EFFECTS OF WATER AVAILABILITY AND VINE HARVESTING FREQUENCY ON THE PRODUCTIVITY OF SWEET POTATO IN SOUTHERN MOZAMBIQUE. III. CROP YIELD/WATER-USE RESPONSE FUNCTIONS

Abstract

In Mozambique the sweet potato (Ipomoea batatas) is grown as both a leafy vegetable, the terminal shoots or vines being progressively harvested during the season, and as a root crop. This paper reports the development of crop yield/water-use production functions using data from two irrigation experiments designed to evaluate the effects of water availability and vine harvesting practices on the productivity and water use (ETc) of sweet potato (cv. TIS 2534) during two contrasting seasons in the south of the country. As the frequency of vine harvesting increased, the water-use efficiency (WUE-ETc) for vine production (dry mass) increased from 1–2 to 4–5 kg ha⁻¹ mm⁻¹ during the rainy season, and from 1 to 9 kg ha⁻¹ mm⁻¹ during the dry season. By contrast, there was a corresponding reduction in the WUE-ETc for storage root production from 14 to 8–9 kg ha⁻¹ mm⁻¹ during the rainy season, and from 23 to 15–17 kg ha⁻¹ mm⁻¹ during the dry season. For the total yields (vines plus storage roots) the WUE-ETc during both seasons were independent of the vine harvesting treatment. Separate lines represented each season, the slopes of which were 13 kg ha⁻¹ mm⁻¹ in the rains, and 24 kg ha⁻¹ mm⁻¹ in the dry season. When, however, crop water-use was normalised using either the seasonal mean daily total of incoming solar radiation, or reference crop evapotranspiration (ETo), but not the saturation deficit of the air, a single common, linear relationship with yield resulted. Possible explanations for this are considered. Plotting relative yields against relative rates of water-use, also gave consistent results that could have general application for predicting the effects of water availability on productivity. Vine yields, when harvested frequently, were less sensitive to drought (yield response factor, Ky=0.7–0.9) than storage roots (Ky=1.2). For total production (vines plus roots), the sensitivity to water stress (Ky) increased, from 0.9–1.0 to 1.2, as the interval between vine harvests increased. Farmers in southern Mozambique trying to maximize total yield during the season, under conditions of water uncertainty, should harvest vines at intervals of not more than 14 days.