Managing Weed Seeds: Encourage Predation!
By Jeff Gunderson

This article was first printed in the July/Aug 2009 issue of the Organic Broadcaster, published by the Midwest Organic and Sustainable Education Service.

As you look out over your farm on a mid-summers day it may seem like a quiet and peaceful place. Well, look closer. Get down there on your hands and knees and really look, because there’s a lot going on that you may be missing. Down there under the crop canopy is a whole host of organisms hard at work disposing of all those weed seeds that have inevitably accumulated in your soil since the dawn of time.

Weed seed predation has traditionally been overlooked as a significant contributor to seed mortality; however, ground dwelling animals can account for a considerable reduction in the weed seed bank. Organisms as diverse as birds, rodents, beetles, crickets, ants and slugs are known to play an integral role in weed management by consuming weed seeds after they have been shed by the plant. In fact, ground beetle communities can consume thousands of weed seeds every day, and up to 70% of the seed produced annually by certain weeds can be consumed by predators if conditions are optimal.

As a process mediated by the ecosystem, predation can be highly variable due to the complex set of factors involved, such as seed size and energy content, local and microclimate, and topography, as well as habitat. Still, research and observation has indicated that management decisions, such as crop rotation and cover cropping strategies, as well as the timing of tillage, can play an important role in the rates of weed seed predation.

Who is Eating What
As stated, many different animals contribute to seed predation, but invertebrates are usually found to be the greatest consumers of weed seeds. One of the most widespread and active predators are beetles of the Carabidae family. These are the common beetles you see scrambling about in your fields, ranging from black to brown to green. Carabids are highly active in agricultural fields, and females lay eggs in weedy areas. They are opportunistic feeders, meaning they eat what they can get, rather than seeking out only the best and most nutritious seeds. In fact, Carabids can be responsible for up to 90% of weed seed predation.

Common field crickets and vertebrates such as mice have also been identified as major seed predators, and at certain times of the year may be the predominant seed consumer in your fields. Predation by mice can even be more consistent throughout the year, as compared to predation by invertebrates, because mice have greater mobility. This allows them to move in and out of fields quickly when little cover exists. The abundance of invertebrates is constrained by their seasonal life spans, making them less common  in fields from late fall through early spring.

Management Makes a Difference
No matter what animal is involved, variance in seed predation rates has been linked to changes in management on the farm. The two most studied practices that can impact predation are cover cropping and tillage. Carabids are known to be fond of cover and are more common in fields where cover cropping is practiced, which provides habitat not only while growing, but also as residue in the following crop. Mice also are more likely to remain foraging in fields with some cover.

While weed seed predation can occur at any time of the year, rates increase from mid-summer to early fall, the time corresponding to weed seed shed. This is also the time when predation by invertebrates becomes important, so one of the most practical ways to manage for weed seed predators is to ensure that something is growing in your fields at this time, whether it is a cover or a cash crop. Predation rates are positively linked to crop canopy light absorption, and if there is a late-growing crop to provide habitat predation rates will increase. For example, underseeding wheat with red clover is known to increase rates of weed seed predation.

However, it isn’t quite that simple. The growth habits of different crops and the management associated with these different crops can affect the mobility of invertebrate predators, and by extension, their ability to access their prey. In fact, predation rates by crickets are higher in the corn or soybean phases of a rotation than in the alfalfa hay phase, which is probably related to the ability of the crickets to move more freely and access seeds more readily in the row crops. The kicker here is that the inclusion of the alfalfa phase increased weed seed predation in the row crop years of the rotation compared to row crops grown without the alfalfa. This indicates that a more diverse rotation is beneficial for invertebrates, and that crop residues from preceding years can provide habitat. On the other hand, be aware that thick cover crop residues can also impede invertebrate movement in the subsequent cash crop, and some disturbance of these residues can be beneficial.

The Effect of Tillage
The timing and frequency of tillage can have significant effects on the rates of weed seed predation occurring in your fields. Evidence exists that tillage may disrupt the foraging of weed seed predators, and predation under no till management is greater in certain circumstances. Tillage may also bury weed seeds, making them unavailable to predators, or directly kill those predators. However, while tillage can have a detrimental effect on weed seed predation, if performed in a timely manner it may prove beneficial by breaking up residues and increasing the mobility and foraging capability of invertebrates. As the greatest rates of weed seed predation occur in mid-summer to early fall when invertebrates are most active, abstaining from tillage at this time is likely to maximize the benefit you receive from these voracious seed eaters. Waiting until spring to perform tillage will conserve habitat for seed predators and ensure that seeds are readily available on the soil surface in the fall, while still helping to break up any heavy residues. If forgoing fall tillage all together is not an option, then try to delay the operation until as late as possible, or try to use an implement that leaves some residue on the surface and won’t bury weed seeds too deeply.

The System as a Whole
Ultimately, weed seed predation is a service that is provided free of charge by a healthy agroecosystem. While it may not be consistent over all years and in every field, by managing to conserve the habitat of weed seed predators a big dent can be put in your weed seed bank.

The main things to remember are to plan for diversity in your rotations by including cover crops, because these crops and their residues provide habitat for seed predators. Diversity across your farm can also help to provide habitat, especially for highly mobile predators such as mice. Additionally, plan your tillage operations to work with the foragers. The mice, crickets, birds and beetles all know the best time to be out in the field foraging, and that’s the time when weeds are going to seed. So if you hold off on tillage during that part of the year, you can preserve habitat and availability of seeds. Of course, this by default means you could be letting weeds go to seed, so use common sense. If you have a weedy field, control by mowing or tillage is going to be more reliable, but if you are green manuring or cover cropping and weeds are within acceptable levels, then waiting until spring, or even later in the season, to terminate those crops will increase the consumption of the weed seeds that are being shed.

As organic and sustainable farmers we take a proactive approach to weed control, grounded in holistic management systems. The underlying goal of these systems is to farm in a way that ensures the ability of the ecosystem to function properly, and therefore cycle nutrients, regulate pests and generally just do as much of the work for us as possible. Agroecosystems are environments that endure a good deal of disturbance, with tillage, seeding, harvest and applying soil amendments. We need to make sure we connect all the ecological dots to compensate for these disturbances. We do this not only so our weed seeds have predators, but also so those predators have predators, and so on up the chain, because that natural regulation is what keeps pest outbreaks and other problems from occurring.

Jeff Gunderson, the new outreach coordinator at MOSES, has a Master's Degree in soil science and a background in agronomy. He may be contacted at jeff@mosesorganic.org

Photo credits: Beetle: Marlin Rice; Cricket: James H. Robinson; Mouse: Phil Meyers