AN EVALUATION OF THE T-SUM METHOD FOR EFFICIENT TIMING OF SPRING NITROGEN APPLICATIONS ON FORAGE PRODUCTION IN SOUTH COASTAL BRITISH COLUMBIA
Seven field trials were conducted over 3 years (1984–1986) at two locations (Agassiz and Oyster River) in south coastal British Columbia to determine forage response to 100 kg N ha−1 applied at various time intervals in the spring according to the accumulation of average air temperatures above 0 °C from 1 Jan. (T-sum). A T-sum of 200 has been reported to be the optimum time for N application in western Europe and the United Kingdom. Both urea and ammonium nitrate were applied at the Oyster River location, while only ammonium nitrate was applied at Agassiz. First-cut forage dry matter production responded to the timing of N application in a variety of ways in the seven trials, with a decrease in growth as N was applied later in the season in most cases. In one trial, dry matter production was lowest at T-100 and T-150 compared to later times of application. Although there were variations among the trials, overall the highest yields occurred when N was applied at T-200 to T-300. Crop quality (%N or crude protein content), however, tended to increase as N was applied later in the season. Recovery of N in the plant and soil at harvest was relatively uniform for all times of N application and the distribution of extractable inorganic N in the soil profile suggested little N leaching. The dominant form of inorganic N found in the soil was ammonium. The cool soil temperatures and flush of plant and microbial activity probably contributed to the apparent lack of leaching and response of the grass to the N applied at various times early in the growing season. The timing of N application in the spring resulted in varying residual effects, whether N was applied or not after the first cut.Key words: N fertilization, yield, quality, timing, N recovery