118 Efficiency of Fall-Banded N: Effects of Application Date, Landscape Position and Inhibitors K.H.D. Tiessen 1 , D.N. Flaten 1 , C.A. Grant 2 , R.E. Karamanos 3 , D.L. Burton 1 , and M.H. Entz 4 E-mail: Don_Flaten@UManitoba.CA 1 Department of Soil Science, University of Manitoba, Winnipeg MB, R3T 2N2 2 Agriculture and Agri-Food Canada, Brandon Research Centre, Brandon MB, R7A 5Y3 3 Western Cooperative Fertilizers Limited, P.O. Box 2500, Calgary AB, T2P 2N1 4 Department of Plant Science, University of Manitoba Abstract We conducted two years of experiments in southern Manitoba to see whether fall-banded N fertilizers require delaying of application date to improve grain yields, as was the case for broadcast and incorporated fertilizers. We also looked at whether losses of fall-banded N are especially large in depressional areas of the field. At each of our four sites, we banded urea (46-0-0) three times in the fall, between September 15 th (early fall) and October 20 th (late fall) depending on weather conditions, and at one time in the spring, mid-row banded at planting. At harvest, we found that in the low landscape positions, crop responses from fall-banded N, relative to spring-banded N, were improved by as much as 40% by delaying application until late in the fall, when soil temperatures had declined to 5 or 6°C. On the other hand, the efficiency of fall-banded urea in the better-drained high landscape positions was not affected by application date in the fall. Presumably, the increased efficiency of late fall-banded N in the low landscape positions was due to less conversion of the urea fertilizer to nitrate prior to winter, reducing over-winter losses. The overall average efficiency of fall-banded N, in terms of grain yield increase as a percent of spring- banded N, was approximately 30% better in the high landscape positions than in the low landscape positions within the same field. This suggests that early fall application of N fertilizer is a viable option on land that is well-drained. However, in wet years, or on poorly drained land where the potential for late fall or early spring flooding is high, producers should wait as long as possible in the fall, or until the spring, to apply nitrogen fertilizer. Introduction To spread their workload, reduce spring tillage operations, and capitalize on lower fertilizer prices, many producers in Manitoba prefer to apply nitrogen (N) fertilizer in the fall rather than in spring. Southern Manitoba historically receives fall rains that make fieldwork difficult and producers are interested in applying N fertilizer as soon as possible after harvest, while soil conditions are still favourable. Unfortunately, early fall applications of ammoniacal fertilizers are expected to form more nitrate prior to the soil freezing than fertilizer applied later in the season (Nyborg et al. 1990), increasing the potential for over-winter and early spring losses of NO 3 - via leaching and denitrification (Yadvinder-Singh et al. 1994). Manitoba Agriculture and Food currently recommends that fall-applied N fertilizers be banded, as opposed to broadcast, and that application be delayed until soil temperatures are below 5EC (Soil Fertility Guide 2001). However, in delaying application there is the increased risk of the producer being caught by an early freeze-up, making field operations impossible. * Published in the Proceedings of the Manitoba Agronomists Conference, Dec. 9-10, 2003, Winnipeg, MB.