C. PHAIKAEW¹, G. NAKAMANEE², S. INTARIT³, S. TUDSRI⁴, Y. ISHII⁵, H. NUMAGUCHI⁵ and E. TSUZUKI⁵

¹Division of Animal Nutrition, Department of Livestock Development, Bangkok
²Pakchong Animal Nutrition Research Center, NakonRachasima
³Lampang Animal Nutrition Research Center, Lampang
⁴Faculty of Agriculture, Kasetsart University, Bangkok, Thailand
⁵Faculty of Agriculture, Miyazaki University, Miyazaki, Japan

Abstract

Two experiments were carried out with the intention of determining the optimum rate of N application for seed production of Paspalum atratum in north-east and northern Thailand. Experiment 1 measured the effect of 5 N rates (0, 100, 200, 300 and 400 kg/ha/yr N) on a fertile clay soil at Pakchong and Experiment 2 measured the effects of 3 N rates (0, 125 and 250 kg/ha/yr N) on infertile sandy soils at 3 sites (Lampang, Chiang Rai and Prae provinces).
The response of seed yield to N application was different in the first and second years after planting. In the first year, N application had no significant effect on pure seed yield or seed quality of P. atratum at any of the sites. In the second year, at the fertile Pakchong site, a rate of 200 kg/ha/yr N produced significantly higher seed yield than other rates. Diminishing seed yields at rates above 200 kg/ha/yr were probably due to the severe lodging of grass by one month before flowering, while low yields at rates below 200 kg/ha/yr were attributed to lower availability of N in the soil. At the 3 infertile sites, seed yields were greater in the second than the first year, and in the second year responded to increases in applied N up to 250 kg/ha/yr N, at which the crop yielded 1459 and 1108 kg/ha of seed at Lampang and Prae, respectively. Seed germination and seed purity increased with increasing N rates at all 3 sites in the second year.
We considered that overall reduced yields in the second year at the lower N input levels were due to a reduction in available soil N resulting from the removal of large amounts of N in vegetation. We concluded that the optimum N rate for seed production of P. atratum in Thailand was approximately 200 kg/ha/yr N on fertile clay soil and at least 250 kg/ha/yr N on infertile sandy soil, with the greater need for high levels in the year after planting.