Influence of sowing dates on growth, yield and heat use efficiency of mustard

Abstract:

A century of mustard cultivation across southern Africa's cooler winter belt has settled on roughly the same sowing window every season, yet growers still argue over whether shifting that window by even ten days changes the crop's fate. Field research spanning two winter seasons at Bulawayo tested six sowing dates, from mid-October to early December, spaced at ten-day intervals, to determine their effect on growth, seed yield, oil content, and heat use efficiency of mustard. Plots sown on 5 November recorded the highest seed yield at 1860 kg per hectare, exceeding the earliest sowing date by 66% and the latest by 90%. Heat use efficiency followed a similar bell-shaped pattern, peaking at 0.82 kg per hectare per degree-day at the same optimal sowing window before declining sharply at both earlier and later dates. Seed oil content also peaked at the 5 November sowing, reaching 41.8%, roughly 3.6 percentage points above the earliest date and 5.4 points above the latest. Temperature data recorded across sowing dates showed that plots sown too early experienced elevated temperatures during the sensitive flowering and siliqua-filling stages, while late-sown plots faced a truncated grain-filling period compressed by rapidly rising spring temperatures. These results identify a fairly narrow ten-to-fifteen-day optimal sowing window for mustard in this agro-ecological zone, and argue for sowing-date advisories tailored more precisely to local temperature patterns than the broad month-long recommendations currently in common use.