There are plenty of opinions about when to terminate cover crops prior to planting cash crops, ranging from weeks before to planting green.
Part of this decision should be based on what benefits you hope to achieve with the cover crop: nitrogen fixation, sequestration, soil moisture management, etc. You should also consider difficulty or speed of cover crop control, herbicide options and crop safety, and how much cover crop residue can be reasonably managed. We are also thinking a lot about the potential for slugs and insect pests, and of course cash crop performance based on all these factors.
Cereal grains such as rye, wheat, and triticale are probably the most common covers in our area. We are also seeing greater adoption of annual ryegrass and some legumes with the clovers probably leading the pack. We often hear about problems killing certain cover crops during spring time and especially cereal grains and annual ryegrass.
We have had a few experiments over the last few years with cereal rye, wheat and annual ryegrass cover crops trying to replicate some of the problems encountered in the “real world." In general, we did not encounter performance issues that we could not predict; meaning that the treatments we thought would work did. In fact, many treatments worked better than they should have. Assuming you choose an effective herbicide program and the application is accurate, sufficient cover crop growth and air temperatures at the time of application are probably the most important variables. Herbicide effectiveness ratings for cover crop control are provided in Table 1.
As examples, back in 2009, we had a test at Landisville where we compared glyphosate (Roundup PowerMax) rates ranging from 16 fl. oz/acre up to 44 fl. oz/acre alone and in combinations with citric acid, atrazine or simazine, standard rates of Touchdown Total, generic Clearout 41 Plus, Gramoxone, and Gramoxone plus atrazine. AMS was included with all glyphosate treatments and NIS with Gramoxone.
Treatments were applied on April 13 between 9 a.m. and 11 a.m. when the cereal rye was 8 to 10 inches tall and the low temperature the night before was 23°F with a high of 53°F the day of application. To make a long story short, all the glyphosate treatments provided more than 90% control by the June 3 evaluation. Gramoxone alone at 3 pt/acre only provided 70% control, but more than 90% control when 1 lb of atrazine or simazine was included.
In an annual ryegrass experiment in 2012 at Rock Springs, we compared glyphosate (Roundup PowerMax 4.5L) rates as low as 11 fl. oz/acre up to 44 fl. oz/acre with or without AMS, at 10 GPA vs 20 GPA, at 4 mph vs. 8 mph, flat fan vs. air induction tips, citric acid or not, and some common tank mixtures like atrazine, Lumax, Balance, Prequel, etc. Keep in mind that the recommended glyphosate rate for annual ryegrass control is generally at least 1.125 lb ae/acre (see comment below).
In the end, most treatments provided 90% control or better by June 4 and only two treatments were less than 90% (reduced rate of 11 fl. oz/acre applied at 10GPA). We had a mild spring in 2012 and herbicides were applied on April 19 to ryegrass that was 8 to 10 inches tall and vigorous at the time of application with a low/high air temperature of 37/71°F. You can view the results of both of these trials on-line in the 2009 or 2012 Field Trials Annual Report .
Bottom line: Make sure the cover crops are actively growing and choose warm sunny days when herbicides are applied.