The relative influence of above and below ground competition on the growth and survival of ryegrass seedlings transplanted into a hill country pasture : a thesis presented in partial fulfilment of the requirements for the degree of Master of Agricultural Science in Plant Science at Massey University

Abstract

In many pasture improvement programmes, for example oversowing in hill country, seedling survival is influenced by competition from the existing vegetation. Competition between pasture plants occurs when resources are limited and may be for factors above or below ground, or both. Technically, the effective separation of above and below ground competition is difficult and considerable problems have been associated with previous studies. A technique developed for field studies combined the treatments of clipping herbage surrounding the transplanted seedling to prevent above ground competition and inserting a metal cylinder (root tube) into the ground to prevent below ground competition, resulting in conditions of shoot, root, full or no competition.

Ryegrass seedlings were transplanted in August 1986 into a pasture in summer dry hill country near Wanganui and subjected to shoot, root, full or no competition from the existing vegetation. The duration of the experiment was three months. The effect of competition on the growth of the ryegrass seedlings was assessed by non destructive measurements (plant height, tiller number) taken at approximately weekly intervals. On three occasions, destructive harvests were made and the dry weight of shoots and roots was recorded.

Below ground competition occurred before, and was more severe than above ground competition, as exemplified by changes in plant size. Ryegrass plants in the treatments with below ground competition were 80 % lighter, 64 % smaller and had 60 % fewer tillers than plants with either shoot competition or no competition. The distribution of plant size was highly skewed, and indicated that the stress plants encountered when subjected to below ground competition was severe. The effect of above ground competition on ryegrass growth was small except when root competition was also present. Shaded plants were usually taller than those that were unshaded. In conclusion, below ground competition, possibly for soil nutrients, was shown to be the major influence on growth and development of transplanted seedlings at the hill country site studied.

The survival of seedlings introduced into pasture was also dependent on environmental factors, especially soil moisture, and therefore important in summer dry hill country. In a second experiment during spring 1986, ryegrass seedlings were grown in tubes and transplanted into a hill pasture at Wanganui. The six treatments consisted of combinations of two planting dates, two tube lengths, two harvest dates and were arranged as a randomised complete block design Seedling survival was high over all treatments (98 %), probably because rainfall during the experimental period was high.