Francisco Molinar

Long-Term Vegetation Productivity and Trend Under Two Stocking Levels on Chihuahuan Desert Rangeland

Abstract Vegetation changes were evaluated over an 11-year period (1995–2005) on 2 light- and 2 conservative-stocked Chihuahuan Desert pastures in south central New Mexico. Grazing treatments were applied to the pastures over a 5-year period from 1997 through 2001. Pastures were not grazed in the 1995–1996 and 2002–2005 periods due to drought. During the 1997–2001 grazing period, grazing use of primary forage species averaged 29% and 40% on light- and conservative-stocked rangelands, respectively. Grazing intensity was consistently higher on conservative-stocked than light-stocked pastures. During our study heavy grazing occurred only in 1 year on pastures with conservative stocking. There were no differences in species or species categories (grasses, forbs, shrubs) of autumn standing crop and basal cover between light-and conservative-stocked pastures. Standing crop of total vegetation and perennial grasses showed large fluctuations among the years due to variable rainfall. Under both treatments, total herbaceous standing crop was unchanged, but perennial grass standing crop declined by over 50% when the last 3 years of study were compared with the first 3 years of study. Broom snakeweed (Gutierrezia sarothrae Pursh), a poisonous half shrub, increased in standing crop and cover during the study. Basal cover of total perennial grasses declined less under light than conservative stocking during the study period. However, climatic conditions exerted the overriding influence on vegetation standing crop and basal cover. Our study indicates that light stocking in the Chihuahuan Desert does not increase perennial grass production compared to conservative grazing but it could have a small benefit in maintaining perennial grass cover during drought. We believe our findings have broad application in the Chihauhuan Desert, but caution they might not apply well to other arid rangeland types.

Long-term impacts of livestock grazing on Chihuahuan Desert rangelands.

Rangeland ecological condition was monitored over a 48 year period on 41 sites on Bureau of Land Management rangelands scattered across 6 counties in southwestern New Mexico. All sites were grazed by livestock during the study period. Sampling occurred in 1952, 1962, 1982, 1992, 1997, 1998, and 1999. A modified Parker 3 step method in conjunction with Dyksterhuis quantitative climax procedures were used to determine rangeland ecological condition. At the end of the 48 year study period (1952-1999), the average rangeland ecological condition score across study sites was the same (P > 0.05) as the beginning of the study (39% versus 41 % remaining climax vegetation, respectively). Major changes (P > 0.05) in rangeland condition occurred within the study period due to annual fluctuations in precipitation. Ecological condition scores increased in the 1980s and early 1990s due to above average precipitation. However, drought in the early to mid 1950s and again in the mid to late 1990s caused rangeland condition scores to decline. At the end of the study (1997-1999), 38% of the sites were in late seral ecological condition, compared to an average of 25% in the 1952 to 1982 period. The amount of rangeland in late seral ecological condition increased while the amount of rangeland in mid seral and early seral condition decreased in the 1990s compared to the 1952-1962 period. The average percent cover of black grama (Bouteloua eriopoda Torr.) and tobosa (Hilaria mutica Buckley), the primary forage grasses in the Chihuahuan Desert, were the same (P > 0.05) in 1952 and 1999. Over the 48 year study period, the average cover of shrubs including honey mesquite (Prosopis glandulosa Torr.) showed no change (P > 0.05). However major increases in honey mesquite basal cover occurred on 1 site and creosotebush (Larria tridentata [Pursh] Nutt.) increased on another. Grazing intensity was evaluated during the last 3 years of study (1997, 1998, 1999). Overall grazing use of forage across sites and years averaged 34% or conservative. Our research shows controlled livestock grazing is sustainable on Chihuahuan Desert rangelands receiving from 26-35 cm annual precipitation.

Long-Term Forage and Cow-Calf Performance and Economic Considerations of Two Stocking Levels on Chihuahuan Desert Rangeland☆

ABSTRACT Forage and cow-calf productivity on two lightly and two conservatively grazed pastures were evaluated over a 15-year period (1997–2011) in the Chihuahuan Desert of south-central New Mexico. Spring-calving Brangus cows were randomly assigned to pastures in January of each year. Pastures were similar in area (1 098 ± 69 ha, mean ± SE) with similar terrain and distance to water. Utilization of primary forage species averaged 27.1 ± 3.0% in lightly stocked pastures and 39.4 ± 4.0% on conservatively stocked pastures. No differences in perennial grass standing crop (163.5 ± 52.2 kg·ha-1) and calf weaning weights (286.1 ± 2.6 kg) were detected (P > 0.10) between light and conservative treatments. Lightly grazed pastures yielded greater (P < 0.05) kg of calf weaned·ha-1 and calf crop percent than conservatively grazed pastures in 1998 due to complete destocking of conservatively grazed pastures during that slight drought (i.e., rainfall was 75% of normal in 1998). After the initial 5 years of study (1997–2001), all pastures were destocked for 4 years (2002–2005) due to drought as rainfall was only 50% or less of normal. Pastures were then restocked for another 6 years (2006–2011). Postdrought, the percentage change in perennial grass standing forage crop (kg·ha-1) was -4.0 and -14.4 ± 2.5 % (P < 0.09) in the light and conservative grazed pastures across the 6 years, respectively. While conservative stocking rates may provide higher net financial returns than light stocking rates during nondrought years as there were more AU per pasture, potential losses from cattle liquidation during short-term (i.e., 1-year) droughts could nullify this advantage. Results suggest that light grazing use of forage is a practical approach for Chihuahuan Desert cow-calf operations to minimize risk of herd liquidation during short-term drought.