Lauren Kolb, University of Maine
Watch the webinar on YouTube at https://www.youtube.com/watch?v=b3M9dnGTC4Q
This video, from the University of Maine Weed Ecology Group, highlights the results of four years of research on weed management in organic spring cereals. Lauren Kolb discusses the limitations of the widely-used spring-tine harrow for weed management, which has a short window of opportunity for effectiveness. Weeds quickly outgrow the white thread stage, when they are most susceptible to being either uprooted or buried. Delays in tine harrowing, due to precipitation or soil condition, can result in unacceptably low rates of control and unnecessary crop damage.
The researchers evaluated the use of increased seeding rates in barley (200 versus 500 plants m-2) and wheat (400 and 600 plants m-2) for increased weed suppression. Elevated seeding rates reduce gaps in the crop row, provide a buffer against tine harrow damage, and increase the rate of canopy formation, leading to greater weed suppression than typical planting rates. This method was compared to sowing cereals in wider rows and cultivating between the rows with sweeps, as is common in row crops like corn and soybean. Yield, weed growth and seed production, and economics were evaluated.
Elevated seeding rates, while providing greater weed suppression than standard seeding rates, did not show a yield benefit. In general, the number of weeds and their competitiveness will dictate how much emphasis needs to be placed on managing weeds. If growers expect their fields to be very weedy, based on what weeds went to seed the previous year, wide rows with inter-row cultivation provide the most economical choice for organic weed management for growers in Northern New England.
Weeds are a constant reminder of previous years’ weed management failures. Without the use of herbicides, organic farmers often see their weed problems increase every year both in number and in diversity of species. Although cereals are quite competitive because of their initial seed size advantage over weed seeds and quick canopy growth, yield reductions due to weeds are common. Grain quality can also be adversely affected, as weeds can harbor insect pests and diseases and compete for essential nutrients. Wet weed seed in the harvested grain can also cause spoilage.
Why are weeds so prevalent in organic cereals? The fundamental agronomic practices used by most organic grain growers―methods developed over the last fifty years of input-intensive production―are poorly suited to organic production, where weed pressure is often very high. These practices―relatively low seeding rates of 120 pounds per acre and wide rows of 7 inches―work in conventional production because herbicides are used to eliminate weeds, thus minimizing the emphasis on crop-weed competition.
Many growers rely on spring-tine harrowing to reduce weeds in organic small grains such as wheat and barley. This cultivating implement uses flexible metal tines to uproot weeds, which then desiccate on the soil surface. Given ideal conditions of dry soil and very small weeds, harrowing can kill over 90% of weeds in the field. However, a wet spring makes timely spring-tine harrowing nearly impossible. Delaying harrowing until field conditions improve reduces efficacy, as weeds are larger and less susceptible to uprooting. Furthermore, spring-tine harrowing treats the entire field uniformly, wherein the tines also harm the crop through uprooting, burial, and foliar damage. Studies in barley have shown an average 10% yield reduction per spring-tine cultivation event. So, although use of the spring-tine harrow can achieve high levels of weed control, there is a trade-off with yield losses due to crop damage.
Organic farmers can achieve modest improvements in crop-weed competition by switching to competitive cultivars that are tall, emerge quickly, and have horizontal leaf carriage; or, they can choose species like oats. Increasing seeding rates to 290 pounds per acre can also increase yield and suppress weed growth. However, this strategy may not be cost-effective due to the high cost of organic seed.
More selective weed control may be achieved using an inter-row hoe and wider row spacing, as seen in row crops like corn or soybeans. The Schmotzer EPP cultivator is one example of a weed management tool designed specifically for controlling weeds within the crop row in small scale organic production. Mounted on a 3-point hitch, the unit is controlled by hydraulic-assisted manual steering. Depending on the size of the crop, working speed can reach 6 miles per hour. Larger-scale cultivators with automated guidance systems can operate at much higher speeds―up to 10 miles per hour―and still maintain accuracy.
Each sweep is mounted to the toolbar with a parallel linkage, allowing the precise depth control essential for variable field surfaces. Weeds are controlled between the row by undercutting or burial, making the efficacy of inter-row hoeing less reliant on soil conditions or weed size. With greater efficacy against larger weeds, inter-row hoeing can be performed multiple times in a season, allowing for control of weeds that would be unaffected by spring-tine harrowing. Because inter-row hoeing selectively targets weeds, crop damage is minimal. Furthermore, inter-row hoeing with the Schmotzer shows promising results for control of creeping perennials like quack grass, which cannot be controlled in-season by spring-tine harrowing or herbicides.
With reduced weed density and weed pressure, cereal grain yield increases. At a cost of $7.52 per acre, inter-row hoeing is a less expensive weed management option than doubling the seeding rate, while providing equivalent yields and weed suppression.
When weed pressure is low, cereals are sufficiently competitive as to not require increases in seeding rate or physical weed control to manage weeds. However, most organic farms have ample weed pressure to merit consideration of this new technology.
References and Citations
- Lötjönen T., and H. J. Mikkola. 2000. Three mechanical weed control techniques in spring cereals. Agriculture and Food Science Finland 9:269. (Available online at: http://ojs.tsv.fi/index.php/AFS/article/view/5668) (verified 3 July 2013)
- Rasmussen, J. 1991. A model for prediction of yield response in weed harrowing. Weed Research 31(6):401. (Available online at: http://dx.doi.org/10.1111/j.1365-3180.1991.tb01780.x) (verified 3 July 2013)
- Rasmussen J., J. I. Kurtzmann, and A. Jensen. 2004. Tolerance of competitive spring barley cultivars to weed harrowing. Weed Research 44(6):446. (Available online at: http://dx.doi.org/10.1111/j.1365-3180.2004.00419.x (verified 3 July 2013)