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On a better note, I've heard of some good yields from some full-season early-maturing soybeans. I do expect the group threes and early fours to shine this year. But, on the average, this is not the case. In future newsletters, I'll spend a good deal of time addressing variety selection.
We also have just begun harvest. The weather has set us back, but I expect to have data rolling in soon. My goal is to make much of the variety test data available in early December.
We survived a year without soybean rust. I can confidently say that we were ready for it. One good thing about the rust scare is that many fungicide trials were conducted this year by Drs. Pat Phipps and Erik Stromberg. I look forward to seeing the data and I'm sure you will too.
I wish you a good and safe harvest season.
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Rain eventually came; between 2 and 8 inches was recorded in various parts of the state during the weekend of October 8 and 9. Although it was too late to help our soybean yield, it wet the soil just in time for small grain planting. The rain would have been more appreciated if it would have stopped over the weekend. But fog, misting rain, and generally very high relative humidity conditions persisted for another 5 days. These warm and wet conditions set up the soybean crop for several problems, largely relating to seed quality. In the next few pages we'll explore these potential problems, their causes and what can be done about it.
Seed Size. The most noticeable problem with the crop is seed size (Fig. 8.8.1, p. 2). Once pods have been set and drought sets in, yield will be lowered primarily through smaller seed. This year, we're seeing 10 to 30% smaller seeds than average, which translates directly into yield. Of course if you have a lot of these small seed, then yields can still exceed 40 bushels per acre.
Outside of yield reduction, are there other problems with small seed? It depends. If you're marketing specialty beans (i.e., food-grade) that require a certain size, then you could have a problem. Or, if you're producing seed for next year's planting, many of the smaller seed will be screened out in the conditioning process. On the other hand, for the standard commodity market, seed size makes little difference.
Small seed are likely to be slightly lower in protein because protein is formed at the end of the season. Conditions such as drought which can shorten the growing season tend to reduce protein levels. Oil content is not usually affected. But, oil yields per bushel may be lower because it is harder to extract oil from smaller seed. Small seed usually result in more millfeed, or hulls, for the processor to market.
Some have asked about late-season drought effects on test weight. Seed size should not affect this, at least not very much. Figure 8.8.2 shows the same volume and same weight of the same variety. The only difference is seed size. Test weight is weight per volume. So, in this example, seed size does not affect test weight.
However, other factors can result in low test weights for soybeans. A bushel of soybean is assumed to weight 60 pounds. Once test weights drop below 54 to 55 pounds, buyers may begin to apply discounts.
Figure 8.8.1. Harvest seed of Vigoro brand V442NRR (left of pencil) compared to planted seed, 2005. Planted seed size was 2700 seed per pound; harvested seed was 4150 seed per pound.
Figure 8.8.2. The same volume and weight of soybean seed of different seed size. The cylinder on the right contains seed of Vigoro brand V442NRR that was planted in the 2005 variety test. The cylinder on the left contains harvested seed produced from that same seed lot.
Phomopsis Seed Decay. When soybeans mature during warm and wet conditions, we can expect seed quality to deteriorate. Conditions during the week after the rain were perfect for this. Because the disease develops more rapidly on plants that are maturing under these conditions, we usually have more problems with early-maturing varieties. But, this year, we have seed decay on our later- maturing varieties as well because of the warmer October temperatures.
Infected seed are shriveled, elongated, and cracked. Severely infected seed may appear white and chalky. Test weight can be lower. High occurrence of these seed can lead to discounts or rejection.
There are a few things that can be done to reduce the disease incidence. It resides in the soil and on infected residue. So, rotation is very important. Later-maturing varieties and later planting dates that delay maturity into the cooler parts of the year will reduce the incidence. Timely harvest is the best management strategy. The longer you leave the soybeans in the field, the worse the disease. So, only plant as many early varieties as you can harvest in a timely manner. Foliar fungicides will decrease the incidence of seed decay if applied from mid pod development to late seed filling stages.
Purple Seed Stain. Purple seed stain is caused by the organism Cercospora kikuchii, the same organism that causes Cercospora blight. Cercospora blight is usually recognized by reddish leaves and reddish purple blotches on the stem and leaf petioles. When severe, defoliation of the upper leaves of the plant will take place. The disease was quite common his year. In many cases, the blotching progressed up the stem and to the pods. You may have noticed the dark, nearly black pods on some varieties. Once it progresses to the pods, there is a higher likelihood that the seed will be stained.
Purple seed stain is very noticeable. The seed will contain pink to pale purple to dark purple splotches, which can cover the entire seed coat. The purple stain itself does not reduce yield, but seed with nearly 100% discoloration may be lower in oil and higher in protein. A lot of staining can result in discounts. Germination of seed with 50% or more staining will likely be delayed.
This year, we are noticing more purple seed stain on the later-maturing varieties and double-crop plantings. But, purple seed stain is not always related to maturity group or planting date. Usually, the disease first appears on the plant during early seed development. If conditions are right (average temperatures over 80o for several days), then the disease will build up rapidly. We definitely had those conditions this year. Other weather factors do not generally affect seed infection. Severity of the infection is largely related to amount of infected leaf debris and residue. Therefore, rotation with a non-legume crop is critical for control.
Other control measures include variety selection, planting high quality seed free of visual staining, and fungicides. Varieties differ in their susceptibility of Cercospora kikuchii, but that information is rarely available in seed catalogs. We routinely evaluate purple seed stain in our variety tests; I'll try to summarize the data after this year's harvest. In 2003, we found little emergence or yield response of fungicide seed treatments when up to 20% of soybean seed contained purple seed stain. However, others have reported some benefit from fungicide seed treatments. Strobilurin fungicides such as Quadris or Headline seem to give some control if applied during pod or seed formation. We will have data available this winter.
Sprouting Seed. One of the most disturbing findings this year was seed sprouting in the pod. In a fungicide trial, we were seeing 10 to 30% of the pods with sprouted seed. This is a very unusual occurrence, but can occur if soybean seed drop below 50% moisture, then increase to 50% or more moisture. We immediately began looking in other fields and contacting county agents to do the same. Soon after our discovery, I heard of reports from North Carolina of sprouting and reports from Pennsylvania of seed swelling in the pod. However, after getting several reports back from county agents and farmers, and after we checked several other fields, we found that the problem was not nearly as severe as in our fungicide trial or as widespread as we may have thought. Below is a summary of our findings.
So, what does this mean? First, the problem is not widespread and should affect our overall seed quality very little. Most of the fields that I looked at with a problem contained less than 1% sprouted seed. At the worst, there could be some lower test weight and seed could contain more foreign material (from the dried up sprouts). However, the light seed will likely be blown out the back of the combine. If the problem is severe, I suggest that the air on the combine be adjusted to remove those light, sprouted seed at harvest. Too sprouted seed in the bin could lead to rejection by the buyer.
Fig. 8.8.7. Dried up sprouted seed in Suffolk, Oct. 20, 2005
Why is it occurring in the later-planted, later-maturing varieties? There are two or three possible explanations. One is that these varieties may have just dropped below 50% moisture. Therefore, the seed needed to absorb less total moisture to raise it back above 50%. A second explanation is that these varieties still had significant amount of leaves, therefore held the moisture within the soybean canopy better. Finally, there was more Cercospora in the later-planted later-maturing varieties.
Again, I don't think that sprouting seed is a big issue, but you should be aware of it and check your late-planted crop.
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Sincerely,
David L. Holshouser
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