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 fertiliz er on the complex pasture swards of the Aspen Parkland region of weste rn Canada. Experiments were conducted, lasting from 1980 to 1992, on f ive contrasting swards representative of permanent pastures in northea st Saskatchewan to determine the long-term effects of N, P, K and S fe rtilizer on herbage yield and species composition. The experimental sw ards consisted primarily of smooth bromegrass (Bromus inermis Leyss.), Kentucky bluegrass (Pea pratensis L), and creeping red fescue (Festuc a rubra L.), in varying amounts. Seven fertilizer treatments were appl ied 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 e ach 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 weed s before drying and weighing. Ground cover composition of all plots wa s measured by a modified line transect technique in 1980, 1987 and 199 2. 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 affe ct the proportion of bromegrass but increased the proportion of fine g rasses in the herbage; adding N and P increased the proportion of coar se grass and at the expense of fine grasses. With respect to sward com position: N decreased the proportion of bromegrass and increased the p roportion of bluegrass and fescue whereas adding N and P had the oppos ite effect. Application of S increased yield 0.9 to 1.8 t ha(-1) and g reatly increased the proportion of bromegrass in the sward and the har vested herbage of the three sites with low levels of soil S. The resid ual 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 r esidual effect on yield in 1992 and a large residual effect on species composition. The results indicate the need for balanced nutrient appl ication to enhance yield and maintain or improve sward species composi tion of pastures in the Aspen Parkland. Fertilizer can be used to impr ove yield on a wide range of sward types.