EFFECT OF UREA AND AMMONIUM NITRATE ON YIELDS AND NITROGEN CONCENTRATION OF TIMOTHY AND BROMEGRASS AND LOSS OF AMMONIA FROM UREA SURFACE APPLICATIONS
Urea and ammonium nitrate were applied at 30, 60, 90 and 120 kg N ha−1 in spring and after cutting the primary growth of timothy and bromegrass in three field experiments. Dry matter yields of timothy and bromegrass and total nitrogen concentration of tissue and nitrogen yields of timothy were determined. Loss of nitrogen as ammonia was monitored on microplots between the end of May and early July. Primary growth yields were usually similar for ammonium nitrate and urea but in the secondary growth timothy fertilized with ammonium nitrate outyielded timothy fertilized with urea in four out of six harvest years. Dry matter response to applied nitrogen was usually curvilinear in primary growth but linear in secondary growth over the nitrogen rates studied. Total nitrogen concentration in primary and secondary growths of timothy increased linearly with nitrogen rate during the initial 2–3 yr; ammonium nitrate and urea were equally effective in all but one harvest year. Total nitrogen production of timothy also increased linearly over the range of 30–120 kg N ha−1, while ammonium nitrate outyielded urea-fertilized timothy during one season in primary and two seasons out of four in secondary growth. Nitrogen losses increased from spring to summer, in general, with 68–75% of the variation explained by air temperature. Urea and ammonium nitrate were considered equivalent nitrogen sources for the primary growth of timothy but urea was less efficient in the secondary growth under summer conditions.Key words: Urea, ammonium nitrate, timothy, bromegrass, ammonia losses