6-1 RISK MANAGEMENT GUIDE FOR ORGANIC PRODUCERS CHAPTER 6 Weed Management JEFF GUNSOLUS DON WYSE KRISTINE MONCADA CARMEN FERNHOLZ n Chapter 5—Weed Biology, we discussed how weeds grow Iand compete with crops. While there inevitably will be a certain level of weeds, it is the grower’s task to make sure that the weeds present do not exceed damaging thresholds that limit crop yields. In this chapter, we will address prac- tical weed management techniques for the organic producer. Weed management for organic crop production falls into two categories: cultural weed control and mechanical weed control. A third type of weed control using chemicals is another option, but will not be discussed in this man- ual as organic herbicides are not commonly used on a large scale in agronomic crops. JOHN DEERE JOHN Figure 6-1. Row crop cultivator. 6-2 RISK MANAGEMENT GUIDE FOR ORGANIC PRODUCERS crops can prevent weeds from COVER CROPS AND Cultural weed adapting to the planting regimen. SMOTHER CROPS control Competitive perennial crops Winter cover crops and smother such as alfalfa are especially ef- crops are two additions to rota- fective in reducing seed banks Cultural weed control includes tions that can have an effect on of annual grasses and broadleaf diversifying rotations, delaying weeds. Winter cover crops can weeds and in suppressing pe- planting, changing planting rate, occupy the niche that exists after rennial weeds like thistle. The timing of nutrient application, a summer crop is harvested and advantage of alfalfa is that it is and using cover crops (see Chap- before the next season’s crop is harvested three or four times dur- ter 13 for more information on planted. They can displace weeds ing the growing season which cover crops). Cultural methods that might otherwise germinate prevents annual weeds from are the first line of defense in in the fall or very early spring. flowering or producing seed and weed management. Winter rye and hairy vetch resi- depletes root reserves of peren- due also has been shown to have ROTATION nials. In addition, its continuous allelopathic effects on some ger- Diversifying a rotation is the cover provides a habitat for ani- minating weeds, but this effect strongest tool against weeds. mals that consume weed seeds. is short-lived and lasts only until Over time, using similar planting Cover crops in rotations can also the residue decomposes. See dates, and cultivation timing will play a role in preventing weed Chapter 13 for more information select for weeds that are adapted infestations. on the benefits and risks of win- to these strategies. Varying crops Because rotation is so impor- ter cover crops. by different planting date (e.g. tant for organic farmers, we have A smother crop is a vigorous- wheat is planted several weeks devoted an entire chapter to the ly-growing crop that growers use before soybean) or growing a pe- topic in Chapter 2. to suppress weeds. Generally, a rennial crop in rotation with row smother crop is not harvested, but plowed down instead. Two examples of summer smother crops used in the Upper Midwest Crop seed size include buckwheat and sudan- Crop varieties vary in seed size and those with larger seed size of- grass (or sorghum-sudangrass). ten have increased competitiveness against weeds. Large seed mass Smother crops may suppress gives an initial head start to the crop at the time it is most critical. some perennial weeds, but a Small-seeded weeds are capable of fast initial growth, but are de- perennial crop such as alfalfa pendent on photosynthesis and outside nutrients. A large crop seed grown for two or more years has its own internal resources and can provide a jump start over generally will be a better choice weeds under the right conditions. Crop seed size is one of many to affect perennial weeds in the factors to consider in crop variety selection. long-term. The primary risk in 6-3 RISK MANAGEMENT GUIDE FOR ORGANIC PRODUCERS using smother crops is that their effectiveness in weed control may be inconsistent and unpre- dictable. Additionally, a smother crop such as buckwheat has po- tential to become a weed itself. DELAYED PLANTING Delayed planting is an option in weed management, but it can reduce crop yields. However, for many organic farmers, delayed planting can be the correct choice Figure 6-2. Sorghum-sudangrass grown as a smother crop. in highly weed-infested fields. early in the season. Producers to decide if a delayed planting Delaying planting allows for need to monitor their fields and strategy is warranted. They also more mechanical weed control be constantly aware of which need to consider if a potential de- operations to be performed prior weed species (see Chapter 7 for crease in yield is justified. to crop planting with the prospect weed identification) are present of fewer weeds in the crop. Organic farmers in the Upper PRODUCER PROFILE Midwest balance the potential yield gains from improved weed SMOTHER CROPS control against potential yield An organic producer from Wadena County uses buckwheat as a losses from delayed planting by smother crop to control Canada thistle and quackgrass. Buckwheat planting corn around May 15 and easily reseeds so he notes that control of buckwheat before it goes to planting soybean between June 1 seed is important to prevent volunteers. Oats are not a good choice and June 15. Cool-season crops to plant after buckwheat because of the danger of seed contamina- like small grains or field pea that tion by potential buckwheat volunteer plants in oats. Buckwheat can are planted early in the spring are be planted in June at a rate of up to 50 pounds/acre. not likely to benefit from delayed A farmer from McLeod County uses sorghum-sudangrass (Figure planting. 6-2) to suppress Canada thistle and quackgrass, but he finds that large-seeded broadleaf weeds like velvet leaf are not effectively Delayed planting can reduce controlled. He plants in the middle of June (no later than June 25th) populations of early-emerging to get a good stand. Sorghum-sudangrass will winter-kill so it can weed species (Figure 6-3). See be tilled in the fall or spring. Sorghum-sudangrass is a warm-season Table 5-11 from the previous crop planted when soils have warmed in June at a rate of 35 to 40 chapter for weeds that emerge pounds/acre if drilled or at 40 to 50 pounds/acre if broadcast. 6-4 RISK MANAGEMENT GUIDE FOR ORGANIC PRODUCERS 100 90 80 70 l 60 50 become more competitive and % contro 40 30 subsequently compete for light and Mid-May 20 water resources. Examples where June 10 this most frequently occurs is with 0 n n n n small grains like wheat and barley, which is why applying manure Cultivatio Cultivatio or compost before planting these RH+cultivatio RH+cultivatio crops is not recommended. For Rotary hoe (RH) Rotary hoe (RH) Lambsquarters Pigweeds crops with high nutrient needs, Figure 6-3. Effect of delayed planting on control of lambsquarters and pig- providing proper levels of nutrients weeds in soybean, 1989-1991, Rosemount, MN. Soybeans were planted mid-May can lead to increased competitive- or early June and treated with the rotary hoe, cultivation or both. Delayed plant- ing usually led to increased weed control, particularly in lambsquarters which ness against weeds. emerges earlier than the pigweeds. Adapted from Buhler and Gunsolus, 1996. The timing of fertilizer ap- plication can be important. When PLANTING RATE NUTRIENT APPLICATION nutrients are applied too early for Increasing the planting rate is Just as nutrients nourish the crop, crop utilization, weeds may be fa- another common strategy for they can also nourish the weeds. vored. Producers may be tempted organic growers. Higher crop There are two issues with compost to delay fertilization. However, the densities can lead to greater and manure application – how the unpredictable release of nutrients competitiveness against weeds. nutrients affect growth of existing from organic fertilizers will make In addition, higher planting rates weeds in the field and the potential using nutrients to manage weeds a can compensate for crop losses introduction of new weed seeds. challenge. that occur during mechanical Compost application in the weed control operations. The spring can stimulate ger- bigger the weed problem, the mination of early-emerg- Table 6-1. Effect of nitrogen on weed growth. Increasing nitrogen levels more effective increasing plant ing weeds. The growth of can have a positive or negative ef- many weeds like foxtails, population will be. Less competi- fect on weed growth depending on tive crops like flax may show a pigweed and lambsquar- the species. Adapted from Davis, 2005. ters is stimulated by nu- greater yield increase. For guide- N INCREASES N INHIBITS lines as to whether to increase trients such as nitrogen GROWTH GROWTH crop plant populations, producers that are intended for crops Velvetleaf Common ragweed should consult the chapters in (Table 6-1). When weeds Foxtail Canada thistle Redroot pigweed this manual for individual crops have a stronger response Lambsquarters to high fertility than the and with local extension person- Giant ragweed nel for optimum planting rates crop does, there will be a Pennsylvania smartweed for their area. negative effect on yield Eastern black nightshade because the weeds will Quackgrass 6-5 RISK MANAGEMENT GUIDE FOR ORGANIC PRODUCERS The method of manure appli- the minimum temperature for cation can also have an effect on composting is 131° F. Therefore, weeds. When manure is injected, some weed seeds will still sur- Mechanical nutrients are placed closer to where vive under common composting the crop (instead of weeds) can use situations, but the overall number Weed Control them.