Article: Tobosa Tiller Defoliation Patterns Under Rotational and Continuous Stocking
Authors: Senock, R.S. Anderson, D.M., Murray, L.W. and Donart, G.B.
Journal: J. Range Management 46:500, November 1993
As we are beginning to see from this literature review, one of the principle benefits of rotational grazing over continuous grazing is the ability to control utilization at the spatial and temporal dimension. This is demonstrated in various experimental and theoretical environments.
In this study, Tobosa grass (Hilaria mutica) was monitored over the course of a two year grazing study conducted at the Jornada Experimental Range 37 km north of Las Cruces, New Mexico. The objective of this study was to “compare the frequency of defoliation and change in height on individually marked tillers of tobosa under high-density seasonal rotational stocking and low-density seasonal continuous stocking.”
Tobosa is a classic “increaser” species, requiring periods of burning or grazing to maintain productivity. . As the article notes:
Culms become coarse and relatively unpalatable with increasing maturity. Without periodic removal of old growth, such as through burning, tobosa culms and litter can accumulate and decrease future biomass production…Relatively high productivity coupled with slow decomposition rates under arid conditions promote litter accumulation and reduces uniform utilization by livestock.
In much of the arid Southwest, this species tends to grow in monotypic stands, requiring grazing strategies that maximize even utilization on the landscape.
Two grazing treatments were established, one continuous and the other rotational, on paddocks approximately 34 ha in size. The height of randomly selected tobosa tillers was measured during each monitoring event, and each tiller was marked with white correction fluid to monitor the frequency of defoliation. Above ground biomass was also measured using a rotary mower. Stock density was 14 times higher in the rotational treatment than the continuous.
The results of the study have positive implications for high stock density rotational grazing. Foremost amongst these results are the comparisons between treatments of grazing uniformity and forage utilization:
Overall, the percentage of tillers grazed at least once in the rotational paddock was greater than 75% in 1985 and 95% in 1986. In contrast, the percentage of tillers grazed at least once in the continuous paddock over the whole season was never greater than 30% and there were several days in each grazing period when no defoliation was recorded…The distributions of tiller heights at the end of grazing indicated uniform grazing in [the rotational treatment].
In regards to the frequency of defoliation, the study concludes:
…there was a higher probability of grazing occurring in [the rotational treatment] compared to the continuously stocked paddock. Within [the rotational treatment], however, the probability of tillers being re-grazed was much less than the probability of a single grazing…Thus the short duration grazing system resulted in only a single defoliation for the large majority of tillers in the rotational paddock…
One caveat to this single defoliation is noted by the authors, as this contradicts other research that suggests higher percentages of multiple defoliations under heavy stocking. Because the tobosa range-type is fairly monotypic, grazing selectivity may be reduced in this particular environment, as little benefit is conferred to the animals by grazing more selectively. In other environments where grass species diversity is higher, animals may have learned to graze more selectively. In such cases, managers must be careful to move animals from a paddock before regrowth begins; failure to do so compromises the manager’s ability to provide adequate recovery.
The management implications of this study are worth noting, and corroborate the large body of evidence being compiled on this blog:
Continuous stocking limits flexibility since stocking rate adjustments cannot provide adequate control over defoliation frequency, especially on individual plants. Therefore, limited control of defoliation can lead to deterioration of vegetation. Rotational strategies provide the opportunity to improve harvest efficiency through better control and frequent adjustment of grazing pressure.
In sum, even control of utilization and harvest efficiency is one of the most experimentally proven benefits of rotational grazing.