Bush-control studies in the drier areas of Kenya. 3. Control of Disperma in semi-desert dwarf shrub grassland.
At 3 sites on the El Barta plains of northern Kenya, the ecological effects of controlling the dominant shrub Disperma sp. near D. eremophilum were studied by comparing the effects on ground cover of clearing with an African hand hoe and 3 tractor-drawn implements, the John Deere Gyramor, the Holt VIIIb Weed Breaker and a rigid-tine cultivator. No method gave complete control, but hand-clearing decreased Disperma cover to 3% initially. Two years later, the respective covers of Disperma and grass were 28% and 31 % compared with 39% and 17% on the untreated plots. On the driest site, the final cover of Disperma was 9 % after hand-clearing, which was significantly less than on the untreated plots. At all sites, the grass cover increased in approximate proportion to the decrease in Disperma cover and withstood re-invasion by Disperma. Clearing in June (early dry season) was generally more effective than clearing in August (mid dry season). In supplementary studies the effects of a foliar spray of 2, 4, 5-T (0.5 lb a.e. with 1 pint of diesel oil in 10 gal of water applied at 2 rates, 8.8 pounds per acre at Baragoi and 7.2 pounds per acre at Labaibor, by a hand-held roller-vane spray pump driven by the tractor power take-off) were similar to those of hand-clearing while burning (on a different site) led to the replacement of Disperma by Aspilia sp.
It is concluded that there is no danger of land denudation if Disperma is eradicated. Where burning is possible this should effectively control it but only a chemical treatment (still to be perfected) seems likely to give control under all conditions. A robust rotary cutter could be used to decrease the cover of Disperma and increase that of grass before burning, A heavy ripper would be useful in dry areas where grass has to be seeded. Hand-clearing is only feasible on small areas, or as a follow-up treatment. More data on the value of Disperma as browse for animals are required before an optimum density can be defined. The animal populations themselves must be controlled to prevent re-invasion by Disperma. In future work, reference should be made to results from the USA on Artemisia tridentata, with which Disperma appears to have similarities. (See Weed Abstr, 13: 732 and 15: 1620). F.s.-D.B.