Two study sites were selected:
1) a sandy loam site, dominant perennial grasses were needle-and-thread Stipa comata, green needlegrassStipa viridula, blue gramaBouteloua gracilis, prairie Junegrass Koeleria cristata and Sandberg bluegrass Poa secunda;
2) a loam site, dominant perennial grasses were western wheatgrass Agropyron smithii, blue grama, Junegrass and needle-and-thread.
Plots (10.1 x 11.9 m, 4 rows of 6) were randomly assigned one of eight treatments (3 replicates each) on each site: five mowing (and removal of clippings) treatments, two herbicide trials (not reported here), and a control (no treatment).
Three plots on each site were mowed to 2.54 cm (1 in) height on one date in 1977 (24-25 May; 9-10 June; 22-23 June; 6-7 July; and 19-20 July) and 1978 (20-21 May; 7-8 June; 21-22 June; 6-7 July; and July 19 or 21).
One half of each plot was sampled for above-ground biomass in alternate years. In 1977 and 1978, soil moisture content was determined, and basal cover estimates were made in August.
Soil moisture content in spring (beginning of the growing season) had a greater effect on forage yields than mowing. Yields were higher in 1978 as a result of higher soil moisture (January-May 1978 precipitation 19-51% above average).
In 1977 on the sandy loam site, early season (May-early June) mowing significantly reduced needle-and-thread yield, perhaps as cutting compounded the effects of the prevailing dry conditions(January-May1977 precipitation 12% below normal). Mowing on 7 July significantly increased sedge Carex spp. yields compared to controls. In 1978, 21 July mowing increased western wheatgrass and bearded wheatgrass Agropyron subsecundum yields. No significant differences in yields were observed for any other species or groups at either site.
In1978, on the sandy loam site, Sandberg bluegrass basal cover was reduced by all mowing treatments, whilst early season mowing increased blue grama cover (yields of both were unaffected).