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B. Frick and E. Johnson - Scott Research Farm

Problem

Crop rotations are considered the key to sound organic farming practice. Why are crop rotations important? What is a good organic crop rotation?

Background

Crop rotation is a planned sequence of different crop types, such as spring-seeded cereals, fallseeded cereals, oilseeds, pulses, perennial legumes and other perennial species. Rotations also include alternating crop types, for instance between barley and wheat, or flax and canola; or alternating cultivars within a crop species, for instance, between Harrington and Brier barley.

Crop rotation is a central component of all sustainable farming systems. Crop rotation offers the most effective, indirect method of minimizing pest, disease and weed problems and maintaining and enhancing soil structure and fertility. Crop rotations can limit build-up of weeds that are favored in a single crop environment. Crop rotations have many benefits, including increased soil microbial activity, which may increase nutrient availability, including phosphorus. When crops are rotated, yields are usually 10 to 15% higher than when grown in monoculture.

Each crop has unique characteristics, and thus requires different kinds of disturbances, such as different seeding times, different tolerance to practices such as post-emergence harrowing, etc. Crop rotation dictates the pattern of these disturbances, that ultimately change weed species' composition within agroecosystems. Thus, in spring-sown crops there is selection against autumn-germinating weed species while the opposite is true of autumn-sown crops. There has been evidence since at least the 1800s that weed incidence varies with crop rotation. An extensive literature survey of over 200 references indicated that weed number, biomass and seed production are reduced in rotations and intercropping situations compared to monoculture.

The greater the differences between crops in a rotational sequence, the better cultural control of pests can be expected. In a Saskatchewan study, the presence of winter wheat in a rotation was the factor with the largest impact on quackgrass growth. When winter wheat established well with adequate moisture, it suppressed quackgrass.

Including alfalfa or other perennial legumes in rotations may be especially helpful in managing weed problems. This solution is limited, in part, by the small number of farms with livestock. This problem might be overcome through creative marketing (to livestock producers or the dehy industry), in the use of alfalfa grown for seed, and of short-term plow-downs when seed prices are favorable. A further constraint results from perennial legumes’ high moisture requirement resulting in severe soil drying. For this reason, introducing perennial legumes should be done cautiously, with on-farm feasibility testing.

Using legumes in rotation began to decline when synthetic nitrogen was introduced in the 1940's. Making rotations shorter (not including perennials, pastures, green manures and increased summerfallow) has reduced soil organic matter, degraded soil physical properties, and increased erosion and external inputs. Once established, forage grasses and legumes within rotations are very effective in suppressing growth of some annual weeds. This is a consequence of leaving the soil surface undisturbed, providing dense crop canopy cover and root development, and mowing. Mowing affects annual weed growth much more severely than it affects forage grasses and legumes.

A Saskatchewan study indicated that weed populations were affected more by frequency of perennial forages in rotation than by any other management factor studied. When forage crops' frequency within a rotation increased, there were more perennial or winter annual weeds such as dandelion, smooth brome, quack grass and narrowleaved hawks-beard -- and fewer annual species. Other factors influencing weed communities less than rotation were the number of tillage passes and fallow frequency. Both tillage and fallow encouraged annual weed growth, but discouraged both perennials and winter annuals.

A good crop rotation is dependent on the site, the manager, field history and the rest of the farm operation, but a guideline for weed management is to include as much diversity as you are comfortable with. Crop rotations can vary timing of disturbances (such as early seeded, late seeded, winter crops, biennials, perennials, and green manures). Crop rotations can account for differences in nutrient requirements. For instance, a three year alfalfa crop might be followed by wheat (which will use nitrogen from alfalfa breakdown), a legume (that would not require a high nitrogen level and would fix atmospheric nitrogen more effectively at low soil nitrogen levels), wheat again, and then oats (with a small nutrient requirement). Rotating crops for disease and insect management can contribute to a healthy crop with less opportunity for weed growth. Crops can be rotated according to competitive ability. For instance, competitive crops such as barley or alfalfa could be grown before less competitive crops such as flax or lentil to start the less competitive crops in as clean a field as possible. Using a competitive crop, or several competitive crops following a less competitive crop may be used for clean up. Weed suppressing cover crops such as fall rye or sweet-clover can replace some of the summerfallow land.

Conclusions

A good crop rotation is important in reducing weeds, diseases, nutrient depletion and in improving crop vigour. An ideal rotation balances requirements of crops and soil, disturbance timing and farmer comfort.

Acknowledgements

Funding provided by the Canada-Saskatchewan Agri-Food Innovation Fund

Originally published in Research Report 2002, Canada-Saskatchewan Agri-Food Innovation Fund