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Issue No. 21, Article 6/August 13, 2004

Down the Backstretch

The crop condition report still shows the corn and soybean crops in Illinois to be in good shape, though how yields will turn out remains a guess. Much of the "extra" corn yield in 2003 came from the rain at the end of August, and much of the soybean crop was beyond help by then, especially in the northern part of the state. Condition scores for both crops were high in mid-August 2003, but yields were set to go up or down, depending on the last weeks of the seed-filling process. That is the case again in 2004.

Concerns about cool temperatures and dry soils are starting to mount. These phenomena are related in the way they affect crops. Cool temperatures mean low rates of water loss through crop plants and so help preserve what soil moisture the crop plants have available. The downside is that cool temperatures also mean low photosynthetic rates and so low rates of seed filling. Unlike other causes of low photosynthetic rates, though, low temperatures usually prolong the seed-filling period. Thus, if we have enough cool days, or if temperatures return to normal with adequate soil moisture later, cool days now may not hurt and could even help yields.

A delay in the seed-filling process due to cool temperatures is usually more beneficial in corn than in soybean. That's because corn benefits more from low night temperatures than soybean, and its photosynthesis rate is higher than soybean's, even when limited by cool temperatures. Still, night temperatures in the upper 40s and daytime highs in the 60s to low 70s are far from ideal for both crops. To give an idea of how much the season can be extended, it will take corn more than twice as many days from now to maturity at day/night temperatures of 72/50 than at 85/65. The positive factor this year is that both crops remain ahead of normal in their development, so getting to maturity before frost is almost certain. Dreams of August harvest are fading, though, except perhaps in some March-planted fields in southern Illinois that are at or very near physiological maturity.

The cool, dry weather should help preserve good canopy health and color in both corn and soybean, and good sunlight amounts will help the crops make the most of what water and temperatures are available. For both crops, it is better for the weather to remain cool if soils remain dry. But one or two more substantial rainfall events and a return to warmer (but not hot) weather with maximum sunlight would put a nice finishing touch on the 2004 season.

With very good growing conditions in May and June, we had reason to believe that the root systems on corn were healthy and at least as large as normal. Root lodging in some fields, however, has led to speculation about root system size and health. The July 13 wind event responsible for much of the lodging came after pollination in most fields, and the goosenecking response that younger plants exhibit is not present in most fields. I had remarked in late June about the fact that brace roots were developing early and that this could be positive or negative, depending on the reason why the lower-stalk sugar supply was sufficient to promote early brace root growth. It's beginning to look like some of this could have been stimulated by early root feeding by corn rootworm.

The ultimate size of corn root systems is under some genetic control, and part of this control occurs when the plant allocates (perhaps simply by having no "better" place to send them) a certain amount of sugars to the lower stalk during vegetative and very early reproductive stages. If growing conditions are good, the sugar supply available for the roots is usually larger. These sugars seem "destined" to be used to grow roots, and it doesn't appear that the plant controls very much the shape of the root system that results. Brace roots are roots that grow from nodes that just happen to be above the soil surface, and they usually develop after the four or five nodes below the soil surface have grown roots. If belowground nodes of roots are damaged, roots at nodes above these will grow more in response, as long as the photosynthetic rate of the plant is not compromised such that sugar supply is limited.

What does this have to do with root lodging? Observations this year indicate that plants that stood well in the wind had good brace roots, while those that did not usually had few or none. As I've explained before, lots of brace roots are not always a positive sign, since this can indicate problems in the underground root system. In this case, though, brace roots were much more effective in bracing the plant than were belowground roots. Large brace roots did not necessarily result from damage to lower nodes of roots, but this could have happened in some fields.

The good early-growing conditions seem to have produced good stalk strength in most fields. Some evidence for this is the fact that winds in mid-July caused very little stalk breakage, and observations now suggest that stalks are stiff enough to provide good protection against late-season breakage, even if their internal health (governed to a large extent by sugar content) is compromised by heavy draw on stalk sugars by the ear. No guarantees, but in most years with high yields and compromised stalk strength, stalk weakness and even breakage start to appear by the time corn is in the dent stage. Much of the early-planted crop is already in dent, or soon will be, and so far stalks are standing well.--Emerson Nafziger

Author:
Emerson Nafziger

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