Irrigation treatments, water use efficiency and crop sustainability in cereal-forage rotations in Mediterranean environment

Agricultural systems based on crop rotation are beneficial to crop sustainability and productivity. Wheat-forage rotations combined with irrigation are the agronomic techniques best able to exploit Mediterranean environmental conditions. This paper describes a long-term field trial to ascertain the effect of combined irrigation and durum wheat-forage rotations on crop yield and soil chemical properties. The two forage crops: annual grass-clover winter binary mixture and perennial lucerne were carried out through 1991-2008 under rainfed and irrigated treatments. The experiments were used to highlight the effect of irrigation and wheat-forage crop rotations on water use efficiency (WUE) and sustainability of organic matter (OM) in topsoil. Irrigation increased the dry matter (DM) of annual binary mixture and lucerne by 49.1% and 66.9%, respectively. Continuous wheat rotation reduced seed yield (SY), stability of production, and crude protein (CP) characteristics of kernel and OM in topsoil. The yearly gain in wheat after forage crops was 0.04 t (ha yr)-1 under rainfed and 0.07 t (ha yr)-1 under irrigation treatments. The CP and soil OM of wheat forage crops rotations, compared with those of continuous wheat under rainfed and irrigated was a 0.8 and 0.5 % increase in CP and 5.1 and 4.4 in OM, respectively. The rotations of annual grass-clover winter binary mixture and lucerne meadow under both irrigated treatments increased the OM over continuous wheat (9.3 % and 8.5 in annual grass-clover winter binary mixture and 12.5 and 9.5 lucerne meadow under rainfed and irrigation, respectively). Irrigation reduced the impact of weather on crop growing, reducing water use efficiency (mean over rotations) for DM production (15.5 in meadow and 17.5 in annual grass-clover winter binary mixture [L water (kg DM)-1]) and wheat SY. However, the agronomic benefits achieved by forage crops in topsoil are exhausted after three years of continuous wheat rotation.