FIELD RESPONSE OF ALFALFA TO HARVEST FREQUENCY, CULTIVAR, CROWN PATHOGENS, AND SOIL FERTILITY .1. SURVIVAL AND YIELD

Declining productivity in older alfalfa (Medicago sativa L.) stands is an important problem in the Canadian prairies. Field trials were esta blished in 1985 at three locations in Saskatchewan to examine the inte raction of (i) harvest frequency, (ii) two alfalfa cultivars, (iii) in oculation with one of three fungal pathogens (Coprinus psychromorbidus , Phoma sclerotioides, or Fusarium spp.), and (iv) K fertilization on survival and forage yield of alfalfa. Harvest and inoculation treatmen ts were initiated in 1986 and continued each year until the tests were terminated in 1989-1990. Plants were dug in spring and fall each year , and total nonstructural carbohydrate levels in the crowns were asses sed using near infrared spectroscopy. Differences in plant numbers, st and density, persistence, and vigor between 'Nordica' (selected for re sistance to C. psychromorbidus) and 'Beaver' were small. Harvest at 10 % bloom (2-3 cuts) did not result in consistent reductions in numbers of plants, stand density, or total nonstructural carbohydrates relativ e to a single-cut treatment. Frequent harvest reduced vigor and first- cut yield of both cultivars in the year following the treatment, but c aused larger reductions for Beaver than for Nordica. Total yields for the frequent harvests over all locations and treatments were 43% highe r than for the single-cut treatment (5.3 vs. 3.7 Mg ha-1). Potassium f ertility and inoculation with fungal pathogens had minimal impact on p lant survival and yield. In conclusion, interactions among the factors which cause stand decline were minimal, and the reductions in plant v igor and first-cut yield resulting from frequent harvest were more tha n offset by the yield from late-season harvests.