Grazing system and stocking rate effects on the productivity, botanical composition and soil surface characteristics of alfalfa-grass pastures

Abstract

A 4-yr experiment was conducted (1991 to 1994) near Brandon, MB, to determine the effects of grazing system (continuous and rotational) and stocking rate [light (1.1 steers ha-1); heavy (2.2 steers ha-1)] on the productivity, botanical composition and soil surface characteristics of an alfalfa (Medicago sativa L.; approximately 70%), meadow bromegrass (Bromus biebersteinii Roem & Schult.; 25%) and Russian wild ryegrass [Psathyrostachys juncea (Fisch.) Nevski; 5%] pasture. Grazing season length was shorter (P < 0.05) for cattle in continuously compared with rotationally stocked pastures in 1991, while in 1993 and 1994 it was shortest (P < 0.05) in heavily stocked continuously grazed pastures. Carrying capacity (steer days ha-1) was greater (P < 0.05) in heavily stocked rotationally grazed pastures compared with other treatments in 1991, 1993 and 1994. In 1992, it was greater (P < 0.05) in heavy than light stocking rate treatments for both rotationally and continuously grazed pastures. Cattle usually gained more (P < 0.05) per day (kg d-1) and during the season (kg hd-1 ) at light than at heavy stocking rates, while total liveweight production (kg ha-1) was greater (P < 0.05) at heavy than at light stocking rates. Forage production and disappearance did not differ (P > 0.05) within grazing systems and stocking rates from 1991 to 1993, but in 1994, production and disappearance were greater (P < 0.05) at heavy than at light stocking rates. Mean seasonal herbage mass available and carry-over were greater (P < 0.05) in lightly stocked pastures than heavily stocked pastures from 1991 to 1994. After the first year of grazing, the proportion of alfalfa increased (P < 0.05), while grasses declined (P < 0.05) within all grazing treatments. In subsequent years, a trend was observed, where alfalfa declined and grasses increased in all pastures, except those stocked heavily and grazed continuously, which by 1994 had the greatest (P < 0.05) percentage of alfalfa. As years progressed, increases (P < 0.05) in basal cover concurrent with declines in bare ground were recorded on all grazing treatments, while litter cover often did not differ (P > 0.05) within either grazing system or stocking rate, except in 1992, when basal cover was lowest (P < 0.05), while litter cover was greatest (P < 0.05) on lightly stocked continuously grazed pastures compared with other treatments. Stocking rates were a key factor to optimizing individual animal performance and/or gain per hectare on alfalfa grass pastures, however differences in the effect of continuous and rotational stocking on pasture productivity were minimal.