The productivity of irrigated legumes in northern Victoria. 1. Effect of irrigation interval

Abstract

An experiment was undertaken to determine the production and water use of pure swards of white clover (Trifolium repens L. cv. Haifa), red clover (Trifolium pratense L. cv. Redquin) and lucerne (Medicago sativa L. cv. Validor) between December 1987 and December 1990. The experiment was a split plot design with 3 irrigation interval treatments, which were the main plots, and 3 legume species, which were the subplots, all of which were replicated 4 times. Irrigation frequency was determined by evaporation minus rainfall (E-R) and was scheduled to occur at intervals of about 40, 80 or 120 mm of cumulative E-R. Annual amounts of harvested dry matter (DM) ranged from 8 to 22 t/ha depending on irrigation and species treatment. There was an interaction (P<0.05) between irrigation interval and species during each irrigation season. White clover DM harvested was reduced (P<0.05) by an average of 23 and 41%, respectively, as irrigation intervals were extended beyond 40 mm E-R (where up to 12 t DM/ha was recorded in an irrigation season). The decline in white clover growth in the less frequently irrigated treatments, relative to the 40 mm E-R irrigation treatment, was most pronounced between December and February, the months of highest evaporative demand, when these swards were 30 and 52% less productive (80 and 120 mm E-R, respectively). This compared with reductions of 16 and 32%, respectively, for red clover and 1 and 4%, respectively, for lucerne during the same period. Red clover, a dense productive stand initially, lost vigour and density in the second summer of this experiment, and from February in the second year was not harvested. However, for the period during which it was harvested, red clover production was reduced (P<0.05) by 21%, on average, when the irrigation interval was 120 mm E-R compared with the 40 and 80 mm E-R irrigation treatments. The production of lucerne was not affected (P>0.05) by the range of irrigation frequencies used in this experiment, although its crown density in the most frequent irrigation treatment was 30% less than in the 2 less frequently irrigated treatments when measurements were terminated in December 1990. However, after the first year, the lucerne swards were all highly productive, producing up to 21 t DM/ha in an irrigation season. For white clover to be productive, frequent irrigation is essential. However, all the indices of plant and sward performance measured during intensive studies showed that frequent irrigation of white clover resulted in earlier onset of water stress relative to the less frequent irrigation intervals. Despite this earlier onset of water stress, the yield advantage of frequent irrigation was large. Red clover and lucerne were less reliant on frequent irrigations to maintain productivity than white clover. However, red clover may need to be re-sown on a regular basis in the northern Victorian environment because of its failure to persist beyond 2 years. It is suggested that lucerne should be considered in preference to white clover because of its greater water use efficiency and its less critical reliance on irrigation management to maintain productivity.