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Surface manure applications can boost weeds

Nutrient management and crop fertilization can influence weed growth and competition with crops.


March 5, 2008
By Donna Fleury

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Nutrient management and crop fertilization can influence weed growth and competition with crops. Previous research shows that nitrogen (N) fertilizer timing and application method affect weed growth and competition with spring wheat. Not only can weeds reduce the amount of N available to crops, but the growth of many species is also enhanced by higher soil N levels. Researchers decided to take these results a step further and compare the effects of nitrogen fertilizer, fresh manure and compost on weed growth and competitive interactions with crops.

Figure 1. Weed seedbank at the conclusion of the four year experiment. Source: Blackshaw, AAFC.
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“Our earlier work with reduced tillage systems confirmed an advantage of banding fertilizer as compared to surface applied in relation to weed biomass,” explains Dr. Bob Blackshaw, weed scientist with Agriculture and Agri-Food Canada (AAFC) in Lethbridge. “There is a big advantage to placing the fertilizer four inches deep in the soil, away from where the weed seeds germinate on the soil surface.” The research showed the accumulative effect of increased weed biomass and competition during a five year period was much lower when fertilizer was banded as compared to surface applied.

Blackshaw launched a new four year study to compare N fertilizer, fresh cattle manure and composted cattle manure, and their effects on weed competition and biomass in wheat. A portion of the beef feedlot manure was composted on site to ensure the manure and compost came from the same source. Granular ammonium nitrate fertilizer was either surface applied or banded four inches (10cm) deep at a rate of 45 pounds per acre (50kg/ha) annually. The manure and compost were surface broadcast annually at a comparable rate of 12 tonnes per acre on a wet weight basis. An unfertilized control was also included and all treatments were repeated for four years.

Weed treatments consisted of a combination of the grass weeds, green foxtail and wild oats, a combination of the broadleaf weeds, common lamb’s quarters and wild mustard, and a weed-free control. “One important factor is with these treatments, we were looking at worst case scenarios and no herbicides were used except on the weed-free control plot,” explains Blackshaw. “That way, the treatments would provide us with a good comparison of weed biomass over four years for the research.”

As part of the study, weed seeds were counted in the manure and compost to account for any impact on the weed seedbank. “Fresh manure may contain viable weed seeds, which can actually add to the weed seedbank, however, compost is expected to contain very low levels if composted properly.”

The compost had almost zero weed seeds, while the manure did have some. “From other research work we’ve completed, we know that most grass seeds die easily during composting, but some broadleaf seeds with hard coats have the potential to survive, such as wild buckwheat, stinkweed, redroot pigweed and round-leaved mallow.” At the end of the four years, the weed seedbank was measured again.

Figure 2. Wheat yield response to nitrogen source in four consecutive years. Source: Blackshaw, AAFC.
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Surface application of N increased weed seedbank
Overall results indicated that the unfertilized control was among the treatments with the lowest weed biomass, indicating weed growth responds positively to higher soil N levels. Weed biomass for both grass and broadleaf weeds was lower with banded N than with surface broadcast N fertilizer, fresh manure or compost. The N amendments affected grass weeds more than broadleaf weeds.

Grass weed biomass was similarly high with broadcast fertilizer and manure in the first three years, but greater with manure in the fourth year. In the first year, grass weed biomass with compost and banded N was similar but in subsequent years, grass weed biomass with compost was similar to or greater than broadcast N or manure, largely due to the timing of nutrient release. “Although the crop benefitted from the nutrients in the manure and compost, so did the weeds,” says Blackshaw. “Under reduced tillage systems, both the weed seeds and nutrients tend to be concentrated at the soil surface, so surface applications tend to feed the weeds along with the crop.”

In comparison, banded applications, where the fertilizer is placed a few inches below the seed, allow the crop to grow down to the fertilizer before the weeds can, gaining an advantage in the first couple of weeks. “The weeds will obviously grow too, but they may emerge and access the fertilizer later, which tips the whole competitive balance to the crop,” explains Blackshaw. “If the crop can emerge and get a head start over the weeds in the first couple of weeks, that can make a huge difference. Weeds may still be present, but they will be small and won’t be competitive or produce much seed.”

Higher yield with banded N
Spring wheat yields were highest with banded N fertilizer. The yields with manure and compost tended to be a bit lower, partly due to weed competition, but also due to the slower release of nutrients. “Although we applied the same amount of nutrients to the plots, whether N fertilizer, manure or compost, they don’t have the same effect on the crop in any one given year,” says Blackshaw.

“There is quite a difference between compost and fresh manure including their rate of release,” explains Blackshaw. Fresh manure tends to release nutrients fairly quickly, making nutrients available for the crop but also for the weeds. However, the nutrient release from compost is much slower initially, making it more challenging to work with. “Composted manure and green manure crops have a longer, slower release and will likely require some fertilizer applied with it to get the most benefit,” explains Blackshaw. Compost will release over three or four years, with only about 20 percent released in the first year. Manure will release about half the nutrients in the first year, releasing the rest over the next one or two years. Both compost and manure do build up the health of the soil over time.

One advantage of manure and compost is there is usually a higher protein content in the spring wheat. “With N fertilizer, the fertilizer is all released in the first few weeks,” explains Blackshaw. “Therefore, later in the season the plant has to remobilize N, as there isn’t really any new N to take up from the soil.” With compost and manure, or crops following a pea crop or plowdown legume forage, there is still new N in the soil available for the plant later in the season, which contributes to higher protein levels. “We always see a half to one percent increase in protein content when we have some type of slow release nutrient in the system.”

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While composting kills most weed seeds, surface application with any fertilizer N source can increase the weed seedbank.

Integrated weed control can help capture the benefits of organic fertilizers
The research shows that fresh and composted manure has the potential to increase spring wheat yield, but also the level of weed competition. “However, we recognize that manure is a valuable resource and provides some other benefits such as improving soil organic matter and protein content,” says Blackshaw. “Therefore, the recommendation is to make sure you have a well-planned integrated weed management program in place, along with a whole package of agronomic practices. Then you can realize the benefits of using manure and compost, while minimizing the weed problems that can come with using them.”

Blackshaw notes that when using manure and compost, growers should consider timing of nutrient application, growing more competitive crops, selecting less weedy fields, and using proper crop and herbicide rotations. Rotating fields where manure and compost are applied, and not applying on the same field every year also may be a good strategy. Make sure pre-seed burnoff is timed correctly, or perhaps consider controlling winter annuals in the fall, and use appropriate in-crop herbicides. “There are benefits to including manure and compost in rotation under reduced tillage, but realize you may be putting more weed pressure on the whole system and have to plan accordingly.” -end-