SEED YIELD AND WATER-USE EFFICIENCY OF WHITE LUPIN AS INFLUENCED BY IRRIGATION, ROW SPACING, AND WEEDS

When grown on sandy soil, spring-seeded sweet white lupin (Lupinus albus L.) is usually subject to moisture stress. Late-germinating broadleaf weeds also compete with the crop and reduce yields. The objectives of this study were to determine the effects of five irrigation levels, two row spacings (15 and 76 cm), and late-germinating weeds on lupin seed yield, seed protein content, and water-use efficiency. The cultivar Ultra was grown on a loamy sand (Udotrantic Haploborolls) and irrigated with a line-source sprinkler system. Full irrigation increased seed yield an average of 553% over non-irrigated controls in 1988, 229% in 1989, and 52% in 1990, but seed protein concentration was reduced 2.5 to 7.9 percentage points. Applied water-use efficiency and crude protein applied water-use efficiency were maximized at 300 to 400 mm or less of total effective water depending upon year. Irrigation increased lupin seed yield primarily by increasing numbers of fertile branches and mainstem pods. Irrigation also increased number of seeds per pod and seed weight. Lack of late season weed control reduced yields by an average of 17% in 1988 and 1989, and 28% in 1990. Yields from narrow rows were 23 to 50% higher than those from wide rows. Generally, weed count was reduced in narrow rows. Substantial benefits of irrigation to white lupin productivity were observed on these soils, but it is unlikely that applications of more than 350 to 400 mm of irrigation water plus rainfall would increase seed yield or water-use efficiency for spring-sown white lupin.