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As grassland resources across the northern prairies continue to decline, it is important to manage what remains as efficiently as possible. The most common practice used to manage grasslands is grazing with livestock. Ranchers and other landowners have varying objectives and options for implementing grazing strategies on lands they manage. To better understand certain aspects of these grazing management options, Ducks Unlimited conducted a study at a 9,500-acre native prairie ranch located on the McPherson-Edmunds County line in north-central SD, during 2009- 2011 to evaluate the impact grazing treatments commonly implemented across the region had on duck production and yearling cattle performance. 

Study of Grazing Treatments

During each year of the study, 1200 yearling heifers of similar weight were randomly placed into herds prescribed to one of three grazing treatments: a four-pasture once-over rotational grazing system (1X), a four-pasture twice-over rotational grazing system (2X), or singlepasture season-long continuous grazing (SL). Each grazing treatment was represented by multiple replicate herds distributed across the ranch each year with each pasture being about 160 acres in size. A similar number of cattle and acres were impacted by each grazing treatment each year resulting in pastures under any given grazing treatment having equal cumulative grazing pressure over the entire ~140-day grazing season (~3.1 acres available per heifer = 1.15 Animal Unit Months (AUM) per acre). For example, a 160-acre pasture grazed in the season-long scenario contained about 50 heifers for the full grazing season, whereas each of the four pastures sequentially grazed in either of the rotational systems contained about 200 heifers, but only for about ¼ of the full grazing season in each pasture – thus all pastures in the study were grazed at the same overall AUM intensity by the end of the grazing season. Weights were obtained from cattle on-site before each herd was released into predetermined pastures near May 1, and again at the end of the grazing season in mid-September. Average Daily Gain (ADG) was then calculated for each grazing treatment. Duck nests were located with traditional nest searching methods of dragging a 100-foot-long chain between two ATVs across grassland vegetation to flush nesting hens. Each discovered nest was monitored weekly until it hatched or was otherwise terminated. Nest searches were conducted equitably across each of the grazing treatments each year throughout the nest initiation period (~May 1 – ~July 3) with about 21 days elapsing between 3 systematic searches in each pasture. Nest success and nest densities were calculated for each grazing treatment.

Results

Heifers in the 1X-rotation had a three-year-mean ADG of 1.22 pounds, 1.51 pounds in the 2X-rotation, and 1.79 pounds in the SL-continuous grazing scenario. A total of 5,066 duck nests were found over the three-year period, of which 31.3% successfully hatched and 68.7% that did not. Of the unsuccessful nests, 90.0% were destroyed by predators, 5.4% were trampled by cattle, and 4.7% were unsuccessful for other reasons (weather, abandonment, etc.). Statistically corrected nest density and nest success values for each grazing treatment were: 72 nests per 100 acres and 13.1% for the 1X-rotation, 80 nests per 100 acres and 15.1% for the 2X-rotation, and 76 nests per 100 acres and 13.3% for the SL grazing scenario. A statistically corrected nest success value of about 15% is generally needed to sustain a duck population at its current level.

Discussion

Each 1X-rotational grazing system utilized four pastures during the grazing season, but livestock only impacted about half the allocated acres during the primary nesting season. The acres that had cattle present during the nesting season showed a moderate reduction in duck nest density and success perhaps due to comparably high cattle stocking density (4 times higher than the SL-grazing scenario) for ~35 consecutive days in each pasture. However, pastures with cattle absent in the rotation during the nesting season showed higher nest density and success, that was similar to adjacent pastures with grazing deferred until after the nesting season. ADG for heifers was the lowest in the 1X-rotation at 1.22 lbs/day perhaps in part due to lower digestibility of the more mature and less palatable stand of vegetation available later in the season in the fourth pasture of the rotation.

Each 1X-rotational grazing system utilized four pastures during the grazing season, but livestock only impacted about half the allocated acres during the primary nesting season. The acres that had cattle present during the nesting season showed a moderate reduction in duck nest density and success perhaps due to comparably high cattle stocking density (4 times higher than the SL-grazing scenario) for ~35 consecutive days in each pasture. However, pastures with cattle absent in the rotation during the nesting season showed higher nest density and success, that was similar to adjacent pastures with grazing deferred until after the nesting season. ADG for heifers was the lowest in the 1X-rotation at 1.22 lbs/day perhaps in part due to lower digestibility of the more mature and less palatable stand of vegetation available later in the season in the fourth pasture of the rotation.

The SL-continuous grazing treatment utilized a lower stock density of ~0.32 heifers/ac maintained throughout the grazing season in each assigned pasture compared to the rotational grazing treatments of ~1.28 heifers/ac in each pasture at the time livestock were present. Overall nest success and densities were similar to the 1Xrotation values but steadily declined by season’s end likely due to the gradual reduction in vegetation height over the course of the season due to continuous grazing. Cattle performed the best in season-long treatments, with ADG significantly higher than those found for both rotational grazing treatments (1.79 lbs/day).

Ranchers and land managers have a variety of different objectives for grazing native grasslands. Each of the treatments evaluated in this study are useful grazing tools and have their own benefits and potential drawbacks in certain instances. For the duration of this study, a four-pasture-twice-over grazing system seemed to be the least detrimental to duck nest survival and resulted in strong weight gains for livestock, whereas the seasonlong continuous grazing strategy provided slightly lower nest success and densities but higher livestock weight gains. However, it should be noted that all pastures included in this study were classified as native range in very good to excellent condition regarding plant health and diverse plant community composition.

Past studies have shown that utilizing season-long grazing practices year after year on the same native pastures having high range condition will lead to decreased pasture health, plant diversity and grassland vigor and ultimately lower potential for livestock performance. Although rotational grazing seemed to have a slight negative influence on nest success and density in pastures where cattle were present during the nesting season at higher densities compared to season-long grazing, it appeared as though pastures within the rotation where cattle were not present somewhat buffered this negative influence. Additionally, using rotational grazing practices to manage native grasslands has many positive benefits and can lead to maintained or increased biodiversity, pasture health, and grassland vigor resulting in sustained profitable grazing operations. 

Randy Meidinger serves as a regional biologist for Ducks Unlimited in Long Lake, South Dakota.

Source: SDGC Newsletter