by Karen Renner 

Manure is a very beneficial resource in crop production systems, but many non-livestock farmers are hesitant to apply manure to their fields because they are worried about weeds. If a farmer spreads manure on his or her fields, will the potential for weed problems in these fields increase? The answer to this question lies in four areas of the livestock system: the feed source, the type of animal, the weed species and the manure-handling system.  

The feed source

When feed is ground and pelletized, very few weed seeds survive, but the few weed seeds that do survive in the feed pellets may become the start of a new weed problem in the field.  

If forages or grain are bought from another farm operation and fed to livestock, it is possible that spreading the manure will introduce a new weed species to the farm fields. Why do I mention ”new” weed species? Fields contain some weed seeds already, and a few more common lambsquarters seeds would never be noticed in most farm fields. However, a few seeds of a new, uncommon weed species could be the start of a new weed problem. The ensiling process reduces weed seed numbers dramatically. After four weeks of ensiling, the number of lambsquarters weed seeds will be almost zero. Most ensiled feed is fed through a rumen digestive system. After passing through both the ensiling process and the rumen, very few seeds of most weed species, including redroot pigweed and common lambsquarters, will survive. 

The type of animal

Chickens destroy 98 percent or more of the weed seeds in feed because of the grinding action of their gizzards. About 25 percent of the weed seeds fed to cattle and hogs were recovered intact in the manure, and 10 to 12 percent of the weed seeds were found intact in horse and sheep manure. Digestion in all livestock species destroys weed seeds, but some weed seeds survive the trip through most livestock and are present intact in the fresh manure.  

The weed species

Weeds with soft seed coats—such as foxtails and other grass species, sweet clover and pepperweed—don’t survive digestion by livestock. Weeds with small, hard seeds—such as common lambsquarters and the pigweeds—pass easily through most animals and are excreted intact and ready to start future weed problems.  

The manure-handling system

The composition of manure varies with the livestock type and age, the livestock feed, and the housing and bedding materials. A survey of fresh manure from 20 New York dairy farms found an average of 40 weed seeds per pound of manure. If a farmer spread 20 tons/acre of this manure, it would be equal to adding 40 weed seeds per square foot to his or her fields. Is this a lot of weed seed? It depends on the field. If a field has fewer than 100 seeds per square foot, another 40 would be a lot of weed seed; if a field has 10,000 seeds per square foot, 40 more would be less than a 1 percent addition to the weed seed bank.  

Across the north central region of the United States, farm fields vary in the number of seeds in the weed seed bank. Remember that one velvetleaf plant seeding out in a corn or soybean field produces 800 or more seeds, so spreading 20 tons of dairy manure with velvetleaf seed will be less of a problem than a scattering of velvetleaf plants going to seed across the field.  

One way to reduce weed seed numbers in manure is to store the manure before spreading. Storing manure reduces weed seed viability (seeds are no longer capable of germinating). In one study, stockpiling manure for three months decreased weed seed viability by 60 percent or more. The warm temperatures and the ammonia gas and uric acid generated in the stack contribute to weed seed decay over time. Similarly, high temperatures in compost piles kill weed seeds. Seed of some weed species will be killed after seven days at 130 degrees F; seed of some tougher weed species requires 30 days or more at 145 degrees F to be killed. Weed seeds will survive in any cool spots in the compost pile. This is why compost must be turned periodically—to break down the organic materials and to expose weed seeds to high temperatures.  

Manure and compost applications provide many benefits to the soil and to the crop. Feed is pelleted or ensiled, feed passes through livestock digestive systems, and various manure storage regimes are available. All of these steps take a toll on weed seed viability. Managing weeds in manured and composted fields requires timely weed control tactics, just as it does in non-manured fields. Always monitor manured fields for new weed species, especially when livestock feed is brought in from out of the area. Good weed management by the farmer will prevent weeds from competing with the crop, and the crop will benefit from the additional nutrients and improved soil quality provided by the manure.  

You can learn more from the recently published “Integrated Weed Management: Fine Tuning the System” (E-3065). You can order it from the MSU Extension bulletin office at www.emdc.msue.msu.edu. One chapter of this bulletin is devoted to manure and compost in integrated weed management systems. 

This article was originally published in the March 2009 issue of the Scoop.

Click here to read the full issue from March 2009.