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Increased Flowering and Yield from a Bromegrass-Timothy Meadow by Timing of Nitrogen¹

Fall application of fertilizer to irrigated meadows the mountain and intermountain region has both utility and agronomic advantages as compared to spring applications. The objectives of these experiments was to compare the efficacy of fall and spring applied N to determine why fall applications on a bromegrass (Bromus inermis Leyss.)-timothy (Phleum pratense L.) meadow produced higher yields and more abundant flowering of bromegrass than spring applications. Fertilizer was broadcast on the surface of small plots in either spring of fall. Yields were sampled with a sickle-bar mower at haying stage, or at regular intervals throughout the season to estimate uninterrupted growth rates. Small plots were clipped by hand to estimate number of shoots, relative species abundance, and flowering culms. Fall fertilization with N stimulated very early, spring growth of bromegrass, which slightly preceded spring fertilization, increased relative abundance of bromegrass inflorescence, and increased dry matter yields by 15% when harvested at customary haying time. A subsequent, more detailed experiment revealed no visible evidence of floral initiation in either bromegrass or timothy in the fall, and fall fertilization had no stimulating effect on early spring growth of timothy. Unfertilized timothy produced a relative abundance of 3% reproductive culms while both spring and fall-fertilized plots had 33% on 22 July. Relative abundance of bromegrass reproductive culms was 2% from unfertilized and spring-fertilized plots and 25% from fall-fertilized lots. Nitrogen fertilization increased the total number of shoots of both bromegrass and timothy, but fall applications favored bromegrass to the detriment of numbers of timothy shoots. Dry matter accumulation rates from spring and fall-fertilized plots were nearly identical (155 kg/ha/day). Increased bromegrass inflorescences resulting from fall fertilization with N has clear implications for increasing seed yield from bromegrass. The applicability of this practice to different environments is not clear. We recommend fall applications of N to bromegrass (compared to spring applications) for dry matter production under arid conditions because fall applications will assure N is readily available to the plant when temperatures are suitable for growth. Fall-applied N advanced the growing season by about 8 days or (1,200 kg/ha) compared to spring-applied N. This practice will likely be detrimental to timothy in mixed bromegrass-timothy stands.

Key Words: Fertilizer timing • Grasses • Smooth bromegrass • Timothy • Floral initiation • Interspecies competition • Growth rates • N uptake • N efficacy

² Associate professors and professor of Agronomy, (senior author is currently agronomist, USDA, SEA, Cooperative Research, Washington, D. C. 20250) Colorado State Univ., CSU Mountain Meadow Res. Ctr., Gunnison, CO 81230, Fort Collins, CO 80523, and Gunnison, CO 81230, respectively.

Received for publication July 30, 1979.