Harvesting first cutting alfalfa and then no-tilling corn into the alfalfa stubble can be challenging and certainly risky. Two of the biggest challenges are 1.) Getting acceptable control of alfalfa after first cutting, and 2.) Even with acceptable control, what impact can the dying alfalfa crop have on corn yield? To start, choose the correct herbicide program and make sure there is adequate regrowth before making the herbicide application. Cutting the alfalfa a bit high and leaving some leaf tissue will probably increase successful control. In some research we conducted at Rock Spring and Landisville, we harvested the alfalfa in early to mid May and no-tilled corn into the stubble shortly thereafter. We compared PRE applications about 5 days after harvest to an EPOST application about 10 days after harvest (6 inches of regrowth) to a MPOST application about 17 days after harvest (10 inches of regrowth). Our herbicide programs focused on 2,4-D and dicamba (Banvel or Clarity) alone or in combination. The combination of these two herbicides is probably the most consistent for control of alfalfa. Glyphosate will add some alfalfa control and is a definite must if annual or perennial grasses are present, but it really doesn't alter the success of application timing. In short, none of the PRE treatments were effective (40% or less). Waiting about 10 days after harvest to make the herbicide application provided 79 to 99% control by early July with the combination of 2,4-D + dicamba providing the best results. By waiting 17 days after harvest when the alfalfa was 10 inches tall, all treatments effectively killed the alfalfa.
Now the bad news; because of dry weather in May, corn yields were severely reduced in all treatments. Yields averaged 8 bu/acre in the untreated checks, 52 bushels in the PRE treatments, 84 bushels in the EPOST timing and 76 bu/acre in the MPOST timings. Yields were fairly similar at the two locations and although we did not include an alfalfa-free check, typical corn yields at both research farms were at or above normal for the regions. So, even with adequate alfalfa control a month into the growing season, we could not recover from the early season competition and the moisture deficit. Although this season is starting out much differently, keep in mind the importance of alfalfa regrowth for adequate control (6 inches is the minimum) and how soil moisture may make the ultimate yield decision.
Alternatively, and still somewhat risky, glyphosate can be used as a preharvest application for stand removal in alfalfa and clover. Glyphosate may be applied to declining stands to help control the legumes and other plant species. This can be particularly helpful for control of perennial grasses including quackgrass. If the crop is to be harvested or grazed by livestock, use up to 1.5 lb ae/acre (2 quarts of a 3lb ae/gal) in alfalfa or 1.125 lb ae/acre (1.5 qt of a 3lb ae/gal) for clover. The minimum interval between application and harvest is 36 hours for alfalfa and 3 days for clover. This treatment will not provide sufficient control of the legume, but it will suppress it and potentially allow for more timely control and reduced competition in no-till corn. In no-till corn, a POST application should be made to control the legume regrowth. The plant growth regulator herbicides (2,4-D, dicamba, clopyralid - Stinger and Hornet, as well as Distinct or Status), should help control alfalfa post in corn. In Roundup Ready corn, glyphosate can also help suppress the legume and a growth regulator herbicide could be tank-mixed with the glyphosate. This approach should at least insure more timely control of the forage, but will not prevent the negative effects of a dry spell early in the growing season.
Source: William Curran, Penn State University