How long before a second defoliation of actively growing grass plants in the Desert Grassland?

Authors: NOELLE, S. - University Of Arizona; LYONS, T. - Bureau Of Land Management; GORLIER, A. - University Of Arizona; MCCLARAN, M. - University Of Arizona; Nichols, Mary; RUYLE, G. - University Of Arizona

Submitted to: Frontiers in Veterinary Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/9/2020
Publication Date: N/A
Citation: N/A

Interpretive Summary: Livestock graze extensively in the southwestern US. If not managed by rotating through pastures, livestock are likely to graze individual plants more than once, thus defoliating the plant. Repeated defoliation of perennial bunchgrass plants during the summer growing season is detrimental. Research was conducted on the Santa Rita Experimental Range south of Tucson, Arizona to determine how many days livestock can remain in a pasture before repeat grazing threatens the viability of grasses. Defoliation events on 1400 individual plants identified in 6 pastures scheduled for summer grazing from June 15 to October 2, 2013 were documented. Plant height and diameter categories were measured before cattle entered each pasture and were re-measured daily or on alternate days throughout the grazing period. Measurements showed that within the Santa Rita Experimental Range, moving livestock after 10 days of grazing during the summer growing season is an effective strategy to avoid second defoliations on individual perennial grass plants while maintaining sufficient forage on the landscape to provide forage during the dormant winter period.

Technical Abstract: In 2007, an adaptive grazing management process began on the Santa Rita Experimental Range (SRER) in southern Arizona. One of the primary ecological objectives was to limit repeated defoliation of perennial bunchgrass plants during the summer growing season. To test the adaptive grazing management outcome, we documented defoliation events on 1400 individual plants identified in 6 pastures scheduled for summer grazing from June 15 to October 2, 2013. Plant height and diameter categories were measured before cattle entered each pasture and were re-measured daily or on alternate days throughout the grazing period. The differences between heights and diameters were used to calculate any regrowth that occurred. Of the 1400 plants marked, 738 (52.7%) were not defoliated, 453 (32.3%) plants were defoliated once, 199 (14.2%) plants were defoliated twice, and 10 (0.7%) plants were defoliated three-times for a total of 881 defoliation events recorded. The majority of first defoliation events and first incidences of second defoliation occurred on day 3. Cattle began switching their grazing strategy from undefoliated plants to new growth of previously defoliated plants by day 9 with 48 of a total 161 previously defoliated plants being regrazed. The highest number of second defoliations were measured on day 11 when 115 plants were observed as having been defoliated for a second time and 5 plants were defoliated for a third time. Arizona cottontop (Digitaria californica) had a higher percentage of both single and multiple defoliation events when compared to the other perennial grass species. Distance from a permanent water source was not found to have a significant impact on the number of defoliations that an individual plant received. Plants defoliated the first time were taller and had a larger circumference (21.0±5.6 cm and 40.8±9.0 cm) than plants that were re-defoliated (15.5±5.6 cm and 31.4±9.0 cm). This study found that the planned 10-day SRER grazing rotation, when followed, would minimize multiple defoliations by cattle on perennial bunchgrasses.