June 2001

Weed Seeds in Manure

A common question that comes up in a manure fertility program is "Will adding manure to cropland result in an increased weed population from seed present in the manure?"

The general answer to that question is that a flush of weed growth on a recently manured field is usually in response to the nutrients present in the manure rather than the addition of new, viable weed seeds to the field. Even so, with herbicide resistance occurring in many parts of agro-Manitoba, certain principles should be kept in mind to minimize the risk of weed spread from field to field or from farm to farm:

• observe weed seed viability in manure - to determine which weeds are most commonly found growing in manure storages;

• small seeds with hard seed coats are the most likely seeds to remain intact when feed is crushed or rolled and are most likely to pass through the digestive system of an animal and remain viable in the manure (common examples are red root pigweed, smartweed and cleavers);

• viable weed seeds are not easily controlled because they can undergo long periods of dormancy (see Table 1 for dormancy periods according to weed species) - manure may break weed seed dormancy or weeds seeds from manured fields may not germinate right away, depending on the availability of nutrients such as nitrogen and the conditions in the manure for seed germination;

• effect of composting on weed seeds: it has been shown that composting manure with adequate water, oxygen and optimum C:N ratios creates a high-temperature environment (50 - 70 degrees C) that destroys some weed seeds (such as wild oats, green foxtail, wild buckwheat, etc.) due to heating and decomposition;

• spread manure on the same field where grain and weed seeds were harvested to minimize introduction of weeds to new parts of a field or farm;

• farms that purchase feed from off-farm sources should take extra care to ensure the use of feed containing weed seeds does not result in the introduction of weed species that are prolific and difficult to control;

• although it may appear that the number of weed seeds introduced to the soil seedbank through manure is high, it may be an insignificant source of weed seeds in fields that already have a large weed seedbank of certain weed species; and

• there may be little or no cause for concern that weed seeds from manure are being added to a field if:

1. these weed species are already present, and

2. the producer plans to manage these weeds with herbicides or other strategies such as tillage, crop rotations, mowing, etc.

Table 1. Relative Viability of Selected Weed Seeds