Female-biased herbivory in fourwing saltbush browsed by cattle

Abstract

Female fourwing saltbush (Atriplex canescens Pursh [Nutt.]) shrubs are more abundant in exclosures than in adjacent grazed pastures at our research site on the shortgrass steppe in Colorado. We hypothesized that female shrubs at this site were being browsed more heavily by cattle than were male shrubs. We conducted a series of 2-year experiments (1997 and 1998) with cattle to measure levels of cattle utilization of male and female shrubs. Overall, utilization of marked leaders was 43.5% in January, 19.7% in April, and 33.4% in September. Percent utilization of marked leaders was consistently and significantly higher on female shrubs both in January (females: 46.5%, males: 40.2%), and September (females: 36.9%, males: 29.9%). In April, differences in utilization of shrub sexes were not significant (females: 20.3%, males: 19.2%). The female-bias in cattle herbivory increased significantly with increasing overall utilization of shrubs. Gender-biased herbivory may have promoted higher mortality among female shrubs, leading to the sex ratio alteration previously observed at this site.