Yesterday a farmer asked what herbicide to use for ragweed control at this time of year. I am not a weed researcher so I asked the expert: Dr. David Jordan with the North Carolina State University. He recommended Cobra, but also cautioned with its use at this stage of development. Cobra cannot be applied within 75 days of harvest. This means that if the farmer sprays it now, he will have to postpone harvest until end of October. 2,4-DB (40 to 65 days pre-harvest interval) and Select (40 days pre-harvest interval) were also recommended for late season broad leaf weed control.

Ragweed mature plant.

Dr. Hillary Mehl’s Peanut Cotton InfoNet (http://webipm.ento.vt.edu/cgi-bin/infonet1.cgi) service shows that peanut accumulated from May 1st and until mid-August across peanut growing region in Virginia from about 1840 heat units (at Waverley) to 1980 (at Skippers). The number of heat units accumulated only from August 1st to mid-August is about 200. Bailey and Sugg require about 2600 heat units to mature; this is approximately 690 more heat units needed to reach maturity. Assuming similar temperatures for the rest of August and September, the 690 heat units can be achieved by the first week of October, or the second week of October if September gets cooler, for a May 1st planted peanut. This means that some herbicides still can be used for weed control in peanut at this time, in particular for late planted peanut; but herbicide label needs to be carefully checked.

Determining the optimum harvest maturity is more complicated than checking the heat units and counting the number of days from planting, as you very well know. Therefore, I recommend that growers take advantage of the pod-blasting clinics that Extension Agents will organize in September. One will be on September 5th at Emporia. I will also send advisories with peanut maturity starting after the first week of September.