Abstract:

Fertilization is one of the important management measures in alfalfa （Medicago sativa） production， but the comparative effect of different topdressing regimes after cutting alfalfa and the optimal fertilization management plan are not clear. Hence， the aim of this research was to investigate the effects on the growth and production of alfalfa， of topdressing time and different combinations of N and P fertilizer applied to stimulate regrowth after cutting. The treatments comprised a complete factorial combination of two application times ［on the day the cut was made （T₀） and 7 d after cutting （T₁）］， three levels of N fertilizer （0， 25 and 50 kg·ha^-1 N， designated N₀， N₂₅ and N₅₀， respectively） and three levels of P fertilizer （0， 30， 60 kg·ha^-1 P₂O₅， designated P₀， P₃₀ and P₆₀， respectively）， with three replicates （54 plots in total）. The alfalfa variety was “Longdong” and the experiment was conducted in the establishment year. It was found that： 1） Topdressing after the first cut promoted the growth of the second crop of alfalfa. The highest alfalfa growth was achieved by applying a small amount of P fertilizer immediately after cutting （T₀N₀P₃₀）， and it was 20.53% higher than the control treatment （T₀N₀P₀）. In addition， topdressing after cutting could increase the alfalfa leaf∶stem. This effect was most evident in the high N-low P treatment combinations （N₅₀P₀ and N₅₀P₃₀） with topdressing at T₁. 2） The yield of the second crop of alfalfa differed significantly （P<0.05） between the various treatments. The highest hay and crude protein yields were 3.58 and 0.94 t·ha^-1， respectively， and these occurred with application of a large amount of N and P fertilizer immediately after cutting （T₀N₅₀P₆₀）， which were significantly different from that of the treatment after 7 days. Topdressing after cutting had no significant effects on alfalfa second cut acid detergent fiber， neutral detergent fiber or relative feeding values （P>0.05）. 3） Root length， root surface area， root volume and root biomass per cm³ of soil in the second cut were significantly affected by N and P topdressing （P<0.05）. The highest values of root length density （2.66 mm·cm^-3） were observed in the no-fertilizer （N₀P₀） treatments； the highest root surface area density （7.75 mm²·cm^-3）^{was observed in treatment T₀N₂₅P₃₀ and the highest root volume densities were observed in T₁N₂₅P₃₀ treatments. Root biomass was rather variable across treatments but was higher than other treatments where no fertilizer was applied （T₀N₀P₀ and T₁N₀P₀）_. In summary， in rainfed agricultural area of Longdong Loess Plateau， topdressing N and P fertilizer after cutting promoted alfalfa regrowth in the establishment year. In practice， a small amount of P₂O₅ or a combination of 50 kg·ha-1 N and 60 kg·ha-1 P₂O₅ can be applied as soon as possible after cutting.}

Key words: Medicago sativa, topdressing after cutting, establishment year, crop production