Abstract
During the second half of the last century, average wheat yields have increased in most parts of the world, including in areas subject to drought. As a case study of factors affecting yield in drought-prone areas, we have studied durum wheat yields and their relation to rainfall in Tunisia, where from north to south there is a gradient in severity and frequency of drought. The mid 1960s delimited two periods: from the early 1900s to the 60s the rate of increase was rather low; after the 60s yields increased at a higher rate. However, yields were quite variable and extremely low in dry years. Recently the rate of yield increase has been slowing down. In northern Tunisia, yield was highly correlated with autumn rainfall, indicating the importance of the early growth stages, whereas variation in the extent of terminal drought was poorly correlated with yield. In the centre and south, where drought occurs frequently, the area sown was highly variable, because farmers often cancel sowing if the first significant autumn rainfalls do not occur. Wheat production was also diminished through a reduction of harvested areas with respect to sown areas, a consequence of severe drought and crop failure. Water use efficiency (yield/growing season rainfall) was stable for durum wheat in northern Tunisia until the late 60s and since then has significantly increased. Given the identified sensitivities to drought, the main factors limiting wheat yield are discussed and analysed in order to give way to more genetic and crop management progress.
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