Tuesday, June 16, 2009
Causes of Seedling Stand Losses in Spring Wheat
Seedling stand loss is defined as the percentage of viable seed that fails to become a healthy plant. In order to understand the causes of stand loss we need to also define seedling vigor. Seedling vigor is defined as those seed properties that determine the potential for rapid, uniform emergence and development of normal seedlings under a wide range of conditions. Causes of seedling stand losses can be categorized in three broad categories - intrinsic attributes, biotic stresses, and abiotic stresses.
Both seed size and grain protein content have been shown to improve seedling vigor in spring and winter wheat seed lots of the same cultivars that have higher seed weight and/or grain protein content will have more seedling vigor. Some of the research, however, suggested that there was no need to remove the smaller seed fraction from a seedlot as long as the seedlot had commercially cleaned as there often wasn't a yield difference at the end of the season despite differences in seedling vigor. Some of the same research, however, did shown that were significant differences in seedling vigor among spring wheat cultivars and the authors of the studies suggested that breeders use it as a selection criterion in their breeding programs.
A cold stress test in addition to the standard germination test is a method to test seedling vigor of a breeding line, variety or seedlot. Corn breeders routinely use this test as a selection criterion in their breeding programs. I am not aware of any spring wheat breeding programs that use the cold stress tests in their breeding program. This can probably be explained by the fact that corn is more often seeded in - for corn - cold soils resulting in protracted germination and seedling emergence.
The physiological age of the seed is also an important parameter that influences seedling vigor. A standard germination test is used to determine the percentage viable seed under ideal conditions. There is also the 'accelerated aging test' to discern seedlots with poor seed vigor and excellent seed vigor. This test is routinely conducted in soybeans and corn but again seldom used in wheat. This too can probably be explained by the fact that corn and soybeans are more often seeded for their species in cool soils, resulting in protracted germination and seeding emergence.
Biotic stresses that cause stand losses include whole host of fugal diseases. Saturated and/or cold soils can aggravate the incidence and severity of a number of fungal seeding diseases including Pythium damping off.
Abiotic stresses that cause stand losses are water, temperature, and/or distance to the soil surface. Excess moisture (anytime the soil water content is above field capacity) depletes the soil of oxygen and germinating seed will quickly die in these anaerobic conditions. High temperatures in excess of 90F can induce a dormancy that will prevent germination. This dormancy is not broken until temperatures drop below 50F. Seeding too deep will prevent to coleoptile to reach the surface, and consequently, the first leaf will not get above ground. The seedling will ultimately die although an etiolated and crinkled up first leaf can often be found just below the soil surface. Crusting of the soil can give a similar result. Seeding too shallow or in a seedbed that is very cloddy poses the risk of poor seed to soil contact. In either case, the seed can not in imbibe enough water for germination to start. This seed will stay viable until a rain improves seed to soil contact and adds water into the upper soil layer. A big risk, however, is that the seed will start to imbibe water but that wind and/or warmth desiccate the seed again. This almost always means the death of the germ.
Judging from some of the fields I have surveyed, the shallow seeding and the anaerobic conditions have contributed equally to the uneven emergence we see in many fields. A clue whether shallow seeding contributed to a delay in emergence is to dig up the seedling and measure the distance from the crown to the tip of the coleoptile. You also count the number of leaves of these seedlings and compare this to the better parts of the field. Given the fact that it takes about 180 GDD for each leaf to appear, you can use the difference in development to reconstruct whether the seed lay in dry dirt; the number of days needed to accumulate difference in GDD should be equal to the number of days between seeding and the first rain received after seeding.