Sustainable fallow management helps preserve crop residue

The practice of fallow has long been an effective risk management tool for producers. In drier parts of the Prairies, fallow helps to build up soil moisture reserves, while in wetter areas it is sometimes used to control problem weed infestations. Fallow has been controversial, in that it can lead to soil erosion and loss of soil organic matter. Fortunately, in the past 20 years improved farming practices have resulted in not only a reduction of fallow acres, but also improved fallow management techniques that minimize negative impacts.

In wetter areas, the need to fallow has been essentially eliminated through improved weed control strategies, such as diversified crop rotations, low soil disturbance seeding methods, reduced tillage and improved herbicides. In drier regions, these same strategies have enabled producers to conserve moisture, thereby reducing the frequency of fallow in their crop rotations.

However, even with these moisture conservation techniques, there are instances where it is too dry to achieve a successful crop every year. This would include areas within the brown soil zone that regularly experience drought, and larger areas extending into the dark brown soil zone that experience periodic drought.

The main goal of sustainable fallow management is to conserve as much residue from the previous crop as possible, while at the same time controlling weed growth. This is especially true this year, when many fields are already low in residue, due to last year's drought. One of the best options is to use herbicides (i.e. chemfallow) instead of tillage to control weeds. Typically, this involves spraying a broadleaf herbicide in late fall or early spring to control winter annuals, followed by two to three applications of a glyphosate/broadleaf herbicide tank mix to control a variety of grassy and broadleaf weeds.

Some adjustment to this strategy may be needed to effectively control more resilient perennial weeds or volunteer plants from a previous herbicide-tolerant crop. One also needs to be aware of the potential negative impact of herbicide carryover on the next crop to be seeded. To deal with these challenges, it is necessary to consider the specific herbicide to be used, the rate and timing of application, and next year's management. Another option is to consider using tillage in combination with herbicides.

While tillage will never conserve as much crop residue as herbicides, there are ways to adjust tillage practices to reduce soil disturbance and improve residue conservation. Some easy ways include reducing tillage speed, using low crown sweeps instead of standard cultivator shovels, and removing harrows. While weed control may be less effective with these changes, this can be enhanced by mounting a dead rod at the back of the cultivator.

If available, wide blade cultivators do an excellent job of conserving residue and are especially suitable for the first tillage operation under dry soil conditions. Special care must be taken to level the blades across the frame, and set the outside tips of each blade slightly lower than the middle point to enhance penetration. Rodweeders are suitable for secondary tillage operations, and must also be operated at proper depth and speed to obtain optimal weed kill and residue conservation. Discers or harrows should not be used for tillage, since they flatten most of the stubble, and bury considerable residue.

Weather and soil moisture conditions are also important factors in deciding whether to use herbicides or tillage to control weeds. Generally speaking, herbicides work better when weeds have been actively growing. Tillage is more effective when soil conditions are on the dry side, and weeds are under heat or drought stress.

Because the 2001 drought produced low amounts of crop residue, producers should carefully consider which fields to fallow this year. If possible, you should not fallow pulse and oilseed stubbles. Even with the best conservation techniques, there may not be enough residue to prevent erosion. Therefore, producers need to consider other ways to enhance erosion protection.

One option is seeding annual barrier strips. This is normally done during a fallow tillage operation, by mounting a small one or two-row seeder at the back of the implement. Some of these seeders were quite popular in the 1980s and may still be available. They are also quite easy and inexpensive to build. The ideal barrier grows reasonably tall and remains standing over winter for both wind erosion protection and snow trapping enhancement. Suitable barrier crops include flax, cereals, sunflower and corn. Ideal seeding time is between late June and early July to allow for significant growth, but late enough to prevent seed production.

Another option is seeding green manure or cover crops across the entire fallow field. Time of seeding can range from early to late summer, but should be done under reasonably good moisture conditions to ensure establishment. The key is to allow enough growth to provide erosion protection, and then terminate growth using herbicides or tillage before too much moisture is removed. This is usually four to six weeks after seeding. If seeding is done in late summer, then it may be possible to leave growth until winter freeze up. One should keep in mind that some moisture loss can be recovered by improved snow trap the following winter. Annual legumes, such as lentils or peas, can also improve soil nutrient status through nitrogen fixation.

Another alternative that can provide similar results is to let weeds grow taller before killing them. It is important, though, that weed growth be terminated before seed set. One also needs to consider that some weeds are only susceptible to herbicide control at earlier growth stages.

The potential for soil erosion is much higher this spring and summer. If you are planning to fallow some land in 2002, consider these options for sustainable fallow management.