TALLGRASS PRAIRIE VEGETATION RESPONSE TO SPRING BURNING DATES, FERTILIZER, AND ATRAZINE

Tallgrass prairies provide an important source of hay and summer forag e in eastern Nebraska, A study was conducted in 1989 and 1990 on 2 lat e seral tallgrass prairies near Lincoln and Virginia, Nebraska to dete rmine if production of selected components of tallgrass prairie commun ities could be altered by burning (not burned, or burned in either ear ly, mid-, or late spring) and applying fertilizer (0 and 67-23 kg N-P ha(-1)) and atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5- triaz ine-2,-diamine] (0 and 2.2 kg a.i. ha(-1). Vegetation was harvested th e year treatments were applied at about 30-day intervals starting in J une and ending in August, Maximum big bluestem (Andropogon gerardii va r. gerardii Vitman) accumulated standing crop (ASC) on unburned areas and areas burned in mid-spring occurred later in 1990 than in 1989, Bu rning in late spring 1989 maintained big bluestem ASC above 1,100 kg h a(-1) through July, whereas big bluestem ASC declined below 840 kg ha( -1) in July on areas where other burn treatments were applied, In 1990 , big bluestem ASC exceeded 1,570 kg ha(-1) in June on areas burned in early and mid-spring and exceeded 1,500 kg ha(-1) in July on areas th at were not burned or burned in mid- or late spring, From July to Augu st 1990 big bluestem ASC declined below 730 kg ha(-1) for all treatmen ts except the late spring burn treatment where ASC was 1,340 kg ha(-1) , Burning in late spring reduced prairie dropseed [Sporobolus heterole pis (A. Gray) A. Gray] and tall dropseed [S. asper (Michx.) Kunth.] AS C by at least 67% in June 1990 compared to areas burned in early and m id-spring, Cool-season grass ASC at Virginia declined 86% in June when burned in late spring compared to areas that were not burned, Fertili zation increased big bluestem ASC by about 23 and 29% in June and July , Vegetation response to atrazine was variable, Atrazine had a negligi ble effect on big bluestem ASC, Burning late seral tallgrass prairie i n late spring increased big bluestem ASC later in the growing season a nd decreased cool-season grasses more effectively than burning earlier in the spring.