Abstract
Methodology was developed utilizing remote sensing techniques to separate and quantitatively measure the various components of alfalfa (Medicago sativa L.) fields containing void areas as well as short grass and weeds. Infrared color film was exposed over mixed hay fields in the Carp and Vernon regions of eastern Ontario in the spring of 3 successive yr (1974–1976). Ground observations were made to ascertain field conditions to confirm the location and the interpretation of dense or sparse alfalfa, tall or short grass, weeds and void areas on the photographs. In 12 representative fields, the percentage of alfalfa, grass and void areas was determined for each year by image area measurements based on optical densities of the photographs. Analysis of soil and alfalfa root samples from these fields confirmed the absence of the root rot pathogen Phytophthora megasperma Drechs. or any other fungi pathogenic to alfalfa. Saprophytic species of Fusarium and Pythium were prevalent in each field. The genera of nematodes found in the samples were not considered harmful to alfalfa. Therefore, an estimated 14% loss of alfalfa was attributed to winter injury during the 3-yr period. The amount of grass increased by 28% and void areas decreased by 14% in these fields.
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