Ontario Field Crop Report – July 20, 2022

Weed pressure in a field where manure has been applied

Managing Weed Seeds in Manure

Where manure is applied there often seems to be higher weed pressure. The question: does manure fertilize the weeds already in the seed bank or are the seeds brought in with the manure?  

Figure 1.  Weed pressure in a field where manure has been applied

Consider that there are up to 15,000 weed seeds per square foot in the soil seed bank according to studies done in North Carolina and Minnesota, and only a portion will germinate at any given time. Spreading manure can add to the seedbank since manure can contain a lot of weed seeds depending on weed pressure from the feed, but the same study revealed that solid manure (per ton) added less than 1 viable weed seed per ft2

Where feed is grown on the farm, the weeds in the manure will most often reflect those already on the farm. Where feed is sourced off-farm there is an opportunity for new and sometimes noxious weeds to appear. Weed seeds in manure come from the feed and after being digested will be spread with manure, however weeds can also be added from spilled feed, bedding and weedy stockpiled manure. Hard-coated small broadleaf seeds are more difficult to destroy than soft-coated, larger grass seeds. Scouting and knowing the weed populations on the farm is the first step to managing weed populations, however reducing viable weeds can occur at the farmstead.

Reducing Viable Weed Seeds in Manure

Heat is the best method for destroying seed viability therefore ensuring that weeds will not germinate. For hard-coated weed seeds, a temperature of 60oC (or 140oF) for 3 days – not necessarily consecutive – will reduce the population of viable weed seeds.   

If feed brought onto the farm is heated during processing it may reduce the weed seed population, but magnitude of reduction would depend on temperature and duration.   

What impact does eating and digestion have on weeds in feed? While often assumed that digested feed will have less viable weed seeds, does a 4-chambered stomach system make a difference? In the study the digestive systems of cattle, horses and pigs provided similar outcomes for weed destruction while the gizzards of poultry act more like a hammermill in destroying weed seeds. After digestion only 7% of weed seeds remain viable for 4-legged livestock while for poultry viable seed is reduced to 0.25%. Table 1: shows the impact of animal digestion on soft and hard-coated weed seeds for livestock and poultry.

Table 1: Impact of livestock and poultry digestion of weed seed viability

Post treatment viabilityPost treatment viability
Large soft-coated weed seeds
(e.g., foxtail, barnyard grass)
Small hard-coated weed seeds
(e.g., pigweed, waterhemp)
Ruminants7 %27 %
Poultry0.25 %3.5 %
Ruminants and monogastric (e.g., hogs) livestock had similar results
Data from Chryseis Modderman – U of Minnesota 2021

Ensiling feed also reduces weed seeds in manure. The ensiling process allows bacteria to ferment plant sugars which reduces pH to allow prolonged feed storage. Viable small hard-coated weed seeds were reduced during the ensiling process. Feed ensiled for several months had less viable seed. Table 2: shows the progression of weed destruction from ensiling and digestion. Digestion of ensiled feed only marginally reduces viable weed seeds over ensiling alone. 

The final strategy for reducing weed seeds in manure is composting. Composting can further reduce viable weed seeds in manure to as low as 2%, but generally between 2 and 10% due to variability in compost piles. Weed seeds that remain viable will often germinate quickly after application. For example, hard-coated seeds such as velvetleaf that normally may wait decades to germinate will often appear the season after application. Weed seed reduction requires proper composting, which takes management. It is not a pile of manure that has remained undisturbed for a year or more. Proper composting is a process that results in manure being heated to 60o to 80°C (140o to 180oF) for a minimum of 4 days with proper moisture at about 50% carbon to nitrogen ratio. Piles are turned until they no longer produce heat. 

Table 2:  Viable weed seeds in weed contaminated feed after treatment

Post treatment viability (small-seeded weeds)Post treatment viability (small-seeded weeds)
Ensiling 1 month
Haylage    59 %Corn silage    40 %
Ensiling 2 months                        13 %
Ensiling 2 months + digestion                        11 %
Ensiling 2 months + digestion + proper composting 2 – 10 %
Figure 2. Digestion of ensiled feed helps reduce viable weed seeds in manure

These strategies help to prevent additional weed pressure but are limited to farms with ensiled feed and/or composting systems. On poultry farms weed seeds will not germinate in solid poultry litter due to high total salts, which will also reduce viable seeds returned to the field.    

Unfortunately weed reduction strategies remain limited for liquid manure systems (deep pit, anaerobic conditions), especially in hog operations where feed is not ensiled and often comes from off-farm sources. Treatment systems such as anaerobic digestion have also not been effective in reducing weed seeds.   

As interest in manure as a fertilizer and organic matter source increases, so does the opportunity to spread problematic weeds. While digestion, ensiling feed and composting manure can limit increasing the weed seed bank, the nutrients from manure, especially nitrogen, help the weeds already in the bank grow more quickly. Controlling weeds in livestock feed crops will help reduce the seed bank. Scouting fields to know which weeds are present and controlling them with a variety of practices will provide the benefits of manure without the potential yield loss from uncontrolled weeds. 

Information in this article is based on research conducted at University of Minnesota and presented by Chryseis Modderman at the North American Manure Expo July 2022. 

Weather DataJuly 11 – 17 2022

LocationYearHighest Temp (°C)Lowest Temp (°C)Rain (mm)Rain (mm) April 1stGDD 0C April 1stGDD 5C April 1stCHU May 1st
10 YR Avg. (2011-20)29.016.712.7340168711491774
10 YR Avg. (2011-20)28.014.716.8283158410511650
10 YR Avg. (2011-20)28.415.114.9306156010321628
10 YR Avg. (2011-20)
10 YR Avg. (2011-20)
Mount Forest202228.411.112.220713248631324
10 YR Avg. (2011-20)26.613.014.129913798711441
10 YR Avg. (2011-20)28.112.811.226914169001451
10 YR Avg. (2011-20)28.914.416.829614879721545
10 YR Avg. (2011-20)26.311.215.625111667341258
10 YR Avg. (2011-20)26.912.516.828112537991346
Thunder Bay202226.55.75.5364975569973
10 YR Avg. (2011-20)
Fort Frances202224.311.025.646810466421108
10 YR Avg. (2011-20)26.712.123.227412077451280
Report compiled by OMAFRA using Environment Canada data. Data quality is verified but accuracy is not guaranteed. Report supplied for general information purposes only. An expanded report is available at www.fieldcropnews.com.