The effect of total water supply, and frequency of application, upon lucerne. I. Dry matter production

The total quantity of water supplied to an established lucerne stand, and the frequency of water application, were varied independently. The total quantity of water supplied during summer, autumn, and early winter was expressed as a proportion of class A pan evaporation (Epan), and ranged from 0.04 to 3.5 Epan. Dry matter production, at total water supply equal to Epan, varied between 8.6 kg ha-1 day-1 in early winter (May–June) and 97 kg ha-1 day-1 in summer (January–February), and was correlated (r = 0.99) with mean minimum temperature. The form of the response curve of log, dry matter production to water supply, relative to Epan, was similar during all seasons. Dry matter production increased approximately logarithmically with increasing water supply up to the equivalent of 0.5 Epan reached a maximum at about 1.5 Epan, and declined above this. A response surface was constructed, based upon total water supply and mean minimum temperature.Frequency of application, at a given total water supply, had no significant effect on dry matter production when total supply exceeded 0.5 Epan in any season. At a total water supply equal to 0.2 Epan, frequent small applications of water (5 mm) produced 50% more dry matter than less frequent large applications (20 and 80 mm) during summer, but only 30% as much as the less frequent applications in the subsequent autumn. Water use efficiency (ratio of dry matter production to total water supplied) was greatest at 0.5 Epan at all frequencies of application in summer. During the 6 months after irrigation treatments ceased, dry matter production was generally related to the amount of available water in the soil, but production was less, even 18 months later, on plots that had previously received the heaviest irrigation (130mm month-1). Frequency of application had no significant effect upon subsequent yield, but frequent small applications in one summer reduced the capacity of lucerne to extract soil water at low water potentials in the following summer.