Rested Grazing Increases Pasture Carrying Capacity

This article was originally published February 2010 on Page A8-A9 in the Prairie Post's Bullbreeders 2010

Previous work at AAFC's Brandon Research Centre, conducted over 10 years, evaluated the inclusion of alfalfa and application of fertilizer, alone or in combination, to bromegrass pastures. The inclusion of alfalfa was identified as a major determinant of profitability. During the course of this study, the presence of alfalfa declined from more than 80% of the biomass available on entry into the pasture to less than 10%. Despite there being little alfalfa left at the end of the trial, researchers were still seeing a benefit to the inclusion of alfalfa. It was suggested that by improving the persistence of alfalfa in the pasture, the benefit to including alfalfa would be even greater.

Winterkill is a primary cause of alfalfa decline in pastures. The risk of winterkill can be reduced by ensuring adequate plant root reserves. To do so, pastures with alfalfa should not be cut or grazed during the critical four to six weeks before frost. This management strategy had not been applied in the previous trial. To implement a pre-frost pasture rest period at the Brandon Research Centre, cattle must be removed from pastures by the beginning of August and will require an alternate source of feed until they can be returned to the pastures following a killing frost.

Swath-grazed annual forages have proved to be a cost-effective means of extending the grazing season and have potential for use when the cow herd has been removed from perennial pasture. With earlier use of the swath-grazed annuals, there should be the added benefit of better utilization of early-season moisture to support greater crop yields.

The Brandon Research Centre, in cooperation with the University of Manitoba, initiated a rested grazing project in 2006 to determine the impact of resting pasture during the critical pre-frost period on pasture carrying capacity and the persistence of alfalfa.

The rested grazing project uses twenty pastures of around 10 hectares each. Eight of the pastures are bromegrass and eight are alfalfa/bromegrass. The remaining four pastures are used for swath-grazed annual production. The perennial pastures were established over eight years. For Phase I of the trial, all the perennial pastures are rotationally grazed by cow-calf pairs from the beginning of June until the end of July. At the end of July, one half of the pastures continues to be rotationally grazed until the available forage is depleted (Phase II). The other half is rested; that is, cow-calf pairs assigned to those pastures are moved to early-seeded annuals for swath grazing (Phase II). Calves are weaned from cows at the end of Phase II. For Phase III, after a killing frost, cows from the un-rested pastures are moved to late-seeded annuals for swath grazing and cows that had been swath grazing early-seeded annuals while pastures were rested are moved back to graze off the residue from the rested pastures. The pattern of pasture use is detailed in Figure 1.

Preliminary results appear in Table 1. During Phase I, both rested and un-rested halves of the pastures have very comparable carrying capacity. For Phase II, the move to using early-seeded annual forage for swath grazing improved the carrying capacity around five fold over continual grazing of the perennial pastures. In contrast, during Phase III, the use of late-seeded annuals for swath grazing improved the carrying capacity by only 1.5 to two fold over the perennial pasture residue after a killing frost.

One could argue that comparing swath-grazed annuals to perennial pasture is unfair. However, comparisons of early-seeded versus late-seeded swath-grazed forages and rested versus un-rested perennial pastures also give an advantage to the rested pasture treatment. Use of early-seeded annuals improved pasture carrying capacity by more than 1.5 to two fold, presumably due to higher yield from better use of spring moisture. Resting the perennial pasture improved its carrying capacity by around 1.5 fold as a result of more re-growth before grazing.

Overall, across all three phases, pasture carrying capacity was increased by around 1.5 to 1.7 fold. The increase was attributed to both resting the perennial pastures and using early-seeded annuals for swath grazing. Furthermore, these results are being observed before there has been much opportunity for improved alfalfa persistence, and with all the same activities and costs. Only the timing of events is changed.

The use of early-seeded swath-grazed annuals as a feed source while resting pastures through the critical pre-frost period can substantially increase the carrying capacity of pastureland, even before improvements in perennial pasture condition become evident. Research is ongoing, and the project is planned to continue until 2012.

Figure 1

pasture at AAFC's Brandon Research Centre

The pasture at AAFC's Brandon Research Centre with a rested paddock on the left and an un-rested paddock on the right at the end of Phase II of the grazing season. Both paddocks contain alfalfa and bromegrass. Of significance is the difference in the amount of forage available (photo taken mid-September), as well as the proportion of alfalfa in the rested pasture.

Dr. Shannon Scott is looking into the effect of resting pasture during the pre-frost period on pasture carrying capacity and the persistence of alfalfa.

Dr. Shannon Scott
Research Scientist - Beef Nutrition Management
Agriculture and Agri-Food Canada
Brandon Research Centre, Brandon, Manitoba