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does composted cow manure have weed seeds

A hot compost pile is composed of 2- to 3-inch layers of high-nitrogen green and brown materials. High-nitrogen materials include raw manure; white clover (Trifolium repens, USDA zones 3 through 10) and the foliage of other legumes, such as peas (Pisum sativum). Green materials include fresh grass clippings, coffee grounds and kitchen scraps. Brown materials include dead leaves, garden debris, sawdust, shredded paper and straw. Layer the materials so the pile consists of approximately 25 percent high-nitrogen, 45 percent green and 30 percent brown or woody materials. Your compost pile should measure at least 3 feet square by 3 feet tall to maximize the heating effects of the decomposing materials.

When most of the organic materials in the compost pile have decomposed, the pile will start cooling down. If it starts cooling too soon, such as within the first two weeks, sprinkle it with water to moisten the ingredients and encourage the beneficial bacteria to continue the decomposition process. After six to eight weeks, the interior of the pile will be cool or barely warm. The finished compost should be dark and crumbly, resembling rich, loamy soil. Place a tarp over the pile to prevent windblown weed seeds from contaminating the new compost.

Building a Compost Pile

Although cow manure is an excellent source of nitrogen and nutrients for the soil, it is also a source of weed seeds and pathogens, such as Escherichia coli and salmonella. Composting the manure in a hot compost pile kills both the weed seeds and bacteria, making it safe for use in the garden. Despite its being thoroughly decomposed, however, you should always wash your hands carefully after handling any compost or composting materials.

The fully decomposed manure is used directly on existing flower and vegetable gardens, dug into the soil before the growing season begins, and mixed with perlite and garden soil for planters and raised beds. Although the weed seeds and pathogens in the compost are dead, garden soil also contains weed seeds. As you dig the compost into the soil, the weed seeds are exposed to warmth and moisture, the two main requirements for sprouting. Thus, while you’ve killed the weed seeds in the manure, the garden may still sprout new weeds.

The compost pile should be moist, but not soaking wet, to start the decomposition process. Monitor the pile, measuring the internal temperature daily with a long-stemmed thermometer. When the pile reaches 155 degrees Fahrenheit, turn it to mix the ingredients and then allow it to heat up again. Mix the pile with a shovel or pitchfork whenever the temperature reaches 155 degrees Fahrenheit. Most weed seeds and pathogens die at 135 degrees Fahrenheit.

This is an eOrganic article and was reviewed for compliance with National Organic Program regulations by members of the eOrganic community. Always check with your organic certification agency before adopting new practices or using new materials. For more information, refer to eOrganic’s articles on organic certification.

"The product of a managed process through which microorganisms break down plant and animal materials into more available forms suitable for application to the soil. Compost must be produced through a process that combines plant and animal materials with an initial C:N ratio of between 25:1 and 40:1. Producers using an in-vessel or static aerated pile system must maintain the composting materials at a temperature between 131 °F and 170 °F for 3 days. Producers using a windrow system must maintain the composting materials at a temperature between 131 °F and 170 °F for 15 days, during which time, the materials must be turned a minimum of five times."
–7 C.F.R. § 205.2 (2000)

Bollen et al. (1998) found that only two of 17 plant pathogens investigated—Olpidium brassicae and one form of Fusarium oxysporum—survived when exposed to small-scale static pile composting of infected plant residues, and then only at greatly reduced levels. Thermal mortality during the active phase of composting was found to be the most important factor affecting pathogen destruction.

References Cited

Several factors contribute to weed seed mortality during composting. In compost systems assembled and managed in accordance with requirements for organic certification, the most important factors are the interaction between weed species, temperature, time, and moisture (Eggley, 1990; Shiralipour and Mcconnell, 1991; Eghball and Lesoing, 2000; Larney and Blackshaw, 2003; Dahlquist et al., 2007). In general, the higher the temperature to which weed seeds are exposed during the active phase of composting, the higher the weed seed mortality. Similarly, the longer the duration of high-temperature exposure, the higher the weed seed mortality. Thus, Dahlquist et al. (2007) estimated that three of the six weed species they examined under controlled laboratory conditions were unaffected by temperatures of 108 °F, but 90% of the seeds of all six species were killed after less than three hours at 140 °F (Table 1). Furthermore, all six species suffered 100% mortality after less than an hour at 158 °F. Similarly, in Texas, Weise et al. (1998) found that, in composting manure at 35% moisture, barnyardgrass, pigweeds, and kochia seeds were killed after three days at 120 °F; Johnsongrass seed was killed with three or more days of exposure at 160 °F; but field bindweed seeds were killed only after seven days at 180 °F.

Susceptibility of weed seeds to thermal mortality, however, is influenced by the moisture content of the compost; weed seeds in a dry environment are able to survive higher temperatures for longer times than seeds in a moist environment. Some (Egley, 1990; Thompson et al., 1997) have suggested that thermal mortality may be greatest for fully imbibed seeds—seeds that have absorbed water and split their seed coat in the process of germination. In Nebraska, Eghball and Lesoing (2000) showed that adding water to beef manure compost greatly enhanced weed seed destruction; moist compost was faster and more effective at killing cocklebur, morningglory, pigweed, sunflower, velvetleaf, foxtail, smooth brome, and shattercane than dry compost, in part due to higher compost temperatures.

Improperly assembled and maintained piles or windrows may not reach high enough temperatures during the active phase of composting for killing all weed seeds and pathogens. Failure to reach adequate temperatures can have several causes:

Finished compost can become recontaminated with weed seeds if weeds are allowed to grow and go to seed on or adjacent to the pile or windrow. Similarly, compost can become contaminated with vegetative reproductive structures from some weeds—Canada thistle and rhizomateous grasses, for example—if they are allowed to grow on or adjacent to the pile. Keep vegetation adjacent to stored compost mowed short, and tarp piles or windrows to prevent contamination by wind-blown weed seeds. When moving or spreading finished compost, avoid picking up soil or other contaminants from under or around the pile or windrow.