A few years ago I was leading an agronomy training seminar and the topic was weed identification. One of the participants said he could identify them all. He went to the first plant and said, “One quart” and the next weed he said “one quart” and the same thing for the next several in a row. He then said, “Why do we need to identify them? A quart of Roundup gets them all” How times have changed! Now it seems like Roundup doesn’t kill anything. Pigweeds, like Palmer amaranth, are a particular problem. Obviously, we need to figure out a way to kill the pigweeds that Roundup no longer does.
Pigweeds at first glance seem like a superweed. They can grow three times as fast as soybeans and can produce up to a 600,000 seeds per plant. If one plant goes to seed per square foot, that can result in over 2 BILLION seeds per acre. However, pigweeds have weaknesses as well. Their seeds are very small, making them very weak in the seedling stage with limited energy reserves. The seeds also require both nitrate and sunlight in order to germinate. Another weakness of pigweeds, when compared to soybeans, is that pigweeds cannot fix nitrogen like soybeans can and must rely on the soil for their fertility needs.
Of course, we could simply switch to different herbicides to kill the weeds. Some feel that soybeans tolerant to dicamba or 2,4-D will solve the problem, but it seems that those two compounds are becoming ineffective at controlling pigweeds in wheat stubble. Does anyone really think that exposing pigweeds to multiple shots of 2,4-D or dicamba per year will result in less herbicide resistance? Wasn’t that the same strategy that gave us pigweeds that glyphosate doesn’t kill? While rotating herbicide mode of action is definitely a wise strategy for preventing herbicide resistance, utilization of non-chemical weed control measures in addition to herbicides is probably even wiser. To many people, this means going back to tillage. However, research has shown that pigweed germination and growth is enhanced by tillage. Tillage also has the obvious drawbacks of reduced soil moisture, reduced soil organic matter, and increased soil erosion. So how can we possibly control pigweeds without herbicides or tillage?
One innovative method of aiding control of pigweeds in soybeans, as well as many other weeds, is to plant the soybeans no-till into the residue of a herbicide killed winter cereal cover like triticale or rye. There are two mechanisms at work here. The first is a phenomenon called crop allelopathy, in which the rye or triticale residue contains chemical compounds that act like natural herbicides that suppress pigweeds but actually seem to stimulate soybeans. Scientists have identified three compounds found in rye and triticale that have this effect. While this may seem far-fetched to some, remember that the herbicide Callisto was derived from an extract of the Mexican bottlebrush or Callistemone plant. The second mechanism takes advantage of the fact that soybeans are a legume and can make their own nitrogen, while weeds cannot. If corn was the previous crop, and was fertilized for optimum yield, usually there is a substantial amount of carryover nitrogen left in the ground at the time of soybean planting. If left alone, this nitrogen delays the time until soybeans nodulate (soybeans must become nitrogen deficient to trigger nitrogen fixation) and feeds weeds. Planting a winter cover crop of rye or triticale will scavenge this nitrogen and convert it into a mulch that can control soil erosion and conserve moisture in addition to controlling weeds. This nitrogen does not disappear, it eventually becomes available again as the mulch rots to benefit the crop after soybeans. The mulch of a cover crop also prevents sunlight from reaching pigweed seeds, and if the mulch is thick enough pigweed seeds can actually starve to death before getting to sunlight. Soybeans, with their large seeds, can emerge through a fairly thick mulch.
There is also an even more innovative method that has been found useful, and that is the inoculation of soybeans (as well as many other crops like sorghum) with a beneficial fungus called mycorrhizal fungi. These fungi occur naturally in prairies, colonizing the roots and acting like root extensions to help draw in both water and nutrients. They have been exterminated from most cropland, as they cannot survive long fallow periods without living roots. Mycorrhizal fungi have gained fame for improving the drought tolerance and nutrient uptake of crops, but have also recently been found to aid weed control. Many weed species, like pigweeds, lambsquarters, and mustards, are not colonized by these fungi and do not benefit from them, while most crop species like corn, soybeans, sorghum, wheat, and alfalfa, are benefitted. Thus inoculation does not actually kill weeds, but instead makes the crop roots more competitive against the weed roots. A recent study by North Dakota State University found that inoculating sunflowers with mycorrhizal fungi reduced weed biomass by 54%. The mycorrhizal fungi have an additional benefit that has also come to light. They secrete a lubricant called glomalin that dramatically improves soil structure and organic matter content. Inoculated soils have better rain infiltration, reduced compaction, and greater root depth, further aiding performance during drought in future seasons.
While they may not replace herbicides, the combination of mycorrhizal inoculation and a winter cover crop can provide a very synergistic effect for not only improved weed control but also the creation of a more drought tolerant field.