From the standpoint of amount of dry matter and nitrogen added to the soil and the eradication of the plants, sweet clover may best be plowed for corn about April 25 to May 5. The five dates of harvest used rank as follows in yield of dry matter for tops and roots: May 3-18, Nov. 1-13, Oct. 1, April 9-13 and Sept. 1. Nitrogen yields rank in much the same order.
Of the total for tops and roots, the tops contained 68 percent of the dry matter and 73 percent of the nitrogen in September, 51 and 48 percent, respectively, in October, 29 and 18 in November, 24 and 28 in April and 69 and 74 in May.
In September the tops contained 3.5 percent nitrogen and the roots 2.8, October 3.0 and 3.4, respectively, November 2.1 and · 3.9, April 5.4 and 4.5 and May 3.4 and 2.6.
In April the roots contained 61 percent as much dry matter and 69 percent as much nitrogen as in November. In April, however, tops and roots together contained 80 percent as much dry matter and 96 percent as much nitrogen as the roots alone in November.
White sweet clover was superior to yellow in yield of dry matter in the tops, while yellow was superior to white for roots. For tops and roots there was little difference. Yellow survived somewhat more than white, but the difference was small.
Plowing Sept. 1 killed nearly all of the sweet clover but with a sacrifice of approximately one-half of the dry matter and nitrogen. Plowing in October and November caused costly eradication problems since approximately 20 plants per square yard survived.
The Pulverator plow was less effective than the ordinary plow in exterminating the clover for September, October, November and April plowings. A depth of plowing recognized as providing good tillage practice is to be preferred.
Draft requirements were in direct proportion to the depth of plowing, and the ordinary plow required practically the same amount of draft as the Pulverator. Draft required increased progressively for September, October and November plowings, dropped sharply for April and increased for May over April.
Pasturing yellow sweet clover from Aug. 15 on reduced the survival nearly 50 percent. Plants buried by hand survived directly in proportion to length of roots for 1, 3, 5 and 7-inch lengths, and inversely in proportion to depths of burying for 3, 5 and 7-inches. Fewer of the small roots survived than of the large ones.
Plants transplanted in normal position had an average of eight buds per plant. On the plants that survived 51 percent of the buds produced shoots. Under field conditions there were approximately two shoots per plant. Large roots had more buds than small ones, but practically the same percentage of buds grew from large roots as from small ones.
Following dormancy buds did not grow at temperatures of 28 and 30° F., increased an average of 0.5 cm. in length in 32 days at 36°, 1 cm. at 40° and 4 cm. at 50°.
Wilkins, F. S. and Collins, E. V.
"Effect of time, depth and method of plowing upon yield and eradication of biennial sweet clovers,"
Research Bulletin (Iowa Agriculture and Home Economics Experiment Station): Vol. 13
, Article 1.
Available at: https://lib.dr.iastate.edu/researchbulletin/vol13/iss162/1