The 2014-15 wheat harvest was delayed in some parts of Kansas due to precipitation after the kernels achieved physiological maturity, which resulted in wetting and drying of mature kernels still in the spike. A direct consequence of wetting and drying of mature wheat kernels is a decrease in test weight, which is a measure of kernels' volume weight or bulk density.
Test weight can be decreased in two different ways in moistened mature wheat kernels.
- First, kernels may swell due to moistening, causing the seed coat to loosen irreversibly. The seed coat later wrinkles with drying and the kernel may not return to its original size, affecting kernel packing and ultimately decreasing test weight.
- Secondly, precipitation can initiate the germination process in the moistened kernel and cause starch to be digested, leaving small voids inside the kernel which decreases test weight. Under these circumstances, pre-harvest sprouting may occur in some varieties.
The extent of the decrease in wheat test weight depends on how many times it rained between optimal harvest time (wheat at harvest maturity) and actual harvest time. The greater the number of rainfall events, the greater the decrease in test weight.
Other factors that may lead to low test weight include suboptimal fertility, such as a nitrogen deficit; root and crown rots; insect damage to foliar and stem tissue; drought, waterlogging, or heat stresses during grain filling; lodging; and diseases that decrease leaf area or grain quality. In 2015, stripe rust, leaf rust, and Fusarium head scab were among the most common disease factors in reducing leaf area or grain quality – and ultimately test weight.
Seed wheat should have a test weight above 57 pounds per bushel for adequate germination under a wide variety of conditions. Seed with slightly lower test weight may be used to sow next year's crop if necessary. If the test weight is below 55 pounds per bushel, however, producers will want to take certain steps to help increase the chances of getting a good stand. Low-test-weight seed usually germinates well, but seedlings tend to have lower vigor than seedlings from seed with higher test weights. Therefore, producers should take special care to try to get a good, uniform stand.
Drill speed. Using a drill speed of 5 mph will help ensure that the seed is placed down in the seed slot, and that the seed slice is closed and firmed properly, making for good seed-soil contact. Getting good seed-soil contact will help the seedlings develop a good primary and secondary root system. Also, when drill speeds are too fast, the openers tend to "ride up at times, resulting in a planting depth that is shallower than intended.
Seeding depth. All wheat should be planted at the proper depth for best stands. But it is especially important that low-test-weight seed is not planted too deeply, since this seed has low emergence vigor to start the growing season. It is equally important not to plant too shallowly. Shallow-planted wheat often has more difficulty establishing a good root system in the fall than wheat planted at the proper depth, and this can be an even greater problem when using low-test-weight seed. Plant low-test-weight seed 1 to no more than 1.5 inches deep.
Seeding rates. Usually, the lower the test weight, the more seeds there are per pound. Producers who use a planting rate based on the number of pounds per acre should not adjust their seeding rate when planting low-test-weight seed. They will end up planting more seeds per acre, but emergence is often somewhat lower with low-test-weight seed, so the stand should come out about normal. If the cause of low test weight include fungal diseases such as Fusarium head scab, which decrease wheat germination rate, an increase in seeding rate may ensure a good and uniform stand.
Seed treatments. Fungicide seed treatments may improve germination or seedling vigor of low-test-weight seed, and protect against certain diseases. See the accompanying article in this issue of the eUpdate for more information.
Seed cleaning. Producers should make every effort to have their seed cleaned as thoroughly as possible to remove scabby kernels and shriveled seed. This may help increase the test weight and improve emergence and seedling vigor. Adjusting the settings during seed cleaning to blow lighter seed away can add 1 to 2 pounds to the seed lot's test weight by removing the small kernels. However, if the majority of the kernels are lighter and shriveled, the potential of gaining much test weight is limited.
Germination testing. Where possible it would desirable to have the seed germination evaluated by a seed-testing lab. The turnaround time for this type of testing is generally 7 to 14 days once the seed-testing lab receives the sample. The variation in the turnaround time depends on the need for pre-chilling treatment prior to the germination test. The need for pre-chilling typically ends around Labor Day weekend. The cost of testing at the Kansas Crop Improvement Association (KCIA) is $17.00 for the standard warm germination test. Growers or others can contact KCIA by phone at 785 532-6118, or by email at [email protected].
On-farm germination tests may be an option for growers who do not have time to have seed evaluated by a seed-testing lab. This topic is addressed in a previous Agronomy e-Update article: https://webapp.agron.ksu.edu/agr_social/eu_article.throck?article_id=618 .