Intensifying a semiarid wheat monocropping with forage rape as a replacement to fallow period in China

The traditional cropping system on the Loess Plateau of China is winter wheat (Triticum aestivum L., W) monocropping. However, this cropping system, with a 3-mo summer fallow (F) period, has exhibited low radiation and precipitation use efficiency. The objectives of this study conducted in 2016-2018 was to investigate system productivity and resource (i.e., water and radiation) use efficiency when winter wheat monocropping was intensified by planting forage rape (Brassica napus L., R) during the summer fallow period. Dry matter yield (DM), water use efficiency (WUE), and radiation use efficiency (RUE) of forage rape and winter wheat in F-W and R-W crop rotations were determined. The results show that forage rape did not change system DM in all years, and winter wheat grain yield as well as DM in 2018, but decreased winter wheat grain yield and DM by 17 and 16% in 2017, respectively. Forage rape increased the system average water use (WU) by 20%, whereas system WUE of DM (WUEDM) was unaffected in both years. Forage rape also improved system intercepted photosynthetically active radiation (IPAR) by 39% but did not alter winter wheat IPAR in both years. In 2017, intensification with forage rape decreased winter wheat RUEDM and RUE of grain (RUEGrain). In conclusion, crop productivity was mainly related to water availability during sowing and the growing season, whereas RUE was related to crop productivity. Therefore, soil water supply needs to be considered before sowing forage rape during the summer fallow in the semiarid region of China as examined in this study.