Northern Prairie Wildlife Research Center
Fire may be used to enhance establishment and, on a periodic rejuvenation schedule, to increase vigor of both cool- and warm-season native grass stands. Preferred times to burn in central North Dakota are as follows. Cool-season native species should be burned in the period between late March and mid-May or from 15 August to 15 September. Warm-season natives should be burned between 15 May and 15 June. Introduced cool-season grasses and legumes should be burned between 15 March and 15 May. Early fall burns might benefit pure stands of introduced cool-season grasses but our experience has shown that fall burns usually reduce or completely eliminate alfalfa and sweetclover in mixed stands in central North Dakota. Increased height and density of introduced grasses as a result of prescribed burning usually last for only a few years. Few burns on native seedlings have been evaluated but effects of fire on true native grasses may last for a longer period of time.
Chemical fertilizer may be used to stimulate seeded grassland stands after establishment (Lavin 1967). As a general rule, nitrogen (N) promotes growth of grasses and phosphorus (P) promotes legumes. Trials in central North Dakota on 6-year and older mixed stands of alfalfa and smooth bromegrass indicated that treatments of less than 100 pounds of N per acre provided increased plant height during 1 or 2 years, whereas application of 250 to 600 pounds of N enhanced grass height for 3 to 6 years, usually in direct relationship to the amount applied (K.F. Higgins, unpublished data). Generally, less P (10 to 15 pounds per acre) is needed on seeded grasslands unless enhancement of legumes or other forte species is desired.
Rejuvenation treatments of native grass seedings after their establishment should be limited to fire, grazing, or haying. Fire is the preferred method unless it cannot be used because of strong social or economic reasons. Generally, these treatments should be applied within a period of a few days. Check local sources for approximate dates. Grazing or haying should not be used on an annual basis on areas managed for optimum wildlife production.
Mechanical treatment (tillage) is the best method for periodically rejuvenating introduced cool-season grasses and legumes such as intermediate wheatgrass, tall wheatgrass, smooth bromegrass, alfalfa, and sweetclover. Trials in central North Dakota involving mowing and mechanical scarifications of grass-legume stands resulted in the conclusion that mowing produced a positive effect on vegetative vigor during the first growing season following treatment and usually favored legumes (K.F. Higgins, unpublished data). Effects from mowing during the second or later growing seasons were not very apparent. Mowing does not remove the lower litter layer and a long-lasting positive effect on plant development is not attained.
Mechanical scarification from either early spring or late summer treatments showed similar effects. Treatments causing the greatest soil disturbance were the most effective. In general, mechanical treatment should completely disturb the soil and plant roots to a depth of about 4 to 6 inches. Light discing or harrowing should follow spiking, chiseling, or shallow (4 inches) plowing in order to smooth the surface of the field. Removal of residual vegetation by haying, grazing, or burning is recommended just before the mechanical treatment to remove excessive plant materials that would hamper tillage operations. In some instances on public lands this removal of excessive vegetation can be done by cooperators in lieu of payment for the mechanical treatment.
Introduced cool-season grasses and native grasses in seeded stands should be watched for signs of vegetative deterioration since seeded vegetation requires rejuvenation to maintain a vigorous condition. Frequency of rejuvenation will vary according to site, success of establishment, soil fertility, moisture, plant species, and other factors. For most areas in the prairie pothole region, we believe the interval of rejuvenation treatments for seeded grasslands may be 5 to 10 years, but it is not possible to prescribe exact schedules. Prescribed burning or a planned grazing system, or a combination of burning and grazing, appear to be methods most suitable for the rejuvenation of seeded native grasses. The best stands of introduced grasses and legumes are maintained by reseeding after a treatment of complete tillage and 1 or 2 years of crop production. Seeded native grasses should be considered as permanent cover and introduced grasses as semipermanent cover.