Effects of winter crops on the earthworm yield and earthworm cast in paddy fields

There is a large market demand for earthworms; thus, earthworm breeding will increase in the future. However, artificial earthworm breeding faces many problems, such as large labor input, high breeding costs, and difficulty in large-scale breeding. Increased earthworm production by planting winter crops may be an effective way to solve these problems. This study incorporated a field investigation and rainfall and accumulated temperature data from 2017 to 2020 to investigate the effects of different winter crops, including fallow (T1), milk vetch (T2), and rapeseed (T3), on the yield and cast of Pheretima guillelmi to provide theoretical and technical support for earthworm production in paddy fields. The results showed that the coefficient of variation (CV) of earthworm yield in different years was positively correlated with the CV of winter crops yield (P<0.01). Continuous rainfall from October to December and low temperatures from December to February from 2018 to 2019 led to a significant decrease in the yields of winter crops and earthworms. The soil organic matter content (average value under three yield levels) was in the order of rapeseed > milk vetch > fallow, and the earthworm yield and earthworm manure yield were significantly positively correlated with the soil organic matter content (P<0.05) and with winter crop yield. The earthworm individuals number per m² of fallow, milk vetch, and rapeseed was 8.4−12.8, 11.2−30.8, and 20.4−49.1, respectively, the average earthworm individuals number per m² was 10.8, 20.7 and 36.3, respectively. The earthworm yield was 290−428 kg·hm⁻², 368−1054 kg·hm⁻², and 763−1845 kg·hm⁻², respectively, and the average earthworm yield were 373 kg·hm⁻², 695 kg·hm^−2, and 1364 kg·hm⁻², respectively. The earthworm cast yield was 1921−3126 kg·hm⁻², 3023−10 116 kg·hm⁻², and 6034−18 958 kg·hm⁻², respectively, and with the averages of 2652 kg·hm⁻², 6438 kg·hm⁻², and 12 545 kg·hm⁻², respectively. The averages were in the order of rapeseed > milk vetch > fallow. The ratio of cast weight to fresh earthworm weight increased with increasing winter crop yield, with an average value of 8.8; the average values for fallow, milk vetch, and rapeseed were 7.1, 9.1, and 9.0, respectively. Variation in earthworm yield and earthworm cast yield between different yield levels of rapeseed was less than that of milk vetch, indicating that rapeseed was beneficial for higher yield stability for earthworms and earthworm casts. In summary, the yield of earthworms and casts was greatly disturbed by rainfall and accumulated temperature, and the high-yield fields of winter crops had high soil organic matter content, which was conducive to increasing the yields of earthworms and earthworm casts and increasing their stability. Planting rapeseed also had a beneficial effect.