Mark VanGessel, Extension Weed Specialist; mjv@udel.edu
When you consider weed control, do you immediately consider herbicides and nothing else? That is common given we have come to rely upon crops that can be sprayed with broad-spectrum herbicides. We have become accustomed to spraying many of those herbicides when it fits our schedule. In addition, since they are very safe to the crop and relatively inexpensive, we know we can spray a second time if necessary.
Unfortunately, for many farms, those days are gone. Due to glyphosate-resistance, additional or alternative herbicides are needed and they are not as effective as glyphosate once was. On the other hand, if you do not have resistant weeds and you want to continue to rely on glyphosate, you need to incorporate multiple non-chemical tactics to reduce the risk of developing herbicide-resistance. Either way, now is the time to plan for next year.
Are you pulling AND removing from the field those Palmer amaranth or common ragweed plants that survived and are in the process of forming viable seeds. If you have too many, are you making plans to harvest those fields in a manner to minimize the spread within the field and to thoroughly clean your combine before you leave that field. If you have fields heavily infested, are you planning to plant corn or sorghum in those fields instead of soybeans? Corn is planted earlier and Palmer amaranth is not growing as quickly, providing a longer period of time to control them. Atrazine is an option for both of these crops and currently it provides excellent control of both Palmer amaranth and common ragweed.
Are you mowing the field edges to limit weeds in those areas from producing seeds and minimize the chance that they spread into your field? Have you been making notes of where the perennial weeds are located? Fall is the best time to treatment most perennial weeds in this region. Glyphosate will control most perennial weeds and you want the weeds actively growing at time of application. So harvesting with the combine header high will minimize damage to the weed and allow it to recover faster. Likewise, you do not want to disturb those weeds for 10 to 14 days after application. Glyphosate needs time to translocate throughout the weed’s root system to provide maximum level of control. So, delay mowing or field work after application; as well as spray early to avoid frost. A fall application in one year is seldom enough to eliminate a perennial weed patch, but two to three years of consistent effort will have a dramatic reduction of number and size of plants in the patch.
Do you have fields (or portions of fields) with severe weed infestations? Does strategic tillage fit into your plans? Strategic tillage is moldboard plowing to bury the weed seeds deep into the soil to reduce the number of plants emerging next spring. Strategic tillage requires the field not be moldboard plowed for at least 4 to 5 years to allow those weed seeds to decay deep in the soil and not brought back to the soil surface. Strategic tillage in the fall with a cover crop planted immediately afterwards minimizes negative impacts on the soil.
Have you arranged for planting a cover crop? Cover crops have many benefits and weed management is one of them. If horseweed is a problem then be sure to use a grass cover crop (cereal rye, small grain, triticale) or early-planted forage radish. Legumes do not produce enough biomass to help suppress fall-emerging horseweed. While a cover crop will reduce the number of horseweed plants and suppress their growth, they will not eliminate it. Therefore, when balancing delayed cover crop termination and needing to control horseweed, you may need to have an Enlist or Xtend soybean variety.
If Palmer amaranth or common ragweed are giving you fits, then plan on terminating your cover crop as close to planting as possible in order to provide the most benefit. An additional one to two weeks of growth in the spring can have significant impact on amount of biomass produced. Is your planter set up to handle larger amounts of biomass or do you need to consider some adjustments now and allow for shop time this winter working on your planter. Before you order and plant your cover crop, how are you going to terminate it? Grass cover crops generally are susceptible to glyphosate. However, annual ryegrass can be difficult to kill with glyphosate in the spring and I do not recommend using annual ryegrass as a cover crop. If you are planting a legume or a brassica, glyphosate alone may not kill them so can you use 2,4-D or dicamba and/or paraquat plus a triazine (like atrazine or metribuzin)? Do you need to make any modifications to your sprayer to terminate taller cover crops?
What herbicides did you use this season? It is critical that you know what herbicide groups you used this year and be sure that you rotate to different, effective and multiple sites of action to manage herbicide-resistance. Be sure you have a good herbicide rotation over 2 to 3 years so you are not overusing any herbicide sites of action.
Are you selecting varieties and hybrids based on your field conditions? If using cover crops and terminating them late, be sure to select for good seedling vigor. The soil temperature is likely to be a few degrees cooler and seed selection should account for that.
This is not an exhaustive list of things to consider for next year’s weed management program, but some steps taken now can have big dividends next year and beyond. It is not too early to start planning and improving your weed management for next year.