Physiological reactions to imposed water deficit by Andropogon gayanus cv. Bisquamulatus and Cenchrus ciliaris cv. Biloela in a mixed fodder crop

Guinea grass (Andropogon gayanus Kunth cv. Bisquamulatus) and Buffel grass (Cenchrus ciliaris L. cv. Biloela) are grown as temporary fodder crops in the Sahelo-Sudanese region of Africa. Differences in their responses to drought conditions may be important in optimizing cattle feeding programmes in integrated farming systems. An experiment conducted in a wind tunnel equipped with a tank forming a weighable lysimeter compared net photosynthesis (A), stomatal conductance (gs) and leaf water potential (Psi leaf) of both species in a mixed sward during three simulated environmental conditions: (1) rainy season, (2) drought period within the rainy season and (3) dry season. With a soil water reserve (SWR in % volume) of 8% during the drought period and the dry season, the Psi leaf and A parameters were significantly lower (P < 0.001) when compared to those recorded with a SWR of 32% in the rainy season. Mean gs values were significantly different during each environmental condition (P < 0.001). No significant difference (P > 0.05) was found in gs and A between the two species during the rainy and the dry season or during a diurnal cycle, although a significant difference (P < 0.05) in Psi leaf existed between the species during the dry season. During this season, Guinea grass was dormant while Buffel grass was growing slowly. When the drought period was imposed during the rainy season (SWR declined from 25 to 5%), A was reduced from 19.8 to 7.7 mu mol m(-2) s(-1) for Guinea grass and from 16.8 to 3.7 for Buffel grass. Psi leaf in both species dropped below -2 MPa while gs declined slowly from 0.70-0.80 to 0.55-0.60 cm s(-1). A was significantly higher (P < 0.05) in Guinea grass during the last days of the drying cycle, but declined rapidly when drought was extended up to the wilting point. Two days after rewatering, all the parameters of both plants were restored to previous levels. It is concluded that Guinea grass was as drought-resistant as Buffel grass and that no competition for water existed between the species in the mixed sward. The study also gives relationships between the physiological parameters of both species and the soil water content during a drying cycle.