Pasture yield and soil water depletion of continuous growing alfalfa in the Loess Plateau of China

For improving soil and water conservation and developing livestock production, an alfalfa (Medicago sativa L.) and crop rotation practice has been widely employed in the dryland region of the Loess Plateau in China. This system typically involves establishing alfalfa for 2–10 years, and then removing it by cutting and returning to a cropping rotation for 2–7 years. The length of alfalfa phase is essential for attaining high pasture yield and remaining enough soil water for subsequent crops. To determine the optimal length of alfalfa phase, one continuous growing alfalfa treatment and nine other treatments, including eight alfalfa/crop rotations and a crop/crop rotation, were studied at the Changwu Agri-ecological Station of the Loess Plateau in China from 1985 to 2001. Results showed that alfalfa yield decreased with time at a rate of 0.629 t ha−1 year−1 and soil water storage in the 0–500 cm profile by 33.5 mm year−1. After alfalfa grew for >8 years, its yield responded very vigorously to seasonal precipitation variations, and the yield age coefficient (YAC), which reflects the effects of alfalfa growing age and soil water deficit on pasture yield, was less than 1.0. Our analysis also showed that the average soil water content within the profile from 300 to 1000 cm was less than 0.18 cm3 cm−3, this was determined to be a threshold value for creating a strong water stress to alfalfa growth. Considering all the factors, this study recommends that the optimal length of alfalfa phase in the alfalfa/crop rotation system should not more than 8 years.