## 4 Discussion  
Dry matter yield is one of the important factors used to evaluate the productivity of forage [26; 27]. In this study, the biomass yield of forage rape increased rapidly from budding to final flowering stage, followed by a gradual increase in the podding stage (Figure 1). Regardless of the transplanting date, the highest dry matter yield was in the podding stage, reaching 6.4 t\(\cdot\)ha\({}^{-1}\) in the 30 October transplant date treatment. This forage yield accumulation is consistent with that of the rape blanket seedlings transplanted technique, which has a maximum forage yield of approximately 6.3 t\(\cdot\)ha\({}^{-1}\) in the Yangtze River delta [28]. The growth rate was highest at the bud period and then decreased after flowering. However, the yield of forage rape in Gansu reaches 11 t\(\cdot\)ha\({}^{-1}\), which is approximately two times higher than the yield in the Yangtze River delta [12]. The difference in yield is likely primarily due to different experimental sites with specific soil characteristics and different varieties and planting methods. For example, the rapeseed in Northwest China is primarily directly sowed mechanically after the harvest of wheat, and the crops show vigorous growth, particularly in the early growth period. In addition, in the northern region, a waterlogging disaster during the growing season is unlikely. However, in this study, low temperatures and continuous rainfall occurred during the transplanting and the seedling growth period, with the cumulative precipitation reaching 414.7 mm in October and November (Table 1), which may also affect the soil nutrient availability, severely inhibiting the growth and yield formation of rapeseed before winter. The nutritional value of forage grass primarily depends on the crude protein content, whereas the digestibility of forage primarily depends on the ADF content [12]. Of the main nutrient measures of forage rape at the podding stage (Figure 2), the crude protein content was 11.6% to 14.0%, the NDF content was 37.7% to 40.7%, and the ADF content was 33.0% to 37.2%, which meet the classification standards for legumes in the United States [29]. At the final flowering stage, the dry matter yield of forage rape was higher in the October transplant date treatments than in the November transplant date treatments, whereas there was no significant difference among transplant date treatments at the podding stage (Figure 1). Relatively few studies have examined the effect of transplanting date on the characteristics of forage rape with RCPPS. With the rape blanket seedling transplant technique, the biomass yield of forage rape transplanted before 20 October is significantly higher than that of rape transplanted in late October in the Yangtze River delta [28]. With the direct-seeding technique, forage rape should be sowed earlier, because the accumulation of dry matter yield is primarily limited by air temperature, particularly in northern China. The temperature always declines rapidly in August in Gansu Province, which significantly inhibits the dry matter formation of rape in the late growth period. From 30 July to 8 August, the biomass yield of rape was significantly decreased by 50% due to the delay of 5 days [10]. In addition, the yield and plant height of forage rape were both higher when sowed in mid-July than in early August in Heilongjiang Province [14]. Notably, with the delay in transplanting in the 20 and 30 November treatments, the crude protein content increased significantly, whereas the ADF content decreased significantly (Figure 2). By contrast, the increase in ADF would reduce the amount of digestible dry matter, influencing the digestibility of forage [29]. The duration of the growing season is decided by the sowing date, which therefore influences the accumulation of biomass yield and nutrients [30]. In Heilongjiang Province, the contents of crude protein, crude fat, carbohydrates, and trace elements in early sown forage rape were all higher than those in late-sown rape [12].