Article: Tallgrass prairie response to grazing system and stocking rate
Authors: Gillen, R.L. McCollum T. Tate, K.W. Hodges, M.E.
Published: Journal of Range Management 51(2), March 1998
This research was conducted in the tallgrass prairie of Oklahoma during the 5 year period from 1989 to 1993. As the article title suggests, research treatments were designed to measure the prairie response to grazing system and stocking rates. The study hypothesis “was that periodic rest periods during the growing season would favor the dominant tallgrass species and this effect would be more pronounced as the stocking rate increased.”
The study was conducted 21 km southwest of Stillwater, Oklahoma at the Oklahoma State University Research Range. Dominant vegetation at the site was little bluestem, big bluestem, and indiangrass. Treatments were assigned randomly to experimental units ranging in size from 14 to 26 ha. Stocking rates ranged from 51.5 animal-unit-days/ha (AUD/ha) to 89.8 AUD/ha, with an average initial beef cattle weight of 208 kg, and cattle numbers per unit were between 10 and 22. Within the above stated ranges, stocking rates were randomly allocated between 1 and 6 levels.
Although the authors claim these stocking rates “represented moderate to very heavy rates for this range type”, based on my own calculations from the numbers provided it seems that the stock density probably did not exceed 1,300 lbs/acre. These stock density numbers are low indeed when we compare them to the reported numbers of ultra-high stock density mob grazing, which often fall between 100,000 and 200,000 lbs/acre.
Recovery periods were 45 days for the first two years, during which time grazing days were 3 to 8 days. Recovery periods were then shortened to 30-35 days, with grazing days shortened to 2 to 5 days. During the study, precipitation was well above long term averages.
Results of this study were somewhat inconclusive.
In 1989, total standing crop was higher for rotational grazing at the lighter stocking rates but this difference disappeared as stocking rate increased. Overall standing crop was much lower in 1993 and was not different between grazing systems. Over 5 years (1989-1993), total standing crop in September was higher in rotation units.
This last point is significant but understated. It suggests that total productivity was greater at the end of each growing season for the rotational treatments; it is puzzling why the authors did not dedicate greater attention and analysis to this data point.
Though much detail is also included about the changes in total standing crop and relative percentages of species and species categories, little insight is offered from this analysis. Overall, total standing crop declined over time and at higher stocking rates. Little bluestem decreased the most, with midgrasses increasing in percent composition in partial response to the decline in little bluestem.
The authors conclude:
Vegetation response to grazing system was minimal in this study…Rotation grazing had no positive impact on the standing crop or relative contribution of any major vegetation component over the study period compared to continuous grazing. Any effects of grazing system were present at both the beginning and end of the study. It could be suggested that higher standing crops of the desirable tallgrasses in rotation pastures at the end of the grazing season would eventually translate into higher vigor and greater relative contributions of these species over time. This did not occur over the 5 years of our study. Shortgrasses were the only component affected by rotation grazing. Rather than diverging, the shortgrass component in the rotation pastures became more similar to that in the continuous pastures over time.
Similar conclusions about variations in stocking rate are also presented:
The standing crop of all major vegetation components, including forbs, declined as stocking rate increased. This was expected as greater numbers of animals translated into higher total forage consumption. The relative contribution of big bluestem, indiangrass, and switchgrass also declined as stocking rate increased which suggested that these species were preferred by livestock. Few changes were attributed to stocking rate over time.
My own response to this research is manifold. The early vegetative response to rotational grazing was favorable, yet converged to the continuous grazing at the end of the study. No data from the middle years of the study were included. Did the seemingly arbitrary shortening of recovery periods have the inadvertent effect of creating a de-facto continuous grazing system?
This question raises the specter of a larger problem that haunts grazing research: scale. Most ranching operations that practice high-density, short duration grazing do so at a scale several orders of magnitude greater than the scale at which this type of research is conducted. This makes larger herds and high densities a practical impossibility, and also, as we have seen, can force a faster rotation through paddocks, compromising the manager’s ability to time-control recovery periods.
Finally, the authors note that “all experimental pastures were in high seral condition when the study was initiated.” High stock densities are most useful in situations of extensive soil capping and over-rested grasses. The tool of animal impact can also be used to promote the cycling of organic material and mulch back into the soil surface. No data was presented about the potential or effect of this tool.