Cover crops should be killed at least a couple of weeks before planting vegetables. That will give the cover time to partially decompose, and time for any cutworm larvae that may be in the crop to die or pupate. If wet weather delays killing or incorporation of cover crops, the time between incorporation and planting may be shorter than normal, or the cover crop may be larger than normal. There are implications for pest, nutrient, and cover crop management.
Black cutworm moths prefer to lay eggs in vegetated areas, including fields with cover crops or weeds. They typically show up in early May. To track black cutworm moth catches in pheromone traps throughout the state, refer to the Purdue Pest & Crop Newsletter at http://extension.entm.purdue.edu/pestcrop/2015/index.html. If larvae are present in the cover crop and they survive until the cash crop is planted or emerges, they may cause serious stand loss. If you plow down a cover crop that is infested with cutworms, you should wait at least 7 days before you plant. If you plant sooner, you may want to use an appropriate pyrethroid insecticide shortly after you plant to protect your crop.
Seed corn maggot adults prefer to lay eggs in soils with high levels of fresh organic matter, such as newly-incorporated cover crops. The longer you can wait after incorporation before planting, the less damage you are likely to experience. Waiting 2-3 weeks will usually suffice. Even more importantly, waiting until soil temperatures reach at least 70°F will greatly reduce the amount of damage. Similarly, waiting to plant will reduce the amount of damage from cabbage maggots on crucifers and onion maggots on onions.
Nitrogen tie-up and release by the decomposing cover crop depend on the ratio of carbon to nitrogen (C:N ratio) in the plant tissue. If the ratio is higher than about 30:1, some soil nitrogen will probably be temporarily tied up as the cover crop begins to decompose, possibly limiting early season crop growth. For grain crops like winter rye, the C:N ratio increases as the crop matures, and by flowering the C:N ratio is higher than 30:1. If your cover crop grows to the point where N tie-up is likely, the problem can be overcome by supplying a small amount of fertilizer with readily available N (20 – 25 lb./A N) close to planting time.
Annual legumes like hairy vetch have a lower C:N ratio throughout the life cycle, and no matter when they are killed are not likely to tie up soil N. However, the longer they grow in the spring before they are killed, the more N they fix. If an annual legume cover crop is allowed to grow longer than usual, the additional N it supplies should be taken into consideration. As a rough estimate, each 1 inch of growth taller than a height of 6 inches will contribute about 2.6 lb./A of N for this season’s crop if the cover crop is incorporated (SAN 2007, pp. 22-23). For example, if you normally work up a crop of hairy vetch when it is 6 inches tall, but this year you don’t get to it until it is 16 inches, about 26 lb./A additional N will be supplied by the vetch, beyond the normal amount from a 6-inch height.
If the cover crop has grown too large to be incorporated using standard equipment, it may be possible to mow it, allow it to dry down a little, and then incorporate. Or, if crop plans allow for no-till or strip-tillage, a killed cover crop need not be entirely incorporated. I wouldn’t recommend adopting either no-till or strip till on a wide scale without careful planning and trials, but this may be a good opportunity to experiment with a small area of reduced tillage.
For reduced tillage systems, cover crops may be killed with herbicide, or winter annuals like winter rye and hairy vetch may be killed by mowing or rolling at the appropriate stage. Winter rye can be mowed after it has bloomed, and may be rolled at the milky dough stage. Hairy vetch can be mowed or rolled at 75% bloom or later, when immature seed pods are visible from the earliest flowers. If not mowed or rolled at the appropriate time, the cover crop will not die and can be very difficult to control once the crop is planted.
References: SAN, 2007. Managing Cover Crops Profitably, 3rd ed. SAN, Beltsville, MD. Available from aws.sare.thinkcreativeinternal.net/Learning-Center/Books/Managing-Cover-Crops-Profitably-3rd-Edition