Comparison of Heifer Weight Gains and Forage Quality for Continuous and Short-Duration Grazing Systems

Abstract

A study of animal performance and forage quality under continuous and short-duration grazing (SDG) systems was conducted with post-weaning heifer calves on smooth bromegrass (Bromus inermis) pasture for 2 years. There were 8 paddocks for each SDG cell. The animals were on pasture from early May through the middle of August in both years. Heifers assigned to the SDG system were moved among the rotational paddocks approximately every 2.5 days. In 1982, the grazing treatments were stocked at equal levels (2.9 animals/ha). The stocking rate was increased to 3.8 animals/ha on the SDG treatment in 1983, while the continuous system remained at 2.9 animals/ha. Animals were weighed and forage samples were collected at the completion of each rotation cycle. Average daily gain (ADG) was similar (continuous 0.48 kg/d vs. SDG 0.47 kg/d, P>.05) in 1982 when both systems were stocked equally. Available forage tended to be greater under the SDG system (3,141 vs. 3,786 kg/ha), but this difference was not signiflcant. Forage quality did not differ (P>.05) between the grazing systems but did decline significantly in both systems during the grazing season. Individual paddocks of the SDG system did not differ significantly in forage quality. In 1983, ADG was similar for both grazing systems (0.56 and 0.52 kg/d, P>.05) and available forage also was similar (2,551 vs. 2,159 kg/ha). Crude protein content of the forage tended to be greater for the SDG system (7.9 vs. 8.5%, P<.05) in 1983. In vitro digestibility and crude protein content were lower, and cellulose and lignin concentrations were higher in forage from paddocks grazed later in the rotation sequence in 1983. The SDG system increased available forage when stocking rates were equal for the grazing systems, and this forage was effectively utilized at a higher stocking rate for the SDG system to produce more grain per ha (165.6 vs. 205.6 kg/ha) without sacrificing individual performance. The use of short-duration grazing (SDG) systems to increase productivity of rangeland has received substantial interest recently (Heitschmidt and Walker 1983, Malechek and Dwyer 1983). This stems primarily from the claim of Savory and Parsons (1980) that a properly managed SDG system will generally result in a doubling of carrying capacity while maintaining equivalent levels of production per animal to those seen under more conventional systems. Experiments where stocking rates were equal for both continuous and SDG systems have resulted in equal animal gains (Sharrow and Krueger 1979). Accompanying this was an increase in available forage under the SDG system (Sharrow 1983). Rotational grazing increases plant growth rate (Chapman et al. 1983) and heavy grazing pressure results in more uniform use of pasture (Briske and Stuth 1982). These plant responses may account for the claims made for increased productivity under the SDG system. The objective of this study was to compare forage responses to continuous and SDG systems. Available forage and quality meaAuthors are research animal scientist, USDA, ARS, Roman L. Hruska US Meat Animal Research Center, Clay Center, Neb. 68933; professor, Department of Animal Science, University of Arizona, Tucson 85721; and research leader, Production Systems Unit, USDA, ARS, Roman L. Hruska US Meat Animal Research Center. L.J. Koong's present address is Associate Director, College of Agriculture, University of Nevada, Reno 89557. Manuscript accepted June 7, 1984. surements were monitored throughout 2 grazing seasons. Effects of grazing sequence through the rotational paddocks on forage parameters was also studied. Performance of the cattle was monitored relative to the rotational cycles. Materials and Methods Replicated continuous and SDG systems were developed by dividing a 64.8-ha smooth bromegrass (Bromus inermis) pasture into 4 16.2-ha cells. Two of these cells were further subdivided into eight 2.0-ha paddocks each with electric fencing. The SDG cells were arranged in a radial design with a central work and watering area. All cells were mowed in early spring, prior to initiation of active growth of the grass, and fertilized in April with 74 kg N/ha each year. Water and a mineral supplement were available to the cattle at all times. Angus, Hereford, Charolais, and Angus X Hereford heifer calves born in September-October of the previous year were used. Breeds were allocated equally between grazing systems and cells, and mean weights were equalized among cells (222 kg in 1982, 231 kg in 1983). In 1982, the continuous and SDG treatments were stocked with 2.9 animals/ ha. Heifers assigned to the SDG system were rotated on a predetermined schedule averaging 2.5 d of grazing per rotational paddock (range 2-4 d). Rotation order through the paddocks remained constant, with 18.5 d of rest (range 17-19 d) after each grazing bout. In 1982, grazing was initiated on 12 May and terminated on 20 August after 5 complete cycles through the rotational paddocks. During the second year of the study, the continuously grazed cells were again stocked at 2.9 heifers/ha. However, the stocking rate of the rotational cells was increased to 3.8 animals/ ha. Grazing was initiated 11 May 1983 and terminated 19 August 1983. The grazing interval per paddock and number of cycles was the same as the previous year. All cattle were weighed after completing each rotational cycle. The available forage in each cell was sampled on the initiation date of the experiment each year and after completing each rotational cycle. Therefore, paddocks were sampled after varying lengths of rest from grazing (018d). Standing forage was clipped with hand shears from 42 quadrats (0.09m2) in each continuously grazed cell and 6 quadrats were clipped in each paddock of the 2 rotationally grazed cells. Forage samples were dried in a forced-air oven at 70?C in 1982 and 45?C in 1983. After grinding to pass a I mm screen, samples were analyzed for dry matter (1000C), crude protein (N X 6.25) and fiber components (Goering and Van Soest 1970). In vitro dry matter digestibility was determined according to Tilley and Terry (1963). All results are reported on a dry matter basis. The data were analyzed as a split-plot design. Grazing systems were the whole plot and variation attributed to replicates within grazing systems served as whole plot error. Cycle or sampling date was the subplot main effect. The interaction of grazing treatment and cycle or sampling date was also tested in the subplot. Subplot error was the variation that remained after other effects in the model were accounted for. Data for 1982 and 1983 were analyzed