Effect of oversown ryegrass (Lolium perenne) and white clover (Trifolium repens) on the genetic structure of New Zealand hill pastures

Abstract

The botanical composition and genetic structure of North Island hill pastures were studied after oversowing. Three pre‐sowing treatments were applied: (a) no herbicide; (b) herbicide to suppress existing pasture; and (c) herbicide to kill existing pasture. Four oversowing treatments were imposed: (1) no oversowing; (2) hill country ryegrass and ‘Prop’ white clover; (3) ‘Yatsyn’ ryegrass and ‘Prop’ white clover; and (4) a mixture of hill country grass species. Five times as many ryegrass seeds were oversown than emerged from buried seed, giving the introduced material an advantage. The ryegrass content was greater at the high‐fertility sites than at the low‐fertility sites, both before and after oversowing. Killing the original pasture tended to increase ryegrass content for 16 months at the low‐fertility sites but increased the ryegrass content for less than 9 months at the high‐fertility sites. The ryegrass content was not significantly different on any of the sown and unsown plots, 16 months after oversowing. Ryegrass populations, in sown plots on the high‐fertility northerly aspect, comprised 70% of the introduced ryegrass and 30% of resident ryegrass, 2 years after oversowing. The number of white clover seeds sown was 70% of that germinating from buried seed, giving the resident material an advantage. As a result, the white clover content was similar in sown and unsown plots, and the introduced white clover germplasm contributed 30% of the clover plant population. At 5 months after oversowing, white clover content was greater under the suppress herbicide. Soil fertility had the greatest influence on both ryegrass and white clover content. Sowing “other grass” species increased their content in the pasture and, after 16 months, Agrostis capillaris was the dominant other grass followed by Dactylis glomerata and Holcus lanatus. The results suggest that the genetic structure of ryegrass populations can be changed at high fertility levels provided no large ryegrass seed pool exists, but changes in the content and genetic composition of white clover are more difficult to achieve. The seedling establishment phase is critical for successful introduction of new genotypes and herbicide had little effect at this time.

Key words:

• Lolium perenne
• Trifolium repens
• Agrostis capillaris
• low‐fertility grass
• weed
• buried seed
• seedling number
• herbicide
• hill pasture
• oversowing
• allozymes