Many factors besides shattering influence the yield of sweet-clover seed. As only those portions of the plants exposed directly to the sunlight set seed abundantly, thin stands usually produce more seed to the acre than heavy stands. When very heavy stands make a large growth, seed is produced only on the upper 24 to 30 inches of the plants, whereas with thinner stands it is produced on the lower branches as well.

The quantity of moisture in the soil at the time the seed is maturing is an important factor also. During hot, dry weather the plants may not be able to absorb from the soil sufficient water to supply the excess required by them for seed production. In this event many of the seed pods will abort and fall when partly mature. Pods abort and fall in a very short time, so that partly shriveled ones seldom are found on the plants, although the extent of the aborting is shown by the number of barren racemos. When such weather conditions prevail, the second crop usually will produce a heavier yield than the first crop. This is due for the most part to the inability of the large plants to obtain sufficient water for seed production. The much smaller plants of the second crop do not require as much moisture as the larger plants of the first crop, as the vegetative growth is seldom more than half as much.

The type of root growth has much to do with the quantity of water the plants are able to obtain during droughty weather. When sweet clover is planted on soil that has a tendency to be wet, the plants will produce a much-branched shallow root system instead of the normal deep roots which are found on well-drained soils. During dry weather the upper layers of soil become so depleted that plants having a very large percentage of their roots in these layers can not obtain a sufficient quantity of moisture to supply their requirements for seed production.

It is often stated that the first crop of sweet clover will produce more seed to the acre than the second crop. This depends very largely upon the thickness of the stand and on weather conditions. In regions where two crops may be grown in a season, the first usually will produce more seed to the acre than the second if the field has a thin stand. When the stand is thick the second crop ordinarily yields more seed. In regions where a crop of hay or pasturage may be obtained in addition to the seed crop, it is seldom an economical procedure to permit the first crop to mature. Not only will sweet clover produce an abundance of nutritious pasturage or a cutting of 1 to 3 tons of hay in addition to the seed, but the difficulty of handling the large, stemmy growth of the first crop for seed is avoided.

Yields of sweet-clover seed have been reduced during the last two seasons by several fungous diseases. Experimental work has not been completed to show the percentage of damage done by these organisms, but in some sections of the country seed yields were reduced considerably. The clover stem borer,[2] which is prevalent in red clover in certain sections of the country, also infests sweet clover. It is probable that this insect did some damage to the seed crop in certain sections of the country in 1916.

[2] Languria mozardi.