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 established in 1985 at three locations in Saskatchewan to examine the interaction of (i) harvest frequency, (ii) two alfalfa cultivars, (iii) inoculation 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 treatments 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 assessed using near infrared spectroscopy. Differences in plant numbers, stand density, persistence, and vigor between 'Nordica' (selected for resistance 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 relative to a single-cut treatment. Frequent harvest reduced vigor and first-cut yield of both cultivars in the year following the treatment, but caused larger reductions for Beaver than for Nordica. Total yields for the frequent harvests over all locations and treatments were 43% higher than for the single-cut treatment (5.3 vs. 3.7 Mg ha-1). Potassium fertility and inoculation with fungal pathogens had minimal impact on plant survival and yield. In conclusion, interactions among the factors which cause stand decline were minimal, and the reductions in plant vigor and first-cut yield resulting from frequent harvest were more than offset by the yield from late-season harvests.