Yield Penalty from Second Year Soybean

John Grove, Agronomic Soil Scientist and Director, Research and Education Center, University of Kentucky

Growers are considering planting soybean after soybean, especially full season soybean after full season soybean, over some acreage in 2017. Many growers have little experience with second year full season soybean, having kept with their existing crop rotations. Other growers, experiencing problems like soybean cyst nematode in some fields, have avoided soybean after soybean on all acres they manage. I observed two extension agricultural economics presentations modeling profit/loss to different crop rotation options, and where second year soybean yield loss relative to first year soybean was set at either 5 or 10 %, in the absence of field data.


One of my long term rotation trials includes the corn-corn-soybean-soybean sequence, with all four rotation components represented each year, since 2008. The 2009 and subsequent seasons allowed comparison of first and second year full season soybean. Each year, a maturity group 4 full season soybean with an excellent disease resistance package was planted in all plots on the same day (always prior to 20 May) at the Spindletop research farm just outside Lexington in 15 inch rows at about 120,000 seed per acre. The plot area was, and is, free of soybean cyst nematode. Soybean grain yield, each year from 2009 through 2016, are given in the table below.

Table 1. Soybean after Soybean Yield compared with Soybean after Corn Yield. Data from Lexington, KY.
	First	Second
Harvest Year	Year Soybean	Year Soybean	Yield Loss
	bushels/acre		%
2009	83.4	78.9	5.4
2010	33.0	34.4	-4.3
2011	54.2	49.1	9.5
2012	28.0	29.2	-4.2
2013	70.4	74.0	-5.1
2014	61.0	60.5	0.9
2015	53.2	46.1	13.3
2016	49.9	48.5	2.8
Average	54.1	52.6	2.3

First year soybean yield ranged from about 30 bushels per acre in dry years (2010 and 2012) to 70 to 80 bushels per acre in moist years (2009 and 2013). Statistically significant yield loss to second year soybean occurred in 2011 and 2015, and I have no idea why this happened in these two years. There was no significant difference to rotation in the other six years – second year soybean even yielded a bit more than first year soybean in three of those six years. Over the eight years, first year soybean yielded more than second year soybean in five of eight years, averaging 2.3 % greater yield overall. I concluded that second year soybean usually yields a bit less than first year soybean, sometimes a lot less. On average, however, the yield loss was not nearly as great as the 10 % growers may factor into an economic analysis of rotation options.

Marestail was more of a problem in soybean.

Though the variation in yield well represents Kentucky’s seasonal weather, readers are reminded that the experiment is optimized for soybean nutrition and weed control. I have observed greater marestail/horseweed (Conyza canadensis (L.) Cronquist) pressure in the experiment’s soybean plots over time, resulting in my annually hand rogueing the trial. Most important, there is no soybean cyst nematode pressure. Before planting second year soybean in any field, take soil samples to determine soybean cyst nematode levels.