サツマイモの飼料向栽培法に関する研究(サツマイモの飼料向利用に関する特集)

抄録

The importance of sweet potato as a food crop has been recently decreased in Japan. Considering its high productivity and vigorous herbage growth in warm season when good fresh roughages tend to be short, the authors tried to make clear through 4 experiments how to grow the crop for producing high yields of both top or herbage and root as a forage crop. Experiment 1. The effect of fertilization upon the yield of top and root, and upon the protein content of herbage. Plots of four treatments, O (no fertilizer), PK, NP and NPK were set up in randomized blocks. Half of nitrogen (ammonium sulphate) was applied on Aug. 5 in the midst of growth. The crops planted on July 5 of 1952 were harvested 7 times from Aug. 10 up to Oct. 20. The top growth of NP and NPK exceeded that of O and PK after late August (Fig. 1 and 2). While, the root growth was delayed in the two plots with nitrogen, but in NPK it could reach the same level of the plots without nitrogen in September (Fig. 4). The contents of both total-N and pure protein-N were increased about 40% in average by the appli-cation of nitrogen (Fig. 5 and 6). From these results, much application of nitrogen is considered not only to raise the yields of top and root toge-ther with adequate amount of potassium but also to increase the protein content of herbage. Experiment 2. The influence of planting and harvesting times on the yield of top and root. Cut stolons were planted twice on May 23 and June 6, 1953. The crops were harvested 4 times from Aug. 19 to Oct. 12. The plots were arranged by split plot design. The top yield increased up to late September and after that rapidly decreased, showing no significant difference between the two planting times (Fig. 7). The root yield had increased up to the last harvest, indicating the superiority of early planting to late one except October (Fig. 8). The contents of both air dried matter and starch of root increased gradually up to Sept. 30, showing no significant difference between the planting times (Fig. 9, 10). Thus, the adequate time of planting may be early June, taking the the preceding crop into consideration. The harvesting time is considered adequate befor the leaves severely fall down however the root growth is not fully completed. Experiment 3. Adequate amount of nitrogen and potassium fertilizers. Three levels of ammonium sulphate, N_0, N_1 and N_2 (0, 2.6 and 5.3kg per a) were combined in split plots with the same levels of potassium sulphate. The crops planted on June 6 of 1953 were harvested 4 times from Aug. 7 to Nov. 12, when we had the earliest frost. The herbage yield showed clear increments with the increase of amount of the fertilizers. While, the root yield reached maximum at the middle level of N and K, and the effect of nitrogen was less than that of potassium (Tables 5 and 6). From these results, the level of K_1N_2, K_2N_1, K_1N_1 or so is considered practical from economical point of view, though the highest yield was obtained in K_2N_2. Experiment 4. Effect of top cutting in the midst of growth upon the yield. It was known from the above experimental results that herbage yield changed to decline in the latter half of growth probably due to fall of leaves under closed condition. So, cutting tops in the midst of growth was tried with and without both base and delayed fertilizers in order to find the contribution of the treatment to diminish the loss by leaf fall and to supply fresh herbage in earlier season than usual. The crops planted on May 25 of 1954 were cut of their tops at a height of about 30cm once on Aug. 10 or Sept. 10 before usual harvest (Oct. 20 in this case). Total top yield of twice cuts exceeded approximately 20-30% that of the check plot (no delayed fertilizer and no cut in the growth midst) except only the plot of no fertilixers. The cutting in August was superior in total herbage yield to that in September. The treatment raised L/S ratio, probab

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