Wayne E. Wyatt, Jeffrey M. Gillespie, Brad C. Venuto and David C. Blouin
Most Louisiana beef producers use the cow-calf system of management in which they maintain breeding herds and produce calves marketed at weaning or shortly thereafter. The systems are forage-based, so pasture management is extremely important. To determine the effectiveness of short-duration, rotational grazing systems, a three-year study (Phase 1 of a six-year project), designed to evaluate pasture stocking methods and rates, was initiated in the spring of 1999 at the Iberia Research Station
Sets of four 16-acre and four 10-acre pasture groups were used. Within each set of four pastures, the cows were either grazed continuously on the same pasture at low (.5 cow per acre), medium (.8 cow per acre) or high (1.1 cows per acre) stocking rates or they were rotated among eight paddocks at the high stocking rate. This design helps determine the effects of stocking rate in continuously stocked pastures and to compare continuous and rotational stocked pastures, when both are stocked at a high rate.
Warm season grass pastures were seeded with annual ryegrass (40 pounds/acre) in the fall of 1998, 1999 and 2000. Pastures annually received 155 pounds/acre of nitrogen in three applications. Potassium and phosphorus soil amendments were applied (each 60 pounds/acre) in June of each year. Pastures were not clipped or hayed during the three-year period. One-eighth of each pasture was fenced to provide a confined drylot area for feeding hay and a protein supplement during periods in which forage availability was insufficient.
Mature, spring-calving, straightbred, Brangus cows (either pregnant or with calf at side) were stocked onto treatment pastures in February 1999. Cows were weighed and scored for body condition in January (pre-calving), April (pre-breeding), July (post-breeding) and October (weaning). The body condition score (BCS) ranges from 1 to 9, with 1 being extremely thin and 9, extremely obese. Cows were checked for pregnancy each November.
Cows were also weighed and scored for body condition the subsequent January. Calves were weighed at birth (January-April 15), in late April or early May, in early July, and in early October (weaning) of each year.
Cow weights and body conditions scores were significantly affected by grazing treatment and generally decreased in response to increased stocking rate. However, they did not differ between continuous and rotational stocking methods at the high stocking rate. See Figures 1 and 2. Mean cow weight tended to be greater for cows grazed at the low rate than for cows grazed at the medium and high rates. While mean body condition score was similar for cows grazed at the low rate and the medium rate, it was lowest for cows grazed at the high rate.
Neither pregnancy rate nor calving rate differed significantly among the grazing treatments. The mean body condition score for each treatment was approximately 5 or greater, and several studies have demonstrated acceptable reproductive performance for cows having a BCS of 5 or higher.
Calf preweaning gain and weaning weight differed among the grazing treatments. Calves suckling cows on the pastures with low stocking rates tended to gain at a faster rate and were heavier at weaning than calves suckling cows on the pastures with a medium stocking rate, and these calves in turn gained faster and were heavier at weaning than calves in the high stocking rate treatments. However, preweaning gains and weaning weights were similar for the continuous and rotational treatments at the high stocking rate.
While individual calf performance is important to producers, a measure of pasture productivity (pounds of calf weaned per acre) may hint at differences in cow-calf production efficiency. Within the continuous stocked treatments, pounds of calf weaned per acre (267, 423 and 506 pounds/acre) increased as stocking rate increased. However, pounds of calf weaned per acre were similar for both the continuous and rotational stocking methods when compared at the high stocking rate.
Stocking rate had more impact on cow weight and body condition score, calf preweaning gain and weaning weight, and pasture productivity (pounds calf weaned per acre) than did stocking method (continuous verses rotational). The additional inputs associated with rotational grazing (fencing materials, labor) do not appear to be warranted in terms of improved production efficiency. The economic analysis of this study is still being conducted and will be available in 2006.
The contrast between continuous and rotational grazing was at the high stocking rate, and it is presumed that the 1.1 cows/acre rate would not be sustainable over an extended period (the drylot period was approximately 120 days each year for both treatments). The effect of stocking rate within the context of rotational stocking and the contrast between continuous and rotational grazing treatments at the more moderate stocking rate is also being evaluated in the second phase of this research.
Wayne E. Wyatt, Professor, Iberia Research Station, Jeanerette, La.; Jeffrey M. Gillespie, Associate Professor, Department of Agricultural Economics and Agribusiness, LSU AgCenter, Baton Rouge, La.; Brad C. Venuto, formerly Assistant Professor, Southeast Research Station, Franklinton, La.; and David C. Blouin, Professor, Department of Experimental Statistics, and Associate Dean, LSU College of Agriculture, Baton Rouge, La.
(This article was published in the winter 2006 issue of Louisiana Agriculture.)