Typically, late August sees more rain than the rest of summer, but when it turns dry and your cover crop seed is in the ground, it’s totally dependent on the moisture in the top few inches to germinate. As we manage late-season irrigation, producers tend to allow the crop to dry the soil profile. This may leave soil moisture at the surface too dry for newly seeded cover crops to germinate.
Producers trying to “water-up” broadcast cover crops or seed spread on the soil surface need to plan irrigation and near future potential rainfalls to create a time period of four to five days with a wet soil surface. This may take two applications on bare soils, while fields with heavy crop residue may be able to get by with one application.
Cover crops that are drilled or disked in benefit from the soil contact to germinate. If soil is dry, apply enough water—0.3 inch in sandy soils or 0.5 inch in loamy sands—to wet the top 4-6 inches of the soil profile.
The cost of applying one or two half-inch applications of water is often under $5 per acre, a small expense compared to the $15 to $25 often seen with cover crop plantings. If cover crops are inter-seeded with corn or soybean in early September and we have a dry spell, the cover crop seed will likely delay germination until rain.
This delay in germination may reduce fall growth, compromising some of the benefits of the cover crop. If it is so dry the cover crop seed will delay germination, there is a good chance the soybean or corn crop will still benefit from water, increasing beans size or adding test weight.
In most areas, rainfall will be sufficient to germinate cover crop seed, but every winter somewhere in Michigan or Indiana, I’ll have a producer tell me how their cover crop didn’t germinate soon enough to get a good fall growth and be beneficial. The big cost is in the land and equipment, which is already made; the small investment in energy and time to water up the cover crop will often over shadow the energy and labor cost in the long run. It is another example of how irrigation can increase the efficiency of most of our resource inputs in the crop.