Seeding Rate Effects On Nitrogen Replacement Value of White Clover in Tall Fescue.

Binary mixtures of legumes and grass reduce dependence on purchased nitrogen fertilizer.  Legume proportions of 30-40% of the sward dry matter are believed necessary for optimum N replacement value (NRV), but it is not clear how legume and grass seeding rate relates to the botanical composition of the final mixture or its stability over time. We conducted a plot-scale replacement series experiment to evaluate performance of varying seeding rates of white clover (Trifolium repens L. cv. ‘Alice’) and a soft-leafed endophyte-free tall fescue (Schedonorus arundinaceus [Schreb.] Dumort. nom. cons. cv. ‘Bariane’) on a pasture site in mid-Michigan. Legume/grass plots were seeded in 2010 using clover:grass seed ratios of 0:100 (8 kg/ha clover), 25:75, 50:50, 75:25, and 100:0 (28 kg/ha fescue) of the monoculture rate by weight.  Monoculture plots of tall fescue were integrated into the trial and fertilized with 0, 50, 100, 150, 200, and 300 kg/ha of N to determine N response curve for calculation of N replacement value of clover. Plots were harvested three times each in 2011 and 2012. Cumulative tall fescue yield increased linearly by 7 and 24 kg/ha for each kg/ha of N applied in 2011 and 2012, respectively. Clover proportion was compared in the third cutting each year.  In 2011, clover percentage was 22.5 to 47.7% in mixtures, but mixtures did not yield more than grass monoculture (P > 0.05). In 2012, clover proportion in mixtures was greatly reduced and did not differ from grass monoculture (P > 0.05), but 50:50 and 0:75 seeding ratios yielded more forage than grass monoculture (P < 0.05).  Seeding 50 to 75% white clover with tall fescue was equivalent to as much as 107 kg of fertilizer N over time, but results were not consistent across years and may depend on death of clover.