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The productivity and pathways of persistence of forage legumesnsown into two contrasting native pasture situations in northeast Thailandnwere studied over four years.Stylosanthes humilis cv. Lawson, S. hamata cv. Vevano, S.nguianensis cv. Endeavour and Macroptilium atropurpureum cv. Siratro werensown in an upland area dominated by Arundinaria ciliata and grazed byncattle at 2.5, 3.5, 4.5, 5.5 and 6.5 animal units (AU) ha-1 per 6 months during eachnwet season until the end of the monsoon.Siratro exhibited highest overall level and persistence ofnyield. This was partly associated with greater longevity of plantsn(c. 40 percent survival per year) apparently linked with avoidance ofndrought via a deep root system; the differentiation of new and persistentnrooted stolons was insignificant. Plant survival after two years wasnnegatively related to stocking rate. Siratro also exhibited satisfactorynflowering and seed formation (c. 70 kg ha-1) under a regime in whichngrazing was terminated at the close of the wet season; although seednproduction was negatively related to stocking rate, seedling regenerationnand seedling survival were independent of stocking rate. A density ofnat least 6 crowns m-2 was maintained at 2.5 AU ha-1 per 6 months, and loss ofnpersistence occurred at higher stocking rates. The persistence ofnyield of Verano was dependent upon seedling recruitment since plantnsurvival was c.17 percent per annum; seedling recruitment wasnsatisfactory in the first three years at levels of c. 60 - 125 seedlings m-2nbut decreased in the fourth year as stresses during the growing seasonnreduced mature plant density and seed formation. Endeavour was unsuccessful; plant survival was c. 9 percent per annum, flowering occurred late andnlittle seed was formed after the termination of grazing; this wasnspatially distributed in unfavourable patterns by the ant Iridomyrmex spp.nThe annual Lawson disappeared since Colletotrichun gloeosporioidesnreduced growth and seed formation. Frequency of cutting hadninconclusive effects on the severity of this disease. Stock showed,nintermediate preference for legumes throughout the growing season.Percent botanical composition, height and yield of the tallnrhizomatous A. ciliata increased with time, independent of stockingnrate. The proportion of ground cover occupied by smaller (15 - 20 cm)ndiameter clumps increased at the higher stocking rates, but yield wasnpoorly related to ground cover. Cattle showed, negative selection fornA. ciliata and consumed leaf but not stem; leaf to stem ratio wasnnegatively (and unusually) related to stocking rate. A large proportionnof A. ciliata tissue was located underground, and growth continued intonthe dry season and was rapid at the onset of the wet season. A. ciliatanreduced the illuminance of the Verano canopy to 0.38 - 0.65 full sunlight, but the twining habit of Siratro enabled its leaves to benplaced in a more favourable light environment.Volunteer native grasses such as Digitaria adscendens andnBrachiaria distachya exhibited rapid seedling growth and an apparentncompetitive effect on legume seedlings at the beginning of the wetnseason before grazing effects were pronounced. These grasses werenselectively eaten throughout, and competition apparently lessenednas the growing season advanced; they seeded successfully due to their low branching habit. n |
