Utilize Biological Processes to Further Reduce Weed Pressure

Introduction

Biological controls play a major role in insect pest management in organic and sustainable farming systems, and in biointensive Integrated Pest Management as described by Dufour (2001). Organic growers use several forms of biological controls:

• Conservation biological control—providing and maintaining suitable habitat for natural enemies of pests. One example is farmscaping—a diversified planting of flowering plants that provide a season-long nectar and pollen supply for predators and parasites of major crop pests.
• Inoculative release—purchasing and releasing small numbers of a natural enemy, which then multiply and gradually bring pest populations down. Examples include the field releases of ladybird beetles, predatory stinkbugs, and Trichogramma wasps against aphids, pest beetle larvae, and caterpillar eggs, respectively.
• Augmentative or inundative release—purchasing and releasing larger numbers of a natural enemy to effect rapid pest knockdown. Examples include greenhouse release of lacewings or ladybird beetles, and spray applications of pathogens of insect pests like Bt or Beauvaria. (Caution: make sure your use of a particular product does not jeopardize your organic certification; see Can I Use This Input On My Organic Farm?)

What about biological weed control? Considerable research has been conducted on biological control of weeds with herbivorous or seed-eating insects, specific microbial pathogens of weeds (“bioherbicides”), and soil microorganisms that have the potential to suppress weed germination, emergence, or growth. At this time, however, few organic vegetable growers utilize weed biocontrol agents per se in their weed management programs. Although ongoing research may expand the role of weed biocontrol products, they will probably not ever achieve the prominence of the many biological insect pest controls that are now commercially available (Hallett, 2005). Why is this so?

Most insect pest outbreaks involve one or two insect species attacking a specific crop. Often, the pests can be controlled through conservation or augmentative release of their specific natural enemies. In contrast, many different weed species appear each year in the field. A specialist biocontrol agent that knocks out one or two species may not significantly reduce overall weed growth, while a generalist agent that attacks most or all of the weeds present would likely damage the crop as well. Furthermore, the efficacy of experimental bioherbicides depends greatly on weather, soil, and other factors that vary widely from farm to farm and from year to year.

Classical biological control—the importation and release of a natural enemy of an exotic pest—is sometimes used to combat invasive exotic plant species, most often in rangeland or natural ecosystems rather than crop fields (see sidebar).

What is the Role of Biological Processes in Ecological Weed Management?

Preventive cultural practices, together with physical controls such as cultivation, flaming, and mulching, normally comprise the bulk of an organic farm’s weed management strategy, with biological products or agents playing at most a minor role. However, biological processes may contribute to the efficacy of practices such as cover cropping, mulching, crop rotation, and farm diversification in reducing weed pressure. Biological processes that can impact weeds include:

• Herbivory—direct consumption of weed seedlings, or foliage or roots of older weeds
• Disease caused by bacteria, fungi, and other microorganisms
• Plant–soil–microorganism interactions that change weed vigor and competitiveness relative to the crop
• Allelopathy—suppression of weed growth by substances released by other plants
• Weed seed consumption
• Weed seed decay

Understanding these processes and how various cultural practices and weed control measures influence them can lead to improved organic weed management strategies. Much of this is still in the research and discovery phase; however some biological processes are sufficiently well understood and documented to be utilized as “little hammers” that enhance the efficacy of the farm’s overall weed management program (Liebman and Gallandt, 1997). Practices such as mulching, cover cropping, reduced tillage, and farmscaping for natural enemies of insect pests can provide a degree of conservation biological weed control by enhancing habitat for weed seed consumers (Menalled et al., 2006). In addition, many diversified farms utilize livestock and poultry as weed consumers, often to significant benefit. Potential farm allies to consider in designing weed management programs for organic vegetables include:

• Livestock and poultry, including weeder geese
• Weed and weed seed consumers already present on the farm
• Allelopathic cover and cash crops and varieties
• Soil microorganisms

Livestock, Poultry, and Weeder Geese

Diversified farming systems that include livestock and/or poultry in addition to vegetables and other annual crops offer expanded weed management options. For example, if a field that has been devoted to annual crop production becomes too weedy, rotating the field into perennial grass–legume pasture or hay for a few years can break the life cycle of the “weeds of cultivation” and reduce their seed banks. Good rotational grazing practices, and/or haying practices can enhance weed control and prevent the buildup of perennial pasture weeds during this period. Many farmers who use such rotations report enhanced productivity as well as reduced weed pressure when the field is returned to vegetable production.

Grazing livestock in fields immediately after vegetable harvest can help curtail weed growth and weed seed production, and running poultry either before or after a vegetable crop can reduce the populations of both weeds and surface weed seeds. The livestock can also be useful in removing diseased crop residues that might otherwise require composting, burning, or deep burial by inversion tillage for disease control.

Livestock can also be used to graze down understory vegetation in orchards, Christmas trees, and other tree plantings (silvopasture), a practice that can accomplish weed management, livestock nutrition, and fertilization (manure) simultaneously.

Repeated intensive grazing can clean up a weed-infested field for future crop production. The weeds should be grazed to the point of severe defoliation at short intervals to deplete underground reserves of perennial weeds. Running hogs with or after the cattle can intensify pressure on perennial weeds, as hogs will root out and consume fleshy roots, rhizomes, and tubers.

Weeder geese are especially effective for removing small grassy weeds from established berry plantings and certain other horticultural crops. Weeder geese are certain African and Chinese varieties of geese that have been used for centuries for selective removal of young grasses and other weeds from established plantings of vegetable or row crops, or from young orchards or vineyards.

Weeder geese require water, shade, and fencing for containment and protection from predators while they are working in the fields. They control weeds most effectively during their first year of life, and should be introduced to the fields at the age of six to eight weeks. Older, second-season geese weed much less actively, so it is common practice to utilize the geese for weed control for one season, and then finish them for meat.

Livestock can either reduce or aggravate weed problems in pastures, depending on how they are managed. Pasture weeds are simply those plants that are toxic or unpalatable to the animals. Cattle or other livestock grazing over large areas for a long time will selectively remove the most palatable species, and the pasture will become more weedy. Correct rotational grazing (management-intensive grazing) with high stocking rates for short durations in each paddock, followed by a suitable recovery period (usually several weeks) generally improves pasture quality and minimizes weed growth. Multispecies grazing can reduce the number of plant species that are rejected by the livestock and become weeds. For example, goats and sheep will consume brushy weeds, like multiflora rose and brambles, that cattle refuse. Good rotational and multispecies grazing practices can enhance overall benefits to vegetable fields that are rotated to pasture for a few years.

In utilizing farm animals for weed management, be sure to consider what weeds are present, and the grazing needs and preferences of the farm’s livestock. Grazing for weed control works best on weeds that are high quality forage, such as johnsongrass, Bermuda grass, quack grass, crabgrass, pigweed, and lambsquarters. It will not work for unpalatable species like horsenettle or some thistles, and grazing large amounts of succulent but toxic weeds such as the docks and young St. Johnswort (=Klamath weed) can endanger livestock health.

Cattle, horses, and bison are true grazers, strongly preferring grasses, clovers, and other highly palatable legumes. They will eat some forbs (other broadleaf plant species, including some common weeds) that are low in volatile essential oils. Goats are browsers, eating a high percentage of shrubs and forbs, including some with higher levels of volatile oils. Sheep are intermediate between cattle and goats in their grazing habits. Swine not only graze, but will dig into the soil to root out and consume fleshy roots of some perennial weeds.

One limitation of livestock as weed biocontrols is that most animals do not digest all the seeds they consume. As a result, their manure can carry or spread weed seeds from field to field. Thus, they control weeds most effectively when they graze before the weeds set seed.

Finally, it is important to protect food safety when utilizing livestock, poultry, or weeder geese as weed management tools in vegetable production. Under USDA Organic Standards, uncomposted manure may not be applied to the field within 120 days prior to harvest of any organic crop to be sold for human consumption. This includes any droppings left in production fields by livestock, poultry, or weeder geese. This 120-day rule is a good guideline for protecting food safety on any vegetable farm, whether or not the operator applies for USDA Organic certification. Running livestock after vegetable harvest and just before cover crop planting, and during the pasture or sod phase of a long term rotation, are effective ways to utilize animals to manage weed control and soil fertility without compromising food safety or organic certification status.

Weed and Weed Seed Consumers Already Present on the Farm

Conservation biological control—habitat protection and enhancement for beneficial organisms—is a vital component of any organic farming system. While the most dramatic pest control benefits will be manifest in reduced insect pest problems, this approach can also help reduce weed pressure. The more diverse the farm ecosystem, and the more ground cover (vegetation and organic mulch) is maintained, the more likely ground beetles and other consumers of the seeds or seedlings of major weeds will be present (Menalled et al., 2006).

Organic producers can conserve and encourage weed seed consumers by:

• Maintaining season-long habitat—mulches, surface residues, cover crops, and strips of grass or other permanent vegetation
• Reducing tillage, especially inversion tillage
• Avoiding the use of broad spectrum insecticides, using botanicals such as pyrethrum only when really necessary, and doing spot treatments for localized pest outbreaks

Allelopathic Crops

Many crops can act directly as weed biocontrol agents by releasing natural substances that suppress or hinder weed seed germination and early growth, an effect known as allelopathy. The substances may be given off by living plant roots, leached from foliage, or released during microbial decay of plant residues. Many cover crops and a few vegetable varieties have been shown to exert significant allelopathic activity against weeds, especially young annual weeds. Well-documented examples include rye, other cereal grains, sorghum, sorghum—sudangrass hybrids, forage radish and other brassicas, and sweet potatoes.

Many allelopathic relationships are quite specific (Schonbeck, 2007). For example, sunflower root exudates inhibit seedling growth of wild mustard and other broadleaf weeds, but have little effect on grasses. Sweet potatoes strongly inhibit yellow nutsedge and velvetleaf, but have relatively little effect on pigweed, morning glory, and coffee senna. In no-till field trials, rye residues are strongly allelopathic against pigweed and lambsquarters, but not ragweed.

Recently killed rye mulch is highly suppressive toward lettuce and other small-seeded vegetables, but much less so on large-seeded vegetables like snap beans, and has no adverse effect on transplanted tomatoes, peppers, and cucurbits. Transplants and large seeds are inherently less susceptible to allelopathic suppression. Furthermore, the allelochemicals given off by a cover crop mulch are usually concentrated near the soil surface, while transplants and large seeds are planted deep enough so that their roots escape exposure to the high allelochemical concentrations near the soil surface.

As specific allelopathic relationships become better understood, crop rotations and cropping systems can be designed to give crops an edge over the major weeds present in a given field. The tolerance of transplanted and large-seeded vegetable crops to most allelopathic cover crop residues  has practical application in fields whose weed floras are dominated by small-seeded annuals that germinate from near the soil surface. For example, tomatoes and other transplanted summer vegetables often do well in a killed rye–vetch mulch, which usually provides several weeks’ effective suppression of summer annual weeds.

Soil Microorganisms and Weeds

The ability of the soil’s microbiota to influence the growth and competitiveness of weeds relative to crops has been a subject of much fascinating research. In field studies, microorganisms have been found on weed seeds (Pitty et al., 1987) and seedlings (Lindquist, 1995) that can attack the weeds at these early life stages, although microbially-mediated decay appears to have a much smaller impact on weed seed banks than do ground beetles and other macroscopic seed predators (Gallandt, 2006). Plant–soil–microbe relationships are highly complex, and research findings have not yet been consistent enough to warrant recommendation of procedures to introduce, encourage, or limit certain soil microbes as weed control tactics. However, findings do indicate that organic farming practices that promote an abundant, balanced, and diversified soil microflora may tip the competitive balance in favor of crops over weeds. This can occur through several mechanisms:

• Healthy soils support healthy, vigorous crops.
• Some weeds are better adapted to stressed soil conditions and unbalanced or depleted soil microfloras than are most crops—good organic management ameliorates this imbalance and restores soil life.
• Many of the most serious weeds of cultivated fields do not benefit from mycorrhizal associations (root symbioses with specific soil fungi) (Jordan et al., 2000; Vatovec et al., 2005), whereas most vegetable, grain, and row crops form highly beneficial symbioses with the mycorrhizal fungi often present in healthy, organically managed soils.
• Soil fungi and bacteria can invade weed seeds as they age in the soil, and may shorten the half life of weed seed banks to some degree.

The potential for reducing weed pressure through specific management practices directed toward the soil microflora remains to be elucidated through further study. This may emerge as a significant "cutting edge" area in ecological weed management research.

Realistic Expectations for Weed Biological Controls

It is not realistic to expect an allelopathic cover crop mulch, a flock of chickens or weeder geese, a guild of weed seed consumers, or other biological agent(s) to control weeds like an herbicide and thereby make cultivation unnecessary. For this reason, some weed scientists hesitate to recommend any of these practices for organic weed management. However, the ecological approach entails the use of multiple tactics to reduce weed pressure to below economically damaging levels (Liebman and Gallandt, 1997). In this context, a biological agent or process that reduces weed growth or weed seed bank populations by even 25 percent (a level of weed reduction often attained or exceeded in research trials with these practices) can make an important contribution, especially when it also provides other benefits, such as nitrogen fixation and soil conservation by a cover crop. Cover crops, conservation of weed seed predators, grazing livestock, and biologically active soils can serve as effective “little hammers” to enhance the overall weed management program.

References and Citations

• Dufour, R. 2001. Biointensive integrated pest management (IPM) [Online]. National Sustainable Agriculture Information Service, ATTRA Publication #PO49. Available at http://www.attra.org/attra-pub/ipm.html (verified 23 March 2010).
• Gallandt., E. R. 2006. How can we target the weed seedbank? Weed Science 54: 588–596. (Available online at: http://dx.doi.org/10.1614/WS-05-063R.1) (verified 23 March 2010).
• Hallett, S. G. 2005. Where are the bioherbicides? Weed Science 53: 404–415. (Available online at: http://dx.doi.org/10.1614/WS-04-157R2) (verified 23 March 2010).
• Jordan, N. R., J. Zhang and S. Huerd.  2000.  Arbuscular-mycorrhizal fungi: Potential roles in weed management.  Weed Research 40: 397–410. (Available online at: http://dx.doi.org/10.1046/j.1365-3180.2000.00207.x) (verified 23 March 2010).
• Liebman, M. and E. R. Gallandt. 1997. Many little hammers: Ecological approaches for management of crop–weed interactions. ed. p. 291–343 In Ecology in agriculture. L. E. Jackson Ed. San Diego: Academic Press.
• Lindquist, J. L., B. D. Maxwell, D. D. Buhler, and J. L. Gunsolus. 1995. Velvetleaf (Abutilon theophrasti) recruitment, survival, seed production and interference in soybean (Glycine max). Weed Science 43: 226–232. (Available online at: http://www.jstor.org/stable/4045488) (verified 23 March 2010).
• Menalled, F. D., M. Liebman and K. Renner. 2006. The ecology of weed seed predation in herbaceous crop systems. ed. p. 297–327 In Handbook of Sustainable Weed Management. H. P. Singh, D. R. Batish and R. K. Kohli Eds. Binghamton, NY: Haworth Press.
• Pitty, A., D. W. Staniforth, and L. H. Tiffany. 1987. Fungi associated with caryopses of Setaria species from field-harvested seeds and from soil under two tillage systems. Weed Science 35: 319–323. (Available online at: http://www.jstor.org/stable/4044591) (verified 23 March 2010).
• Schonbeck, M. 2007. Biological Control of Weeds [Interactive online course]. Module H. In Integrated pest management for organic crops. Cooperative Extension Curriculum Project. Available via: http://www.sare.org/Learning-Center/Courses-and-Curricula/Southern-SARE-s-Integrated-Pest-Management-for-Organic-Crops-Course (verified 23 March 2010).
• Vatovec, C., N. Jordan and S. Huerd.  2005.  Responsiveness of certain agronomic weed species to arbuscular mycorrhizal fungi.  Renewable Agriculture and Food Systems 20(3): 181–189. (Available online at: http://dx.doi.org/10.1079/RAF2005115) (verified 23 March 2010).