The effects of sward height and nitrogen fertilizer application on changes in sward composition, white clover growth and the stock carrying capacity of an upland perennial ryegrass/white clover sward grazed by sheep for four years
Barthram, G.T.; Grant, S.A.; Elston, D.A.
Grass and Forage Science 47(4): 326-341
ISSN/ISBN: 0142-5242 DOI: 10.1111/j.1365-2494.1992.tb02278.x
Perennial ryegrass/white clover pastures grazed by sheep and receiving either no fertilizer N (N-0) or 120 kg N ha-1 year-1 (N-1) were maintained with surface heights of 2.5, 3.5 and 5.0 cm for over four years. The treatments were replicated. The white clover (WC) population was greatest in the N-0 treatment, and declined during the study. Between-year variation in WC was negatively related to rainfall and positively related to temperature. WC as a proportion of the total plant population decreased during the summer in the N-1 treatment. The perennial ryegrass (PRG) population was greater in the N-1 treatment, declined during the study and both within and between years was positively related to temperature. The population density of the unsown grasses was highest in the N-fertilized treatment and in the swards maintained at the lowest heights (these treatments also had the highest stocking rate); it increased during the study, within-years being positively related to temperature and between-years being positively related to rainfall. The WC stolon extension rate was largely unaffected by N fertilizer application and was greatest in the taller swards. Leaf appearance rate was unaffected by N fertilizer application and sward height; it was positively related to temperature and negatively related to rainfall. Branching rate was greater in the N-0 treatment with significant sward height effects confined to a negative relationship with local sward height within treatment plots on one occasion; it was negatively related to rainfall. The ground level red:far red light ratio was negatively related to local sward height. The total live weight of sheep carried in the N-0 treatments was 0.7 of that in the N-1 treatments. Expected photomorphogenic responses by WC were confined to stolon extension. It was concluded that on the poorly drained clay-loam soil used in this study the effects of sheep, in interaction with climatic factors, had an overriding effect on clover branching rate and the ultimate species composition.