Michael Starling, David Weaver, and Wes Wood

Up to one-half of the soybean crop in the southern United States is grown as a second crop after small grains. In the Deep South, it is common to plant soybean following full-season corn. In this cropping system, planting date is delayed from the optimal mid-May to mid or late July. Because soybeans flower in response to daylength, planting late results in limited vegetative growth. Research has shown that this lack of vegetative growth is a major reason why late-planted soybeans yield less than full-season beans.

There are two possible approaches to the problem of limited vegetative growth in late-planted beans. Application of nitrogen fertilizer may promote growth of young plants by supplying young plants with additional N before nodulating bacteria begin supplying the plant with N. Research has also shown that indeterminate growth habit (most soybeans in the Southeast are determinate) may promote more vegetative growth in late-planted soybeans.

No studies have been conducted in the southeastern United States to evaluate the effects of nitrogen fertilizer and growth habit on late-planted, double-cropped soybeans. An AAES study was conducted to determine the interactive effects of starter-N (0 and 45 pounds per acre) and growth habit (determinate and indeterminate) on soybean growth and yield following corn in a late-planted, double-cropped system.

Experiments were conducted in seven southern Alabama environments: Brewton Experiment Field in Brewton (1996), E. V. Smith Research Center Field Crops Unit in Shorter (1996), Gulf Coast Substation in Fairhope (1995 and 1996), Monroeville Experiment Field in Monroeville (1996), and the Wiregrass Substation in Headland (1995 and 1996).

Three Maturity Group VIII soybean genotypes--Au86-2397I (indeterminate), Au86-2397D (determinate), and Cook--were planted with two starter N rates (0 and 45 pounds per acre). Two of the genotypes (Au86-2397D and Au86-2397I) are near isolines that differ only in growth habit, and the other was a determinate check cultivar (Cook).

All sites were managed as doubled-cropped soybean production systems following full-season corn. Pioneer 3563 corn was grown for maximum yield. Land preparation after corn harvest consisted of chisel plowing followed by one or two passes with a disk harrow to form an adequate seedbed. Soybeans were planted in mid-July using a grain drill with row spacing of six inches. Seeding rate was six seeds per foot of row.

Plant height, dry matter yield, and plant N content were both increased at the R1 stage of development (first flower) (see table). Starter-N resulted in a 5% increase in plant N concentration. There was some reduction in nodule number and nodule dry weight in the N-treated plots (data not shown), but there was no negative effect on total plant N concentration at the R1 stage of growth. There was no effect of genotype on plant N concentration (see table).

Nitrogen and Genotype Effects on Growth and Yield of Late-planted Soybean

Genotype	Lbs. N/acre			Lbs. N/acre					Lbs. N/acre			Lbs. N/acre			Lbs. N/acre
	0	45	Avg.	0	45	Avg.			0	45	Avg.	0	45	Avg.	0	45	Avg.
	Plant height at R1			Dry matter yield at R1					Plant N concentration			Plant height at R8			Soybean grain yield
	in.			lbs./acre					pct.			in.			bu./acre
Cook	14	15	15	2,209	2,138		2,174		3.24	3.43	3.33	18	20	19	26.3	29.9	28.1
Au86-2397D	13	14	14	1,827	2,290		1,737		3.33	3.43	3.38	17	18	18	25.8	27.2	26.6
Au86-2397I	13	15	14	1,702	2,149		1,925		3.17	3.38	3.27	26	27	27	27.3	29.9	28.1
Average	13	15	--	1,907	2,192		--		3.25	3.41	- -	20	22	--	26.8	29.0	--

Plant maturity date was not affected by genotype or N application (data not shown). Plant height at the R8 development stage (plant maturity) was affected by both genotype and N application (see table). As expected, the indeterminate Au86-2397I had greater plant height than Au86-2397D at R8. Application of starter-N slightly increased R8 plant height. The magnitude of difference in R8 plant height between N treatments was similar to that observed at R1.

Grain yield was affected by both genotype and N rate (see table). The indeterminate Au86-2397I was higher yielding than the determinate Au86-2397D when averaged across all environments, averaging 28.1 bushels per acre compared to 26.6 bushels per acre for Au86-2397D (see table). Starter-N increased grain yield an average of 2.2 bushels per acre when averaged across all environments. The greatest increase came in the highest yielding environment (Wiregrass Substation in 1996) where application of starter-N boosted grain yield from 32.3 bushels per acre to 36.6 bushels per acre.

In this experiment, application of starter-N increased vegetative growth. This resulted in higher yields for the N-treated plots. The indeterminate genotype also had a yield advantage in this experiment. Further research is needed for other locations and a wider range of late planting dates that were not addressed in this study.