Within-paddock variation in pasture growth: landscape and soil factors

In cropping systems the continuing development of precision farming promises much in the way of efficient resource (input) use. An extension to pasture agronomy is some way off. In order to gain some background information on within-paddock variation in pasture growth and the main factors responsible, two paddocks in southern NSW (one with naturalised pasture, 'Yaven' and the other improved, 'Cootamundra') were closely monitored over the major growth period (winter-spring) in 1998. Sampling locations (~ 100) were approximately evenly spaced over the 60 ha of the Yaven paddock and 40 ha of the Cootamundra paddock. In May, soils were sampled for pH, available phosphorus (Olsen P), exchangeable cations, electrical conductivity and organic carbon as well as soil depth and profile description. At Yaven the percentage of gravel (> 2mm) was also measured. At each sampling location pasture growth (June ' November), peak spring dry matter and spring botanical composition were measured. Growth over the measurement period ranged from 470 ' 9560 kg/ha for Yaven and 1830 ' 15,720 kg/ha for Cootamundra. Similarly there was wide variation in all of the landscape and soil characteristics measured. All subsets multiple regression was used to identify those site characteristics that were related to productivity. For both sites a significant amount of variation could be accounted for by site characteristics. For instance at Yaven 30% of the variation in growth could be accounted for by two variables: % gravel in the profile and soil pH. At the more uniform Cootamundra site, landscape and soil factors (only two: calcium as a proportion of exchangeable cations and position in paddock, Northing) explained 24% of variation in growth. However, if the species growing at each site were taken into account then the proportion of variation accounted for, increased by 30 % and 9 %, respectively. These results are discussed in relation to the complex species associations