Weed Seed And Animal Manure -- Separating Fact from Poop

By W. Carroll Johnson, III, Ph.D

This is not a proposed topic for Discovery Channel’s “Dirty Jobs.” And, I am not to be confused with the show’s host. Rather, I will attempt to clarify the relationship between animal manure used as fertilizer and weed seeds. Animal manure is highly touted as an alternative to synthetic fertilizers. In the eyes of farmers, animal manure is both a disposal headache and a value added product. In the context of food plots, animal manure can be a cheaper source of essential plant nutrients than synthetic fertilizers and help build valuable organic matter in mineral soils.
Recent articles and public discussions on using manure in food plots clearly described the benefits of this alternate fertilizer source. I will continue the discussion by trying to clarify the commonly misunderstood relationship between animal manure and weed seed.

For the purposes of this discussion, animal manure will be lumped into two broad categories; manure from livestock and manure from poultry. Litter is a mixture of manure and bedding material. Livestock manure has the potential to be heavily laced with weed seed. This is primarily due to two reasons. (1.) Livestock are typically grazers and many weeds that they find when grazing are palatable. (2.) The digestive system of ruminants does not readily affect the viability of weed seed. Weed seed ingested by livestock pass through the animal and are excreted in a viable state. An excellent example of this phenomenon is the rangeland weed, tropical soda apple. This weed is a serious pest of rangeland in Florida. Cattle eat the tasty tropic soda apple fruit and the seed are excreted in manure. This weed is on the Federal Noxious Weed List and infestations outside Florida usually occur in stockyards or feed lots where calves originating from Florida are temporarily located. Fortunately, these localized infestations can be effectively monitored and controlled.

It is a common misconception that poultry manure and litter are laced with weed seed. This is generally not true due to three reasons. (1.) The digestive system of poultry, i.e. the gizzard, destroys the weed seed. (2.) Commercial poultry are contained (not free-ranging) and fed a carefully prescribed diet of grains of exacting quality standards. Simply, commercial poultry never see a weed seed, much less eat it. (3.) Poultry litter is often composted and the heat generated during the process kills weed seed.

Table 1. Viability of weed seed collected in animal feces1.
Germination of weed seed recovered from animal feces2(%)
Weed species Calves Horses Sheep Hogs Chickens
Velvetleaf 45.1 16.6 12.1 38.1 1.9
Field bindweed 38.7 10.4 15.4 51.2 0.0
White sweetclover 16.6 19.1 8.2 31.1 0.0
Red sorrel 7.2 10.1 9.5 4.2 0.0
Pennsylvania smartweed 26.4 22.2 8.4 7.1 0.0
Wild rose 13.3 6.8 7.6 22.7 0.0
Hoary cress 14.2 5.2 13.6 14.0 0.0
Average across
weed species
23.1 12.9 10.7 24.1 0.3
1Source of data: Harmon, G. W. and F. D. Keim. 1934. The percentage and viability of weed seeds recovered in the feces of farm animals and their longevity when buried in manure. J. Am. Soc. Agron. 26:762-767.
2One thousand seed of each weed species were mixed with grain and fed to test animals.

To illustrate the differences in weed seed in livestock manure versus poultry manure, refer to Table 1. This is data from Nebraska published in 1934, so this is not a new discovery but still relevant today. In this research study, 1000 seed from each of seven weed species were mixed with clean, ground feed grains and fed to different animal species. Feces from each animal were collected and seed removed by washing and sieving the manure. The recovered seed were placed in a seed germinator to test for viability. As the data clearly shows, weed seed are capable of surviving digestion and excretion from cattle, horses, sheep, and hogs with germination varying among the livestock from 10.7 to 24.1%. In contrast, weed seed recovered from poultry manure germinated only 0.3%. In more recent research conducted at Auburn University in the early 1990’s, exhaustive trials showed that poultry litter collected from numerous commercial broiler houses contained no viable weed seed. These two experiments clearly show that poultry litter contains miniscule amounts of viable weed seed, if any at all.

Poultry litter is generated in massive quantities and stored in large piles for composting, either outdoors or in open-sided shelters. Weed seed can be deposited by wind or birds in the stored litter. The resulting weeds eventually produce their own seed. However this type of contamination is generally considered to be insignificant.

Despite overwhelming evidence that poultry litter does not contain weed seed, weeds still seem to be more prevalent where poultry litter is used for fertilizer compared to synthetic fertilizer. Poultry litter stimulates the germination of weed seed that are already present in the soil. This stimulation appears to be due to the high level of fertility provided by the poultry manure and presence of chemical compounds in the decaying manure that stimulate weed seed germination. In particular, pigweed seed are stimulated by poultry manure.

The implications of this phenomenon may alter what forages are planted. The tendency for weed problems to be worse on sites were poultry litter is used may sway food plot managers to plant forages that have robust weed-control options. An example is Imperial Whitetail Clover, on which Slay® herbicide can be used for broadleaf weed control. Multispecies forage blends have fewer weed control options and perhaps should not be planted where sites are fertilized with poultry litter. The intent of this discussion is to clarify the relationship between weed seed and animal manure.

Livestock manure should not be used as a fertilizer since weed seed readily contaminate the manure. Using poultry litter for fertilizer is an agronomically sound practice. However, be prepared for intense weed infestations due to the sudden stimulation of weed seed already in the soil. Basically, using poultry litter is a trade-off: cheaper fertilizer versus more weeds. The choice is yours.