Big sacaton Sporobolus wrightii is a perennial bunch grass native to southwest USA and northern Mexico. Land managers have traditionally (for over a century) removed dead foliage by autumn/winter burning or mowing aiming to stimulate new growth and enhance forage quality. Although primarily for livestock, this has implications for native grazing animals. Until now however, no studies evaluating burning or mowing and season effects on big sacaton forage quantity and quality had been undertaken.
In January 1980, on a 500 ha big sacaton grassland near Tucson, (31º47’N, 110º 37’W), Arizona, southwest USA, an experiment was initiated. The grassland had been lightly grazed by cattle and horses in autumn, winter or spring from 1976-1980, and moderately cattle-grazed in winter from 1935-1975. Prescribed autumn or winter burns had been implemented at 5 to l0 year intervals between 1935 and 1975.
A 2 ha area was fenced. Ten, 0.3 x 2.9 m areas were randomly located in 15 x 15 m plots. Grass was cut at ground level, weighed, dried and reweighed to calculate forage dry weight (fuel load) and water content.
Plots were burned (headfires) or mowed (to 5 cm stubble height) in winter (6 February), summer (10 July) or autumn (2 October) in 1980, 1981 and 1982.
Twenty, 0.3 x 2.9 m areas were sampled every 6 weeks for 3 years after treatment. Big sacaton plants in four were cut at ground level, separated into green and dead material, and weighed to calculate green and dead forage. Subsamples of each were analysed for total nitrogen (values multiplied by 6.25 to give a measure of crude protein content).
Big sacaton biomass (green and dead forage) varied considerably among years and seasons (2,800-5,000 kg/ha). In winter and summer, green material comprised less than 15% of biomass with water content of 10-30%. In autumn, green forage averaged 55% of biomass, and water content 48%.
Plants burned or mowed in summer produced new leaves within 3 days, and in winter within 20 days. Winter or summer treatment appeared (deceptively) to promote leaf production (compared to controls), with a ‘carpet of green leaves’ from April-August on winter plots and in July-August on summer plots.
Plants treated in autumn took 215-245 days to produce new leaves, probably attributable to the crown being exposed (by burning or mowing) in winter when temperatures may fall below freezing.
Green and dead material following summer treatments were similar to that on untreated controls within 2-3 growing seasons. After three growing seasons, big sacaton on autumn and winter treatment plots had still not fully recovered.
Crude protein in green forage was 3-5% greater in treated plants than in untreated plants for 6 weeks after treatment, but after this no differences were detectable.
Thus burning or mowing in any season removed green forage (otherwise potentially available to grazing animals) and reduced the amount of green material for two or more growing seasons. Autumn mowing or burning, especially, retarded regrowth.