Long-term effects of fertilizer on yield and species composition of contrasting pasture swards in the Aspen Parkland of the Northern Great Plains

Little is known about the effects of long-term application of fertilizer on the complex pasture swards of the Aspen Parkland region of western Canada. Experiments were conducted, lasting from 1980 to 1992, on five contrasting swards representative of permanent pastures in northeast Saskatchewan to determine the long-term effects of N, P, K and S fertilizer on herbage yield and species composition. The experimental swards consisted primarily of smooth bromegrass (Bromus inermis Leyss.), Kentucky bluegrass (Poa pratensis L), and creeping red fescue (Festuca rubra L.), in varying amounts. Seven fertilizer treatments were applied at each site from 1980 to 1991 (shown as N-P-K-S in kg ha−1): 0-0-0-0, 45-0-0-0, 45-20-0-0, 90-0-0-0, 90-20-0-0, 90-20-50-20, 180-20-0-0. Another treatment (90-20-0-20) was added in 1984. An area within each plot was protected from grazing with wire exclosures and harvested in July and September. Harvested samples were classified according to coarse-leafed grasses (mostly smooth bromegrass), fine-leafed grasses (mostly Kentucky bluegrass and creeping red fescue), legumes and weeds before drying and weighing. Ground cover composition of all plots was measured by a modified line transect technique in 1980, 1987 and 1992. All swards, regardless of species composition, responded positively to fertilizer. The yield increase obtained from applying 45 or 90 kg N ha−1 (0.8 and 0.7 t ha−1, respectively) was more than doubled by adding 20 kg P ha−1 with the N. Nitrogen applied alone did not affect the proportion of bromegrass but increased the proportion of fine grasses in the herbage; adding N and P increased the proportion of coarse grass and at the expense of fine grasses. With respect to sward composition, N decreased the proportion of bromegrass and increased the proportion of bluegrass and fescue whereas adding N and P had the opposite effect. Application of S increased yield 0.9 to 1.8 t ha−1 and greatly increased the proportion of bromegrass in the sward and the harvested herbage of the three sites with low levels of soil S. The residual effect of the N and P treatments on yield was small but the effect on species composition of the herbage was substantial; S produced a residual effect on yield in 1992 and a large residual effect on species composition. The results indicate the need for balanced nutrient application to enhance yield and maintain or improve sward species composition of pastures in the Aspen Parkland. Fertilizer can be used to improve yield on a wide range of sward types. Key words: Bromegrass, Bromus inermis, bluegrass, Poa pratensis, fescue, Festuca rubra, legume, N, P, K, S, grazing, species diversity