Update:
The trial was successfully completed as intended.
In 2019, the second year of the rye water use study was completed. The study was designed around 7 rye termination timings relative to soybean planting. Rye was planted the previous fall and terminated in spring with glyphosate. Rye was terminated once per week in the spring until two weeks after soybean planting (plus checks which contained only rye, only soybean, rye and soybean together, and no crop/fallow). Access tubes were placed in each plot for a neutron probe to read soil moisture levels at various depths in the soil profile. Soil moisture readings were taken from each plot at each depth once per week for the duration of the growing season. Each treatment of the trial was replicated 4 times using a randomized complete block design.
The spring of 2019 was quite dry in Carrington through the end of May, while late-season moisture was plentiful. Rye water use (in non-terminated treatments) peaked around June 24 when rye in the grain filling stage. Due to timely rain around May 20th the rye water use did not differ from the fallow treatment until the last week of May. Soybeans were planted on May 20. Soybean water use did not differ from the fallow treatments until around July 1. Even with a wet fall, soybean water use deficit was still greater than the rye, but peaked on Aug 23. Water deficit from the rye-only plots returned to fallow levels by early Sept. This whole process illustrates why rye and soybeans work well together. Soybean water use did not differ from fallow treatments until after the rye water use peaked, essentially creating a soil moisture 'hand-off". When rye was terminated prior to soybean planting, the water use trend was the same as having soybeans alone. This is due to shutting the rye water use down prior to its water use surge.
Soybeans responded to the differing rye terminations. When rye was terminated prior to soybean planting, soybean yield was not impacted. When rye was not terminated (both crops grown to maturity), soybean yields were reduced by 50%. With the timely rains in June, July, and August all other treatments had similar soybean yields as the soybean-only check. This is the first year that this has happened since 2014 at Carrington. In 2018 (the first year of measuring water use) we saw significant yield reductions when the soil moisture reached 1.5" of water deficit in the top 2 feet. We never reached this deficit in 2019 except in treatments where the rye was not terminated. This further provides evidence that soybean risk can be predicted by measuring soil moisture status. More work is planned in order to build a case for the soil moisture threshold which causes soybean yield loss, in order to time terminations more precisely.