Short Duration Grazing at the Texas Experimental Ranch: Effects on Aboveground Net Primary Production and Seasonal Growth Dynamics

Abstract

The effects of short duration grazing on aboveground net primary productivity (ANPP) and seasonal growth dynamics were evaluated. Total ANPP was estimated to be 234 g/m2 in 1978 in the ungrazed control plot as compared to 330 g/m2 in the grazed treatment plot. ANPP estimates in 1979 were 352 and 268 g/m2 in the ungrazed and grazed treatments, respectively. Close examination of growth dynamics suggest that under certain environmental conditions grazing accelerated vegetative growth. The effect of livestock grazing on rangeland vegetation has been most often related to changes in standing crop resulting from changes in species composition (Sims et al. 1978, Sims and Singh 1978, Van Poollen and Lacey 1979). But information regarding the effects of grazing on the dynamics of aboveground net primary production (ANPP) are seriously lacking. A primary reason for the lack of such studies can be related to experimental design. With year-long and seasonal grazing systems, fenced exclosures or portable cages must be utilized to prevent livestock consumption of forage. Exclusion of livestock grazing creates an artificial environment because periodic grazing may alter the growth response of a plant in contrast to a non-grazed plant (McNaughton 1979). This problem may in part be alleviated in a short duration grazing system (SDG) if the period of grazing is short. Utilizing frequent harvest techniques, the effects of grazing on the dynamics of aboveground growth may be evaluated and estimates of ANPP calculated. The primary objective of this study was to quantify the shortterm effects of a SDG system on aboveground biomass dynamics and ANPP. It has been hypothesized that an increase in AN PP will occur at optimal levels of herbivory (Dyer 1975, McNaughton 1979) and Heitschmidt et al. (1982a) hypothesized that an increase in ANPP may be a major factor whereby a significant increase in livestock carrying capacity may be realized following implementation of a SDG system utilizing the method of grazing advocated by Savory (1978). Treatment and Study Area The study was conducted at the Texas Experimental Ranch located (994 14'W, 330 20'N) in Throckmorton County during the 1978 and 1979 growing seasons. The ranch consists of approximately 2900 ha of native grasslands. Continuous weather data since 1961 (unpublished), indicates a highly variable climate of warm wet springs and falls, mild winters, and hot summers. Mean daily maximum temperatures range from 13? C in January to 360 C Authors are associate professor and former research associates, Department of Range Science, Texas A&M University Research and Extension Center, Box 1658, Vernon 76384. Appreciation is expressed to the Swen R. Swenson Cattle Company for providing the land, livestock, and facilities for this research project. Report is published with approval of the Director, Texas Agricultural Experiment Station as TA 16357. in July. Mean daily minimum temperatures range from -3?C to 21?C in January and July, respectively. Average precipitation is 690 mm bimodally distributed (Fig. 1). Average frost-free growing season is 233 days extending from March to November. Study plots were located in an ungrazed area and in one pasture of a 10-pasture SDG treatment. The SDG treatment was stocked with Hereford/Angus crossbred growing heifers. Because of an inadequate labor force during winter months, the trial was run only from early April to early October each year. Stocking rate during these 6 months was 0.24 ha/AUM. A stocking rate near 0.54 ha/ AU M is considered moderate for mature cows under yearlong continuous grazing at the ranch (Heitschmidt et al. 1982b). Length of each grazing event ranged from a minimum of 3 days during periods of rapid vegetative growth to a maximum of 7 days during summer dormancy. Periods of rest ranged from 35 to 42 days. Both treatment plots were 60 X 60 m subdivided into two 30 X 60 m replicates. Slope was approximately 1% with a southwest exposure. The ungrazed plot was located approximately 30 m south of the grazed plot. Prior to initiating the study in 1978, both areas were periodically grazed at a moderate rate of stocking during various grazing trials. The ungrazed treatment plots had not been grazed for approximately I year. The grazed plots had not been grazed for approximately six months.