Growth responses of Brachiaria decumbens cv. Basilisk and Brachiaria brizantha cv. Marandu to phosphorus supply

A glasshouse experiment was conducted at Campo Grande, the Mato Grosso do Sul State, Brazil to compare growth response of Brachiaria decumbens (following BD) and B. brizantha (following BB) to different quantities of phosphorus (following P) supply. Seedlings of the two species were transplanted in pots filled with an Oxisol applied with 0,25, 50and 200kg/ha of P. The plants were harvested after 33, 62, 103 and l32 days of growth. Then dry weight of stem, leaf and root, leaf area, P contents of shoot and root, and P content of residual pot soil were measured. Crop growth rate (CGR), net assimilation rate (NAR), efficiency of applied P, acquisition rate of applied P, P use efficiency (g of forage production per g of P uptake); and P acquisition efficiency (g of P uptake per g of root weight) were calculated. P supply increased dry weight and CGR mainly through enlargement of leaf area. The effect of P supply on NAB was less than that on leaf area. NAR of BB was higher than that of BD at all P rates in early growth period. However, BB decreased its CGR and NAR rapidly at the later growth period. Therefore, it was suggested that the forage of BB should be utilized at the earlier stage than BD to avoid forage losses. Moreover, the drop of CGR and NAR of BB also suggests the necessity of a maintenance fertilizer other than P for this species. BB also showed higher efficiency of applied P and P use efficiency at a low P supply, which might have relevance to the high responsiveness of BB to fertilizer application. On the other hand, BD showed higher acquisition rate and higher P acquisition efficiency per unit root weight than BB at all rates of P supply, and it seems that those characteristics permit BD to adapt to low fertile soils. The relationship between P acquisition efficiency and root morphology of both species is also discussed