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Determination of Pasture Biomass Using Four Indirect Methods

Dep. of Agronomy, 2101 Agronomy Hall

James R. Russell

Dep. of Animal Sci., 301 Kildee Hall, Iowa State Univ., Ames, IA 50011

^* Corresponding author (hormone{at}iastate.edu).

The most accurate method for determining canopy biomass of pastures for forage availability is by cutting and weighing forage from known areas. Faster methods that require less time and labor would help producers to monitor forage availability in pastures on a daily or weekly basis. Indirect methods rely on calibrations performed on pure or evenly distributed plant compositions to determine forage biomass. However, microclimates developed by varying landscape positions and soil morphological characteristics of pastures may cause uneven plant and species distributions. This study was performed to compare the ability of a modified Robel pole, rising plate meter, canopy height stick, and Li-Cor LAI ZOOO leaf canopy analyzer to determine forage availability in pastures with varying species composition from four areas. Swards consisted of pure warm-season grass stands, cool-season grass stands, legume stands, and grass-legume mixtures. Instrument readings were compared with forage availability determined by clipping and were measured for accuracy, or closeness to clipped weight. For all observations, coefficients of determination (r²), were 0.63, 0.59, 0.55, and 0.32 for the modified Robel pole, rising plate meter, canopy height stick, and leaf canopy analyzer, respectively. For modified Robel pole readings, r² was highest for observations in red clover (Trifolium pratense L.) (r² = 0.83), smooth bromegrass (Bromus inermis Leyss.) (r² = 0.82), and alfalfa (Medicago sativa L. (r² = 0.76) swards, whereas the rising plate meter r² values were highest for observations in tall fescue (Festuca arundinacea Schreb.) (r² = 0.85), alfalfa (r² = 0.84), and red clover (r² = 0.73) swards. Grass observations also had their highest r² values with the modified Robel pole and rising plate meter at 0.63 and 0.59, respectively. The modified Robel pole proved to be the most accurate method used over a variety of species.

Received for publication December 5, 1996.

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