As spring activities get started it is tempting to kill cover crops early so you have one less thing to worry about.
However, to reap agronomic and environmental benefits from cover crops, it is important to let them grow until you get ready to plant the next crop.
Our current recommendation is to kill the cover crops about 2 weeks prior to planting, or before they get too tall (2–3 feet), whichever comes first. Some producers try new things like planting into green or very tall cover crops, but this is something that we don’t have a lot of experience with yet.
The main topic of this article, however, is why to let those cover crops grow until shortly before planting.
1. Especially if we apply manure, nitrate leaching is greatest in the spring. The cover crops are needed to soak up the nitrate or it will leach below the rooting zone, be lost for agricultural production, and contaminate groundwater.
A summary of cover crop samples taking on Pennsylvania farms in the previous 3 years showed cover crops could take up more than 200 lbs/A of N. A regression analysis revealed that the cover crops contained about 60 lbs of N per ton of dry matter.
So letting that cover crop grow a ton or two more per acre can keep a lot of N in the system. This protects our ground and surface waters. The nitrogen will be released upon degradation of the cover crop biomass, as long as the cover crop is terminated prior to boot stage.
For more information on cover crop nitrogen release and growth stage of cereals, see the current Agronomy Guide (http://extension.psu.edu/agronomy-guide/cm/sec10). Check page 86 of the new Agronomy Guide to determine growth stage of cereals:
2. Mulch is an essential component of sustainable no-tillage systems. The mulch gives you erosion control, increased infiltration, and reduced evaporation during the summer and is the food source for soil organisms.
If the cover crop is killed early, little of it will remain 6 weeks after planting when drought periods and heavy rains become more prevalent, and there will be little to sustain the soil biological community.
3. The root system grows with the above-ground biomass. The root system is very important to provide improved aggregation and soil tilth. So giving cover crops time to grow also to helps maintain and improve soil quality.
If a farmer wishes to start planting early because they have a large acreage to cover, it is possible to synchronize killing the cover crops with expected planting date. So if the farmer expects to plant 200 acres every week over a three-week period, it's possible to kill some cover crops earlier than others to accommodate the planting schedule.
Remember, the benefits of cover crops depend on the growth we get out of them.