Utilization patterns by Angora goats within the plant canopies of two Acacia shrubs

Abstract

Uneven distribution of livestock in large pastures results in some areas receiving more use than the average and some areas receiving little or no use. Six 2-ha experimental pastures on a shallow ridge site were stocked with 2, 4, or 6 Angora goats per ha to reflect different levels of use found in large pastures of south Texas. Two additional pastures on a sandy loam site were stocked with 2 goats per ha. Utilization estimates were made in each pasture using a twig diameter-weight relationship. Estimates of utilization of guajillo (Acacia berlandieri) and blackbrush (A. rigidula) were made in canopy strata which the goats could reach in a quadrupedal stance (low), a bipedal stance (middle), and from the zone above the bipedal stance (high). These measurements were repeated 3 times during the grazing season. Nonlinear regressions of diameter on weight (Y = aXb) collected from plants in control pastures provided a better fit than log-log regressions in almost every instance. Fit index values, which are analogous to R2 values for linear equations, ranged from 0.82 to 0.94 for nonlinear equations and from 0.62 to 0.88 for the log-log regressions. Goats exhibited different grazing strategies by using the canopy strata differently for the 2 plant species. Percent utilization in the middle strata was higher than in either of the other 2 canopy strata within each grazing treatment and for each plant species. Cumulative use in the middle strata for guajillo was 79% compared to 63% in the low and 28% in the high strata. Blackbrush also had highest use in the middle strata with 39% use compared to 27 and 9% for the low and high canopies, respectively. By the third sampling period, use of guajillo in the 2 lowest canopy strata declined and use of blackbrush increased over the first 2 sampling periods. Average grazed twig diameter within each grazing treatment did not vary significantly in the low strata throughout the growing season. On heavily used sites, averaged grazed twig diameter increased in the 2 highest canopy layers as the season progressed. The size of grazed twigs in the middle zone on the heaviest grazed sites was significantly higher than in any other canopy strata.