SPECTRAL REFLECTANCE MEASUREMENTS OF ALFALFA UNDER SHEEP GRAZING

Lamb grazing experiments conducted on alfalfa (Medicago sativa L.) fields require numerous measurements of phytomass in order to identify optimum conditions for lamb weight gain. Our objective was to test the ability of spectral reflectance measurements with a portable hand-held radiometer to predict alfalfa phytomass. We used vegetative indices consisting of linear combinations of the near infrared (NIR) and red wavelength intervals, such as the NIR/Red ratio and normalized difference (ND). Reflectance measurements were taken during two grazing trials where alfalfa phytomass ranged from 200 g m-2 initially to negligible phytomass after 15 d of intensive grazing. A portion of the alfalfa was desiccated due to frost damage during the second trial. The ND and NIR/Red were well correlated with alfalfa phytomass (r = 0.87-0.97). Measurements taken at solar zenith angles (57 and 69-degrees) were found to produce similar ND/phytomass regression coefficients. The desiccated alfalfa increased red reflectance, which consequently lowered the ND and NIR/Red values. The ND was preferable to NIR/Red because it was more sensitive to low phytomass levels that are characteristic of grazing studies. An ND of 0.55 identified a threshold phytomass level, below which continued grazing caused a decrease in lamb weight gain. Handheld radiometric measurements are a quick, accurate, nondestructive means of estimating alfalfa phytomass in pasture grazing experiments.