In planning for upcoming spring wheat planting, growers should consider ways to get the best possible stand and prevent stand losses. University of Minnesota wheat researchers say seedling-stand-loss causes can be categorized in three broad categories: intrinsic attributes, biotic stresses and abiotic stresses. They say that 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, suggests that there was no need to remove the smaller seed fraction from a seedlot as long as the seedlot has been 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, does show there were significant differences in seedling vigor among spring wheat cultivars.
The physiological age of the seed is also an important parameter that influences seedling vigor, says Jochum Wiersma, University of Minnesota Extension small-grains specialist. “A standard germination test is used to determine the percentage of viable seed under ideal conditions,” he says. “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 can probably be explained by the fact that corn and soybeans are more often seeded in soils that are well below the minimum temperatures needed for germination, resulting in protracted germination and seeding emergence, while wheat will germinate at 40° F temperatures," Wiersma says.
"Seedling vigor tests, such as the accelerated aging test, don't improve the outcome of the seeding operation and stand establishment all that much," he adds.
Researchers say biotic stresses that cause stand losses include a 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 soil moisture depletes the soil of oxygen and germinating seed will quickly die in these anaerobic conditions, says Wiersma.
High temperatures in excess of 90° F can induce a dormancy that will prevent germination. This dormancy is not broken until temperatures drop below 50°. “Seeding too deep will prevent the coleoptile to reach the surface, and consequently, the first leaf will not get above ground,” says Wiersma. “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 cannot 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. 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.”