The Influence of Grazing Pressure on Rooting Dynamics of Caucasian Bluestem

Abstract

Caucasian bluestem (Bothriochloa caucasica (Trin.) C.E. Hubb.) is a warm-season grass introduced from Eurasia that is currently used for reseeding farmland and depleted range in the Southern Great Plains. Although this species is thought to be grazing tolerant, little specific information is available concerning its response to grazing. Variable (put-and-take) stocking was used to maintain heavy (3 to 8 steers/ha) and light (2.5 to 4.5 steers/ha) grazing treatments during mid May to late September from 1983 to 1985. Seasonal changes in root mass and root length to a depth of 60 cm were measured the first 2 years, and end-of-season root length was measured the third year. Leaf area index (LAI) was measured during the first 2 years. Peak root mass was 27 and 46% less in heavily relative to lightly grazed swards in 1983 and 1984, respectively. Total root length for heavily grazed swards was 33 and 45% less than lengths of lightly grazed swards in 1983 and 1984, respectively. Heavy grazing resulted in a relatively larger reduction in LAI than in either root mass or length, and thus the ratio of absorbing root surface to transpiring leaf surface was greater for heavily grazed than lightly grazed plants. This increased ratio may explain our previous observation that heavy grazing resulted in an improved water status of leaf tissue. End-of-season total root length over the 3-year period (15 to 18 and 24 to 28 km/m2 for heavily and lightly grazed swards, respectively) was remarkably consistent given the variable climatic conditions over the study period.