, including high-yield growers, about why I didn’t make the 300-bushel yield.

“I’m on 36-inch rows, and one high-yield grower thinks I’d do better with twin rows. I do not have a way to spray fungicides on small plots, and that might have hurt my yields. You also could make the argument that maybe that’s the limit of the land’s potential. This is a flatwoods soil. But I have a good system for watering. In the future, we need to do this trial in more locations with more yield goals.”

Harris says he mistakenly planted 48,000 seed in one area of the plot and made 269 bushels per acre. In high populations, he says, the nitrogen is significantly lower in the ear leaf, but P and K levels remained high.

“I repeated the study in 2014, when we had terrible early season problems, and I think climate had a lot to do with the results. The area had high phosphorus, so I didn’t apply any phosphorus. We peaked out at about 200 bushels.”

There are a lot of ideas today about how to make 300-bushel corn, says Harris.

“One is to add potassium to your starter, and another one is to put nutrients in the furrow, but my preferred placement is two-by-two. As for micronutrients, we need to be careful not to put too much boron on corn. I’d prefer to be in the medium to high range with the tissue analysis for 300-bushel corn. We need to pay attention to micronutrients, but we don’t need to let them get out of hand – we just need to keep them in the recommended range.”

As for timing, Harris says his recommendation is for two splits on nitrogen – at planting and at sidedress. “And I’m not sure that’s enough. When I’ve looked at three and four splits, four did the best. I don’t think split nitrogen is the key to making high yields, but splitting definitely makes you more efficient.”