Weed Seeds and Manure

Do manure applications cause greater weed pressure?

When manure is applied to a field it often results in an increased flush of weeds. The question is often asked whether this results from weed seed coming through the livestock into the manure, or is it a response to the added nutrients. Although the consensus seems to be that weed pressure increases with added fertility, there are a number of simple principles that will help minimize the risk of weed spread from manure to the field.

1. Spread manure on the same fields where the feed was harvested to minimize the introduction of new weed species.
2. Where feed is purchased from off-farm sources, scout fields where manure is applied for "new to the field" weeds. If discovered, make sure to address these weeds in the herbicide or weed control strategy, especially for prolific and hard to control species.
3. Weed species with small and hard seed coats are the most likely to survive livestock digestive systems and manure storages. The most common of these are pigweed, smartweed and velvetleaf. Viable weed seeds are not easy to control because they can remain dormant for a long time. Field conditions such as fertility, temperature and tillage can affect viability and time to germination. Table 1, "Relative Viability of Selected Weed Seeds", shows the approximate number of seeds per plant and seed longevity for some common weeds.
4. Introduction of weed seeds from manure is less significant if the seed bank for those weeds is already high. It is important that weed control strategies include weeds that may come from manure application. These strategies could include herbicides, tillage, mowing, crop rotation, or a combination of all.
5. Composting manure results in a high temperature environment (50 - 70 °C) that will destroy most weed seeds. From experience though, some seeds such as tomatoes and velvetleaf, will survive the composting process. However, because the composting process often breaks seed dormancy these species will usually germinate at a single time therefore making them easier to manage then if they germinate in multiple flushes.
6. Observe manure storages and surrounding areas. Which weeds are growing in the manure pile or liquid storage? The weeds here have proven to be viable in manure and can be transported to the field.

Source: Manitoba Agriculture
*Conn et al., 2006. Weed Science, Vol. 54, No. 3, pp 464-470
**Masin et al., 2006. Weed Research, Vol 46, No. 5, pp 362-370.

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