Soybean. The old recommendation for terminating irrigation on soybeans is the R6 stage (seed fill). However, rooting depth in SEMO may be hampered by hard pans and frequent rains. Therefore, it may be appropriate to keep irrigating soybeans later into the season. Figure 1 shows that yields continued to increase with later irrigation termination dates. These data are from a sandy soil, but the silt soil responded in a similar manner. A yield loss of about ¾ bushel/acre per day was shown to occur when irrigation is terminated too soon on a sandy soil. Yield increase was associated with added seed weight. Therefore, irrigation should go on until the beans are not only touching, but the shoulders are beginning to bunch up in the corners. The canopy should have yellow leaves. For typical soybeans (3,000 seeds/lb), if final seed weight is less then 14 gram per hundred seed, then irrigation may have been terminated too soon.

Fig. 1. Soybean yield versus the date of last irrigation for sandy soil.

Fig. 2. From L to R, a 8-, 12-, and 15-gm per 100 seed.

Corn. The Milk Line (ML) starts at dent and moves down the kernel towards the cob. When it reaches this point Black Layer (BL) occurs. It is generally reported that this takes 20 to 30 days, and ½ ML to BL takes about 10 to 15 days. This progression has been slower in the hybrids observed in our study, and therefore, cutting off irrigation at ½ ML is too early. At the last irrigation corn should be at the ¾ ML stage and the husk beginning to loose most of its green color.

Fig. 3. Relative Yield versus Heat Units, 2003 & 2005

2006 Field Day Report