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Effects of Nitrogen on Heading and on Other Components of Bromegrass Seed Yield in the Subarctic¹

Five rates of nitrogen, from 0 to 270 kg/ha (0 to 240 Ib/A), were topdressed in spring on a 3-year-old stand of ‘Polar’ bromegrass (predominantly Bromus inermis Leyss. x B. pumpellianus Scribn.) in subarctic, southcentral Alaska, and compared during the same and the tollowing year for influence on factors that affect seed yield. In the year of application, spikelets per panicle and weight of seed per panicle increased with increasing rates of N. Different rates of N did not influence panicles per unit area or seeds per spikelet in the year of application. Although differences were not statistically significant, seed yields in the year of different N applications displayed a pattern suggestive of N influence but only as engendered through intra-panicle yield components.

In the following year, when all plots received N in spring at 100 kg/ha (90 Ib/A), the various levels of N applied the previous year affected markedly both panicles per unit area and seed yields per hectare but did not influence seed weight per panicle. These results indicate that for subarctic areas late summer N application should promote higher bromegrass seed yields the following year than application in spring of the year of harvest.

The number of panicles per 1.4 m² was approximately equivalent to the seed yield in kilograms/hectare (panicles/l.5 yd² = seed yield in pounds/acre) in both years when spring N application during the same year was 90 kg/ha (80 Ib/A) or more. At lower rates of N, this formula over-estimated seed yields because of lowered seed yield per panicle.

Key Words: Alaska • Floral induction • Floral initiation • Bromus inermis • Bromus pumpellianus • Photoperiod • Latitude

² Research Agronomist, Crops Research Division, ARS, USDA, Palmer, Alaska, 99645.

Received for publication March 5, 1970.