Plant dynamics of New Zealand tussock grassland infested with Hieracium pilosella. I. Effects of seasonal grazing, fertilizer and overdrilling.
Field trials were conducted in Mackenzie Country, South Island, New Zealand in 1975-82 at a low fertility site dominated by Festuca novae-zelandiae with light H. pilosella infestation and on a moderately fertile site dominated by H. pilosella with abundant F. novae-zelandiae and Agrostis capillaris and occasional plants of H. praealtum. Pasture was left ungrazed or rotationally grazed or set stocked in spring, summer or autumn in combination with the following pasture improvement treatments: oversowing with a Trifolium hybridum, T. repens, T. pratense, Dactylis glomerata and Phleum pratense mixture + 200 kg sulphur molybdic superphosphate/ha per year; cross-drilling 15 legumes with 13 grasses + 200 kg S superphosphate. On the unimproved control treatment at both sites, H. pilosella maintained or increased its dominance in the subsequent 6-7 years. With fertilizer, overdrilled legumes increased and suppressed H. pilosella within 2 years on the moderate productivity site. On the low fertility site, legume swards took 5-6 years to begin suppressing H. pilosella which had increased from an initially low infestation.