Crop Management

By Greg Stewart, OMAFRA Corn Specialist and Dr. Thys Tollenaar, University of Guelph

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The following is a list of crop development and management highlights from presentations given at the 2001 Southwest AgConference.

1) Numerous studies on narrow row (less than 30 inches) corn across North America over the past decade have been inconclusive as to the actual yield benefit to expect. Some results indicate as much as 8-10 per cent advantage while others predict no yield advantage to rows less than 30 inches. It is probably difficult to justify narrow row equipment costs based solely on increased corn yields, but the picture improves somewhat if equipment harmonization produces savings or efficiencies (e.g., same 20-inch row planter for corn, soys, edible beans)

2) As a theoretical rule of thumb, achieving canopy cover (maximum light interception) 1 day earlier in the season translates into 2 bu/ac additional yield. This concept applies for rows that canopy sooner because they are planted closer together, but it also applies to corn that is planted earlier, experiences less stress, enjoys balanced fertility, etc. How much more quickly do 20-inch rows reach maximum light interception than 30-inch rows? The difference is actually not that great: perhaps 1-3 days, thus theoretically boosting yields by 2 to 6 bu/ac.

3) Once both 20-inch and 30-inch row corn canopies are fully developed, they intercept the same amount of light and there is no additional yield gain to be obtained. As rows widen beyond 30 inches, however, the ability for the canopy to intercept a high percentage of the sunlight starts to fall off. Selecting ultrawide rows (greater than 40 inches) is a sure way to generate a 10 per cent yield loss.

4) Confused over GDUs or GDDs versus Ontario CHUs? Both are based on temperature, but they are calculated quite differently. There is no single factor that will allow you to convert one to the other, but if you must: GDU x 1.3 is approximately equal to CHU. Also, there is no advantage to the GDU approach; in fact, research indicates that the CHU system is slightly more accurate at predicting corn crop development.

5) What are the advantages to early silking in corn hybrids? Simply put they are:

a) allow for the life cycle to be completed - maturity before killing frost

b) higher solar radiation during grain filling - delayed silking results in grain filling that occurs during a period of lower light intensity and reduces yield potential

c) warmer temperature during grain filling. Generally, earlier silking is an advantage and usually means earlier to black layer - but there are differences in how rapidly different hybrids fill their grain. Some later silking hybrids may still be capable of filling more rapidly and reaching maturity as quickly as earlier silking hybrids.
6) On the hybrid selection front, considerable progress is being made in the ability to rank hybrids from 'unbalanced' strip trial data. By unbalanced, we mean the type of data that is gathered from a large number of on-farm strip trials where hybrids that you would like to compare or rank don't actually appear side by side in the same test that often. Weikai Yan and Dr. Tony Hunt from the Department of Plant Agriculture at the University of Guelph have developed a system where this data can be analyzed to give an indication of the hybrid or cultivars' relative performance similar to a situation where they all appeared in a performance trial. This exciting new approach can give considerably more power to our hybrid selection process. Of course, one of the keys to making it all work is to have those hybrids with significant potential appear in a reasonable number of on-farm strip trials. Be sure to put a good range of proven performers and emerging stars from various companies in your hybrid trials this year.

7) The theoretical maximum yield for corn is calculated by Dr. Tollenaar to be about 470 bu/ac. Record yields over the past few decades have approached 400 bu/ac. Must you plant some freakish 'racehorse'-type hybrid to achieve these record yields? An analysis of the hybrids used to obtain some of these record yields actually indicates that they often could be considered 'workhorse' hybrids which yielded as well or better than the average in poorer, low yielding environments. High yield potential, however, is still one of the most important considerations for corn growers. The point to remember is that nearly all elite, high potential hybrids will have their defects. Good hybrid selection requires that you obtain as much of this information as possible and select for genetic diversity. If you are selecting two racehorse hybrids for your early planting, be sure that they complement each other and do not have the same inherent weakness, e.g., low stalk strength.

8) The most economic rate of nitrogen for a corn crop depends on many factors, including weather conditions, soil texture and organic matter, previous crops, manure applications, etc. The soil nitrate test has been used with some success to evaluate the impact of these different factors on the amount of nitrogen that will be made available by the soil, and hence, how much fertilizer N should be applied to reach maximum economic returns from the corn crop. This approach to predicting nitrogen requirements (as well as other approaches) is most meaningful to growers when accompanied by some in-field verification to assess the actual amount of nitrogen required by the crop. This requires a series of plots or strips receiving different rates of nitrogen. There are numerous ways to set up these strips. Here are three suggestions:

Flag Colour	Sidedress Applicator N Rate ( lbs N/acre )
White	0
Yellow	45
Orange	90

a) Full Rate and Zero. Apply a rate that you are confident is all the crop will need to maximize yields, and also apply a zero rate. The relationship between Full Rate yields and Zero nitrogen yields can be used to indicate what the optimum rate would have been.

b) Full Rate and Best Guess Less. Here, apply the full rate and a second rate which is a reduced rate based on some predictive tool such as a soil nitrate test, credits from red clover, etc. You may not be able to determine what the most economical rate should have been, but you will be able to know whether your reduced N rate saved you more in nitrogen costs than it cost you in corn yield.

c) Multiple Rate Strips will allow you to determine with some reliability what the optimum rate should have been. This system requires the calibration of the nitrogen applicator for two distinct rates. These two rates will vary somewhat, depending on how much total nitrogen you want to apply. For illustration purposes, we will pick Rate #1 to be 45 lbs N /acre and Rate #2 to be 95 lbs N/acre. Based on these two pre-calibrated rates, you can develop a simple flag system to take to the field and lay out a series of N-rate plots.

Using these two rates and employing two passes with the applicator, you can now apply five unique rates: 0, 45, 95, 140 (45 + 95), and 190 (95 + 95). This approach can be much more accurate and less time-consuming than calibrating your equipment for 4 or 5 individual rates, and it provides a better range of yield response values to determine the optimum N rate for that crop. In using any of the above techniques, be sure to repeat the strips several times to improve reliability.