## 1 Introduction  
The summer rice-winter rapeseed (_Brassica napus_ L.) rotation is the traditional cultivation system in the Yangtze River delta, in which the rice crop cycle is from June to October and afterwards, the winter rape typically grows from November to May. It is likely that oilseed rapeseed would increase rice production and improve soil fertility when used as a previous crop in a rotation system [1]. However, considering the late harvest of the preceding crop and low planting profit, traditional rapeseed is often classified as an uncertain crop compared with the local main crop of rice [2]. Therefore, because of the delayed planting date of rapeseed, low air temperature and high precipitation always occur, which inhibit the vegetative growth of rapeseed seedlings and lead to the poor development of pod formation and grain filling in the late period [3,4]. The planting costs of rapeseed have also increased substantially because the price of agricultural production material and labor resources has increased rapidly in the Yangtze River delta. In addition,large quantities of low-cost, high-quality imported oilseeds have negatively affected the development of domestic rapeseed production, in which rapeseed is grown conventionally for grain [5; 6]. The hybrid varieties of low erucic acid and glusosiolate ('double-low') rape are high-yielding forage crops characterized by their high regrowth capacity [7] and nutritive value [8]. Thus, forage rape can be a substantial part of the feed rotation in pasture, particularly in autumn-winter when pasture growth is limited, and it therefore increases the economic benefits of rapeseed production. Forage rape has been promoted in a large area in northern China, including in Heilongjiang, Shanxi, and Gansu provinces [9]. In the northern region, forage rape typically grows from August to October, which is beneficial for improving the productivity in the complementary forage rotation of wheat-forage rape or bean-forage rape. In addition to enriching the species of forage in these agricultural areas, forage rape improves resource use efficiency, including that of temperature, water, and light, compared with fallow fields [10; 11]. Forage rape is also beneficial because it reduces soil erosion and soil pathogens and improves soil characteristics [7; 12]. In 2017, the National Agricultural Technology Extension Service Center issued 'Winter Rapeseeed Production Technical Guidance Opinions 2017-2018', which first clarified the multiple uses of rape for oil, forage, flowers, honey, and fertilizer [13]. To improve rapeseed production, innovative uses of rape need to be developed, consistent with local conditions. In the loess hilly region, the combined uses of rapeseed for forage, sightseeing, and grain have promoted the development of multifunctional agriculture, which brings great economic and ecological benefits [9]. In the cold region of Northeast China, the first innovation in growing forage rapeseed was to plant two crops a year [14]. However, few studies have examined the cultivation of forage rape in the Yangtze River delta. In the rice-rape rotation system in the Yangtze River delta, the planting date of rapes is restricted by various factors such as stubble configuration, weather, and soil conditions. The timing of transplanting inevitably affects the growth characteristics of rapes. Delayed sowing date with insufficient accumulated temperature would directly inhibit seed germination and early seedling growth, thus affecting the accumulation and transport of photosynthetic products, leading to low biomass and grain yield [15; 16]. The ridge culture place planting system (RCPPS) is a promising technique for planting rapeseed that has been widely adopted in the Yangtze River delta [17]. With the RCPPS, the stress of waterlogging damage can be avoided, especially in the seedling stages. Furthermore, early seed sowing and seedling raising can facilitate the growth of rapeseed, particularly alleviating the inhibition of rape growth by rice stubble [17; 18]. [PERSON] et al.