You can't spray new alfalfa too early

Wednesday, November 5, 2014

Research shows that, to get good weed control, you need to start spraying alfalfa as early as the 0.5 trifoliate. Doing it later can cost you yield

by PAT LYNCH

This is a lesson I relearned this past spring. A seed sales person sent me a picture of an alfalfa stand where parts of the field were good and other parts were poor and weedy. The customer was wondering why the alfalfa was so poor. I told him "the answer is pretty simple. The weeds crowded out the alfalfa."

The phone explanation was not good enough for the grower. So I went to the field. Once there, we got on our knees and started to pull weeds. Once we pulled the weeds, you could see some small alfalfa plants. In the weed-free areas, the alfalfa was good.

We visited with the farmer and I told him the field was sprayed too late. He said: "No. I called the custom spray operator and he came." He wasn't really sure what stage the alfalfa was at when he called. So I asked him how long it was between calling and spraying. He said: "Not that long, maybe a week." To which I replied that, for the weed pressure there, he called too late and he sprayed too late. The weeds were shepherd's purse. They had seed pods. This indicated they were well established when sprayed. It was probably sprayed three or four weeks too late.

I told this to Paul Wight of Pickseed and Paul said he sees this often. He became a bit excited as he explained: "You know, farmers don't get the 'weed-free period' concept of weed control in forages. They know about the weed-free period in corn and soys, but not in forages. Too often, they let weeds get too big in forages before they spray."

Paul gave me information from the University of Wisconsin indicating that, under heavy weed pressure situations, you needed good weed control from the 0.5 trifoliate to the seventh trifoliate. If you had seen the weeds in this field, you would agree.

Part of the problem is that I and others have been telling you for years that you should spray alfalfa at the second to the fourth trifoliate. This is based on the research to register Embutox. That research was with a nurse crop. Weeds are more competitive in direct-seeded alfalfa and must be removed earlier. The reality is that once you recognize that alfalfa is at this stage and then make arrangements to spray, the weeds have grown. I am going to change my recommendations to spray at the 0.5 to second trifoliate stage.

The other issue with this field with shepherd's purse was rate. The producer was told he could cut the rate of Embutox by 50 per cent if he used an oil. This does not work. It may work where the weeds are small and at light pressure, but not on big weeds.

Embutox is the butyric form of 2, 4–D. Weeds can change this formulation into 2, 4–D amine, which kills weeds. Small alfalfa plants cannot change the butyric form to the amine form. Once alfalfa plants are at the fifth trifoliate, they can change butyric form to amine form. So spray early to reduce the "herbicide effect" on new seedings.

One problem with only using 2,4-D butyric is no mustard control. I was part of the team that researched and registered the Embutox and MCPA mix. This mix is harder on alfalfa than Embutox alone. So if you do not have common mustard, there is no reason to add MCPA.

Remember this article when Roundup Ready alfalfa is planted. When Roundup Ready corn and soybeans were introduced, growers waited until the weeds were big before they sprayed. This cost yield.

Let's not make the same mistake with Roundup Ready alfalfa. Spray early. BF

Consulting agronomist Pat Lynch, CCA (ON) formerly worked with the Ontario agriculture ministry and with Cargill.