Weed Seeds in Manure

Cornell University has studied the importance of weed seed in cow manure (Mt.  Pleasant and Schlather, 1994).  The authors tested manure from 20 dairy farms by collecting manure from fresh droppings in the barn or from piles of cow manure just prior to being spread in the field.  They found apparently viable seed from 13 grass and 35 broadleaf species.

Lambsquarters seed was in the manure of more than half the farms, yellow foxtail on 35 percent, common chickweed and dandelion on 30 percent, and wild mustard, redroot pigweed, and barnyardgrass on 25 percent.  One farm had 400,000 seeds per ton of manure, most of which were common lambsquarters seeds.  Four farms had no weed seed in the manure and the rest averaged more than 75,000 seeds per ton of manure.

They also sampled an additional six farms with high populations of velvetleaf but found this weed in only one of 12 manure samples (one sample each from the milking cow barn and heifer barn).  Three barns on these farms had manure with no weed seeds and the other barns averaged 133,000 seeds per ton of manure.

If 30 tons per acre of manure with 75,000 seeds per ton were applied, the seed bank would increase by 2.25 million seeds per acre.  Is this a serious situation?  This depends on the current number of weed seeds in the seed bank.  UW agronomists report they have no precise estimate of seed bank numbers in Wisconsin, but the Cropping Systems trials at Arlington and Walworth counties average about 15 million per acre.

In this case, adding more than 2 million seeds in manure would increase the seed bank about 15 percent, an amount 'hat could be noticeable.  However, in weedier fields, the impact would be less evident while in relatively clean fields, it would be greater.

An equally serious consideration is the introduction of new weed species on farms in manure.  This can happen anytime feed grains, hay, silage or bedding is purchased from others.  Recent concern of cottonseed coming up from the southern states contaminated with cocklebur fruits illustrates this point, but thankfully, it appears that few new cocklebur infestations have resulted.  In contrast, velvetleaf became widely distributed in New York when farmers purchased feed grains from the Midwest in the 1970s and UW agronomists believe the migration of velvetleaf northward in Wisconsin has primarily been the result of contaminated feeds.

-Don Drost, Barron County agricultural agent

Article in the Country Today Paper,  6/20/01