Performance of a Eucalyptus grandis × E. camaldulensis hybrid clone as influenced by a cowpea cover-crop

Abstract

In 1990, a trial was initiated to determine the impact of a cowpea (Vigna unguiculata (L.) Walp.), when established as a forestry cover-crop, on the growth of a Eucalyptusgrandis×E. camaldulensis hybrid (GC304) at a sub-tropical site in KwaZulu-Natal, South Africa. Treatments included a weedy and manually weeded check, the planting of cowpeas in a single row (33,000 seeds per hectare), with or without a pre-germination herbicide spray, and the planting of cowpeas in a double row (66,000 seeds per hectare), with or without a pre-germination herbicide spray, replicated five times. The pre-germination herbicide spray was made up of a mixture of paraquat and metolachlor. Economic and growth responses were assessed when the trial was felled at 7 years of age. Initial growth benefits (first detected from 130 days after planting) were maintained until the trial was felled, with the manually weeded check and the two cowpea treatments which were planted with a pre-germination herbicide outperforming the rest of the treatments. These three treatments realised a significant increase in potential merchantable volume per hectare of between 16 and 22% over that of the weedy check, confirming that on this site some form of vegetation management would be needed due to the severity of weed competition. Although the volume for the manually weeded check was the highest, it only produced the third highest returns after all the weed control input costs were deducted. The planting of cowpeas together with a pre-germination herbicide realised the greatest profit due to the lower cost associated with fewer weeding operations required as a result of the combined effect of the cowpeas and herbicides on suppressing weed growth.