Rotational vs. Continuous Grazing Affects Animal Performance on Annual Grass-Subclover Pasture

Abstract

Performance of Romney ewes and their lambs grazing an annual grass-subclover hill pasture was evaluated under both fivepaddock rotational and continuous grazing management treatments during 1977 and 1978. Live weight gains of ewes and lambs tended to be greater under rotational than under continuous grazing during the spring or fall green-feed periods. During the summer dry-feed period, however, sheep maintained thefr body weight better under continuous than under rotational grazing. These data support the hypothesis that rotational grazing most effectively improves animal performance during the green-feed period, perhaps through its reguhtion of pasture production. A common goal of rotational grazing systems is to increase forage production by controlling the frequency and intensity of pasture defoliation. Increased herbage production from rotational compared to continuous grazing management systems has been reported for perennial ryegrass (Ruane and Raftery 1964, Young and Newton 1975) and for perennial ryegrass-white clover (Trifolium repens) pastures (Marsh and Laidlaw 1978). This increase is thought to result from the maintenance of a more nearly optimum Leaf Area Index (LA1 = leaf area per unit ground area) than is achieved under continuous grazing (Smetham 1973). But as Young and Newton (1975) pointed out, increased forage production will result in superior animal production only if the additional forage available is consumed. Lambourne (1956) concluded that where feed supply was insufficient under continuous grazing, the extra feed produced under rotational grazing led to improved lamb growth. When feed supply was already adequate on continuously grazed pastures, the additional forage produced by rotational grazing became rank, causing a decline in forage quality. Lamb growth suffered. This same principle is well illustrated by the stocking rate X grazing management interaction discussed by Robinson and Simpson (1975). They noted that animal performance under rotational grazing only exceeded that of continuous grazing at intermediate to high stocking rates. When stocking rates were either very low or extremely high, rotational grazing tended to reduce animal performance. A second goal of rotational grazing is to improve the efficiency by which forage is harvested and converted to salable animal products. Continuous grazing allows livestock a considerable amount of dietary selectivity (Heady 1961). This selectivity, while allowing animals to choose a high quality diet (Weir and Tore11 1959), often results in overutilization of the most preferred plants allowing less preferred plants to become rank and of low quality (Sharrow and Krueger 1979, Smetham 1973). Undesirable shifts in species composition of the sward may follow as unpalatable species increase and as low growing plants such as clovers are shaded out (Smetham 1973). The high animal density for a short period of time employed by rotational grazing systems tends to increase grazing pressure, thus suppressing dietary selectivity by animals. This favors a more even utilization of all forage produced. The nonuse period immediately following grazing favors the growth of clovers and provides for production of high quality grass regrowth for grazing during the next rotational cycle (Smetham 1973). Since dietary selectivity may be suppressed under rotational grazing, it is important that ample high quality forage be present on each paddock if high animal performance is desired. Heady (1961) expressed concern about the implications of reduced dietary selectivity of livestock if rotational grazing systems were applied to the California annual grassland. He suggests that livestock production would suffer if animals were concentrated on small areas even for short periods of time due to reduced quantity and quality of forage available per animal. U Author is associate professor, Department of Rangeland Resources, Oregon State Although both continuous grazing and rotational grazing sysniversity, Corvallis 97331. terns are widely used in western Oregon, acomparison of these two Paper No. 6210. Oregon Agricultural This riport was submitted as Technical I E: rperiment Station, Corvallis. ~.nn.rrint rwx=ivrd lnnunrv IS 19X2. .._ .._ 1_..r. .__-..__ __..__., __. .._-. systems has not been made on annual grass-subclover pastures. The objective of this paper is to discuss the effects of rotational and JOURNAL OF RANGE MANAGEMENT 36(5), September 1983 593 continuous grazing on the performance of ewes and lambs grazing annual grass-subclover under heavy stocking rates. Table 1. Speeiaa composition (% of weight) and standing crop (kg//ha) of herbage present prior to stocking of pastures grazed continuously (C) and rotationally (R). Materials and Methods Information presented in this paper is taken from the final 2 years of a 6-year study. Animal performance data from the initial 4 years of the study were previously reported by Sharrow and Krueger ( 1979). 18 May 1977 I5 April V978 Research was conducted during 1977 and 1978 at the Soap Creek Ranch, approximately 20 km north of Corvallis, Ore. Elevation is approximately 100 m. The climate is maritime with cool, rainy winters and hot, dry summers. Mean annual precipitation is 103 cm. Species C R C R Vulpia myuros (rattail fescue) 62 61 48 52 Bromus mollis (softchess) 9 9 II 13 L&urn perenne (perennial ryegrass) 4 1 3 T Tk~olium subterraneum (subclover) 20 19 32 29 Misc. grass T 2 2 I