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What the Crops Need Now

June 17, 2011

By Emerson Nafziger, University of Illinois Extension
(Courtesy of the Bulletin)

 
With nearly all of the corn and more than 90% of the soybeans planted, our thoughts and concerns turn to what it will take to keep the 2011 crops healthy and thriving or at least out of serious trouble.
 
The corn crop in Illinois was rated at 69% good to excellent on June 12, up slightly from 66% the week before. The soybean rating went from 61% good to excellent on June 5 to 66% on June 12. These aren't "highest-ever" type ratings, but they do reflect the improvement in crop appearance following the warm weather and abundant sunshine during the first third of June. The crop greened up and started to grow rapidly during this period, before slowing down again during the cooler temperatures of the past week.
 
Growing degree-day accumulations, which were slightly below normal through May, have turned above normal. At Urbana, we accumulated 189 GDD in the 7 days from June 3 through June 9. That amounts to almost 30% of the normal June total of 641 GDD, and it's a rate of accumulation seldom seen outside hot spells in July or August. While the crop in June responds well to such warm temperatures, high GDD accumulations come mostly from warm nights, and we would prefer average accumulation rates (more moderate night temperatures) in July and August.
 
Many have commented on the rapid recent growth in what had been a rather laggard crop through most of May. In the planting date study here at Urbana, the March 31 planting has by now received more than 950 GDD and is in high gear with regard to growth; adding each leaf stage after V10 (after about 850 GDD) requires only about 50 GDD, and we can expect growth rates to be 2 to 4 inches per day depending on temperatures at night, when most of the visible height increase takes place. The reduced GDD per leaf stage is due to rapid stem elongation, which pushes leaves out of the whorl quickly. Plantings done in early, mid- and late May have accumulated about 730, 560, and 320 GDD, respectively, so they are at or near stages V8, V6, and V3.
 
Rainfall is, unfortunately, following what has become a rather familiar pattern, with heavy rain in spots over much of the state in recent weeks. Most of the northern half of Illinois received above-normal rainfall in the past month, with parts of western Illinois receiving twice the normal amounts. Southern Illinois got a break that allowed most of the planting to get done, but rainfall has been heavy in many areas this week (June 14-15), and crops are going to suffer from standing water once again.
 
Damage from standing water is visible as drowned-out spots (particularly in soybean) and, in corn, as yellowing and stunting. As we saw in 2010, saturated soils cause both root damage and loss of nitrogen, meaning that plants cannot take up enough N to keep healthy color. While pale leaves cannot photosynthesize at high rates, pale color is only a symptom of the poorly functioning root system that results from low soil oxygen. In such cases, while supplemental N may provide some benefit, it will not do so until and unless soils dry enough to allow roots access to oxygen again.
 
One small advantage the 2011 crop has over 2010 during heavy rains in June is that this year's crop is not as large and is not growing as fast. Last year's crop died quickly, or had its root system permanently damaged, when soils became saturated during warm temperatures in June, at a time when plants were much larger than normal. We expect better prospects for recovery this year, but of course the recovery will come only if we get some good drying weather before roots are permanently damaged. It will help recovery if temperatures remain moderate during the drying process.
 
Saturated soils both bring dangers to the root system from lack of oxygen and prevent the root system from growing deeper into the soil. Roots need not be deep to support current growth, but later, when the crop might have to take up water and nutrients from deeper in the soil, lack of root penetration now could create problems. It's for this reason that conditions drier than average in June usually prove to be favorable for corn yield and wet Junes can sometimes mean lower yields. Nitrogen also remains in place better under drier soil conditions. Still, the problem of wet soils in June can be largely overcome, and good yields can result if soils alternate between wetter (not saturated) and drier in turns during the rest of the season. Larger plants will dry the soil much faster than smaller plants, so if the too-wet phase can end soon, recovery should follow.
 
We have had several reports of "floppy" corn in the past two weeks from areas where soils had been dry for some time and where plants were starting to grow rapidly. This is almost always a case in which the nodal root system is trying, and failing, to grow out into the soil. This is usually because surface soils are too dry to support root growth. In some cases a physical barrier, such as sidewall drying and hardening following no-till, also contributes. Plants with this problem have essentially outgrown the capacity of the seminal root system to support growth, and so they may fall over, due to lack of both physical support and water. Plants may also turn purple as sugars accumulate; they accumulate because they aren't being used to support root growth or for top growth as water uptake declines. The only cure is to have the soil near the base of the stem get wet, thereby allowing roots to penetrate and grow.
 
There are always concerns about how poor growing conditions or stress during early vegetative growth--V5 to V8 or so--might result in permanent damage to yield potential. The concerns arise from the knowledge that the ear starts to develop during this period and from the assumption that full potential can be reached only when conditions are ideal. While we would rather have the crop experience no stress during the entire season, there is little evidence to suggest that yields are often curtailed by what happens during vegetative growth. It's very difficult to do an experiment in "controlled stress" where we can actually test this, but observations, together with what experiments have been run, suggest that as long as the height and leaf area of the crop aren't compromised, say by extreme water stress, crops that undergo some stress during early to mid-vegetative growth can yield very well.
 
In part, the resistance of corn to stress during mid-vegetative growth or earlier to mild or moderate stress is related to counterbalancing, positive effects that accompany the stress. As an example, dry weather in June usually is accompanied by a lot of sunshine, so daily photosynthetic rates may be high even though leaves curl some in late afternoon. High temperatures help speed crop development, even though they tend to favor top growth over root growth. As a final note, though, much of the stress tolerance of young corn plants stems from the fact that corn, a crop of tropical origin, is physiologically resilient and not easily damaged by the vicissitudes of a typical growing season.
 
The big concern in 2011 will be, as always, whether the corn and soybean crops will have enough water available at the critical stages of development. For corn this will be before, during, and after pollination, as well as through the grain-filling period. For soybean, it will be from full flowering through pod-setting and then during seed-filling. This year the period from early July through mid-August will be critical for corn; for soybean, all of August will be most important. We depend on water from rainfall, of course, but other critical factors in water supply will include soil types and condition and the health of root systems.

 


 

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RELATED TOPICS: Corn, Weather, Soybeans, Agronomy, Crops

 
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