Replacing summer fallow with annual forage improves crude protein productivity and water use efficiency of the summer fallow-winter wheat cropping system

Diversifying the summer fallow (F)-winter wheat (Triticum aestivum L.) (W) cropping system by replacing summer fallow with forage crops could increase forage yield and precipitation use efficiency in the Loess Plateau region of China. However, its influence on protein productivity is unclear and it is unknown whether improvement in protein productivity will increase soil water extraction and economic input. A three-year (2011–2013) field experiment was conducted to investigate the rotational effect on system productivity, economic benefit, and sustainability when summer fallow was replaced with forage rape (Brassica napus L.) (R) and common vetch (Vicia sativa L.) (V). Results showed that the rotation system with forage crops did not alter soil water storage of subsequent winter wheat. Compared with the FWFW rotation system, the rotation system with forage crops improved precipitation use efficiency of crude protein yield (PUE_CP) and system water use efficiency of crude protein yield (WUE_CP) by 20 and 28% respectively. The greatest crude protein productivity (CP_yield) was achieved with the RWRW rotation system, followed by the FWRW and RWVW systems, and CP_yield with these systems was 51, 21, and 36% greater than that with the FWFW system, respectively (P < 0.05). In contrast, net income in the FWFW rotation system had the greatest value (U.S. $2,005 ha⁻¹), 27, 20, 34, and 29% greater than that in the FWVW, RWVW, FWRW, and RWRW systems, respectively (P < 0.05). Consequently, the alternate rotation system of FWFW and RWRW is recommended for local farmers when considering both profitability and sustainability.