By Terry Kowalchuk MSc. P.Ag., Provincial Specialist Forage Crops, Regina
Integrating perennial forages into annual crop rotations can reduce herbicide resistant weeds and interrupt pest and disease cycles. They can also improve soil quality and reduce fertilizer costs.
Forage legumes fix nitrogen (N) from the atmosphere. Fixation potential varies with growing conditions including moisture conditions, the amount of legume present, the species and the quality of the stand are critical factors in fixation potential and subsequent yield benefit. On average, N fixation from alfalfa can contribute 45 pounds of nitrogen per acre annually.
The N release rate to the following crop depends on the amount of nitrogen fixed and the level of organic matter present in the soil. Moisture, temperature and soil conditions also affect microbial activity and the N release rate. The total benefit to subsequent crops may range from 100 to 200 pounds of N per acre, over a two to three-year period following stand termination.
A Saskatchewan study on the effect of legumes in rotation on phosphorus (P) uptake, found that wheat grain yield was improved from 32 per cent to 60 per cent by using alfalfa and red clover in rotation compared to annual grains. The soil available P that was measured after wheat and canola harvest was not affected by different crop rotations at all sites. This was despite the greater removal by forage legumes during the first two years of the four-year crop rotation period and the enhanced P removal by wheat and canola crops grown following the forage legumes. This indicates that forage legumes are able to maintain soil P fertility in the face of greater removal by crops in rotation, at least in the short-term. However, with minimum P additions after four years of continuous cropping levels at all sites were low, indicating the importance of maintaining P fertility over time.
Forage in rotation also contributes to soil health. There is an increasing recognition that living root systems of perennial forages most closely mirror native prairie ecosystems resulting in more biologically active soils. Adding perennials to a rotation extends the growing season for living plants and exudates from the roots to help proliferate mycorrhizal fungi and other symbiotic organisms in the soil.
In addition to enhancing microbial biomass, forage crops also help store carbon in the soil. Studies of land within the U.S. Conservation Reserve program showed that land seeded back to perennial grasses increased soil carbon levels by about one tonne per hectare per year over a five-year period, which is higher than most annual forage systems. Increasing soil organic carbon improves soil tilth, reduces erosion risk by increasing aggregation and improves water infiltration and moisture holding capacity.
Weed suppression by perennial forages in rotation is well documented. A survey conducted in Manitoba found that 83 per cent of producers indicated fewer weeds in annual crops after alfalfa, compared to rotations with annual crops only. Good control of wild oat, green foxtail and Canada thistle was observed for up to three years after alfalfa, although the alfalfa stands did have higher dandelion and shepherd’s purse populations. Perennial forage in rotation continues to be good practice for addressing herbicide resistant weeds, especially with the rise of Group 2 resistance weeds since this survey work was conducted.
Less work has been done on the effects of perennial forages in breaking insect and disease cycles. This is partly due to the myriad of potential insects or pathogens that interact with crops on an annual basis, which makes it difficult to draw causal relationships between crop types. In general, perennial forage crops break disease cycles by removing host plants from the rotation for a longer term, thus reducing the level of pathogens in the soil. For example, studies on cereal crops following three-year forage hay stands have shown reduced occurrences of common root rot.
Generally, production benefits from including forages into crop rotations are optimized after two or three years, but forage production is often extended to account for establishment costs. Often the goal of forage production is to maximize forage stand life and to terminate the stand when production declines. Depending on market and annual crop production conditions, rotating forages more quickly through a crop rotation with annual crops may provide greater agronomic and economic benefits compared to establishing forages for longer periods of time.