Small but Mighty: Weed Seeds in Manure

July 2022

By Chryseis Modderman, U of MN Extension Educator

U of MN Extension graphic

Weed seeds in manure can be problematic when they result in overgrown, weedy fields after manure application. And seeds are in no short supply: a survey found that fresh manure on dairy farms had an average of 75,000 seeds per ton. But, luckily, there are some measures that can be taken to reduce the viability of those seeds.

First of all, don’t assume that animal digestion will take care of the problem. Though it will reduce weed seed viability, simply feeding the material to livestock will not eliminate all seed. Grass and soft-coated broadleaf seeds are more easily destroyed in digestion than hard-coated seeds. In one study conducted on rumen animals, such as cattle, 27% of hard-coated seeds remained viable after digestion. The gizzard digestive system of poultry is highly effective at destroying weed seeds, and only 3.5% of hard-coated seeds fed to ducks were recovered and found viable in a similar study.

So what can you do to reduce weed seed viability beyond the gut? In general, heat is the enemy of weed seed survival. The benchmark for good seed mortality is 140⁰F (60⁰C) sustained for three days. Hot temperatures that fall below that mark or a shorter duration will still kill some seeds, but not as thoroughly. How you subject the seeds to heat is up to you, but below are a few suggestions.

Minimize weed seeds in feed and forage by ensiling

What goes in, must come out; so killing seeds before they get to the animal is a good strategy. One way to do that is to ensile the feed (if appropriate for the feed type). The fermentation and heat generated during ensiling is quite effective for killing weed seeds. One study found that just one month after seed-contaminated haylage was stored, viability of the toughest seeds dropped by 41%; and in corn silage, the drop was even greater at 60%. Logically, seed viability continues to decrease as silage storage time increases. Eight weeks of ensiling was shown to kill up to 87% of viable seed; and when feed went through both ensiling and rumen digestion, the seed mortality increased to 89%.

Minimize weed seeds in manure by composting

What if ensiling isn’t feasible? What if your manure is already contaminated with weed seeds? In those cases, composting is a very effective method for killing weed seeds – more effective than ensiling.

Internal heat generated by properly composting manure will kill most weed seeds – even the hard-seeded ones. The key word here is “properly.” Aged manure is not composted manure. One more time for those in the back: aged manure is not composted manure. Proper composting requires active management and must be monitored and aerated for correct weed-killing conditions to develop.

Temperature and moisture are the two most crucial elements for seed mortality in compost. Studies have shown that sustaining the compost at that benchmark of 140⁰F for three days can reduce weed seed viability 90-98%, so long as a minimum of 35% moisture is maintained. Another study found that overall duration was important and that it took between 21 and 50 days of composting for best results.

Even under the most diligent composting program, there can be seeds that survive. It is theorized that, since manure is not a uniform product, this mortality escape is due to cooler pockets that do not sustain high temperatures for long enough. Therefore, just because manure has been composted does not necessarily mean it is totally free of weed seeds.

Scout fields that receive manure containing weed seeds

Remember, even if the feed was ensiled and the manure was composted before spreading, it’s still possible for weed seeds to remain viable. A 98% reduction in viability seems sufficient, but even low seed survival rates can be problematic. It’s a numbers game: 2% survival of 75,000 seeds would leave 1,500 viable seeds remaining per ton. Applied at 8 tons per acre, that would increase the weed seedbank by 12,000 seeds per acre! Therefore, it is crucial to scout fields that receive manure to head off any severe weed infestation.