Estimating production and utilization of jojoba

Abstract

Jojoba (Simmondsia chinensis) is a major evergreen browse species for livestock and wildlife throughout its range from central Arizona to northwest Mexico and Baja California. Current guidelines for grazing management are based on utilization levels as estimated from determining the percentage of twigs grazed. Utilization can be estimated more accurately from twig diameter measurements. On 3 sites in southern Arizona, leaf weight, stem weight and total weight were correlated with the square of twig internode diameter, having average r2 values of 0.81, 0.73, and 0.83, respectively, for small diameter twigs (less than or equal to 3mm) most frequently browsed. Estimates of twig weight from regression equations for the 3 sites varied less than 0.3 g and low standard errors of estimate (less than or equal to 0.33) indicate twig diameter measurements can give precise estimates of twig weight. Percent utilization of current year's growth can be calculated from estimates of twig weight remaining and twig weight removed by grazing from diameter measurements at initiation of current year's growth and at the point of grazing, respectively. On 2 sites, mean grazed twigs and mean weight utilization were similar for shrubs moderately grazed by cattle. However, regressions of weight utilization on percent twigs grazed indicated that percent twigs grazed could overestimate weight utilization of total twigs and underestimate weight utilization of current year's twigs, especially when utilization is high. An alternative to basing management of jojoba on time-consuming utilization measurements and arbitrary utilization limits is to monitor size of marked shrubs and manage for stable or gradually increasing shrub size.