H.J. “Rick” Mascagni, Jr. and Brenda Tubana

Introduction

            Nitrogen (N) fertilization is a critical cultural practice required for producing maximum corn yield. Many factors, including soil type and crop management systems, determine optimum N rates. Nitrogen is typically knifed-in soon after the crop has emerged and an adequate stand established. Growers often times split N fertilizer applications as part of their management system or, in some cases, due to uncontrollable factors such as excessive or lack of rainfall, may produce soil conditions conducive to N fertilizer loss through denitrification and/or inefficient plant N uptake.  If N is topdressed with a fertilizer containing urea losses may occur due to volatization, which depends to a large extent on climatic and soil factors. If irrigated or rainfall occurs (0.5 inch or greater) within about three days, the fertilize is incorporated and no or minimal volatization losses will occur  Sometimes N applications are delayed or omitted due to inclement weather, while at other times, growers apply the recommended N rate for an expected yield potential. However, as the crop develops yield potential may be higher than expected and additional N may be required. In each of the above situations the question arises, how late can N fertilizer be applied and be effective? The N fertilizer source is also an important component of an effective fertility program. Products are also available such as urease inhibitors (i.e., Agrotain) that minimize urea volatization losses for 7 to 10 days. The objective of this trial was to evaluate N applications, N sources, and an urease inhibitor at different growth stages on a Mississippi River clay soil.

 Procedures

            A field experiment was conducted in 2011 on Sharkey clay at the Northeast Research Station near St. Joseph to evaluate the influence of N rate, timing, and fertilizer source on corn yield and N fertilizer use efficiency (NFUE). Early-season N rates were injected at about the four-leaf growth stage (April 15) as 30-0-0-2 solution (UAN) at N rates of 0, 150, 210, and 240 lb/acre. Urea,with and without Agrotain (3 qts/ton urea), was also hand-broadcast at the rate of 150 lb N/acre at the four-leaf growth stage. For the early-season N rate of 150 lb/acre using 30-0-0-2, a supplemental N rate of 60 lb/acre was applied at about the 12-leaf (May 22) and early-silk growth stages (June 2). Urea was hand-broadcast and UAN was hand-dribbled (to simulate a dribble application) at the 12-leaf and early-silk applications. There were a total of 10 treatments (see Table 2). Pioneer 31P42 was planted on March 23 at 32,000 seed/acre. The trial was furrow irrigated. Cotton was the previous crop and all LSU AgCenter recommended cultural practices were followed.

             The experimental design was a randomized complete block with five replications. Grain yield, yield components, plant N, seed N, NFUE, and remote sensing data were determined. Grain yield was determined by machine harvest from the two middle rows of four-row plots and reported at 15.5% moisture. Yield components, ears/acre, kernel weight (g/100 seed), and ear size (kernels/ear) were also determined from the two middle rows.  Ear-leaf samples were collected at the early-silk growth stage to determine the influence of treatments on the N status of the plant. Seed samples were also collected at harvest. Total N was determined in the plant tissue and harvested seed by the LSU AgCenter’s Soil and Plant Testing Lab. Seed-N uptake (lb N/acre) was calculated by multiplying seed-N concentration by grain yield. NFUE was calculated using the following formula: (seed-N uptake for a given N rate – seed-N uptake for the no-N control) / N rate.   Remote sensing data using a SPAD meter were also determined at the 4- and 12-leaf growth stages. Statistical analyses were performed using the GLM procedure of SAS using a probability level of 0.10.

 Results and Discussion

            Rainfall only totaled 4.9 inches in May and June (Table 1). Furrow irrigations were applied on May 22 and June 2. Across treatments, yields ranged from 15.9 (control) to 197.5 bu/acre (Table 2).

             At early season, urea, urea + Agrotain, and UAN were compared at the 150 lb N/ace rate. Yield responses to treatments had the following rank: UAN > urea + Agrotain > urea (Table 2). There was a 11 day interval between application and rainfall. The late application of 60 lb N/acre increased yields similarly for both the 12-leaf and early-silk applications. There was little difference in yield between sources. Kernel weight was slightly higher for the late compared to early-season applications. When comparing equivalent N rates applied either once early season or split between early season and 12 leaf or early-silk growth stages, yields were similar.

 

Leaf and seed N data are presented in Table 3. Leaf N, seed N, seed N uptake, and NFUE responses to early-season N treatments were highest for UAN compared to urea + Agrotain and urea. There were small differences between the 12-leaf and early-silk N applications for each N trait evaluated. When comparing equivalent N rates, there were small differences between single and split applications for any N trait. Nitrogen fertilizer use efficiency (NFUE) was extremely high, ranging from 0.46 to 0.68. The highest NFUE value occurred for the 150 lb N/a UAN treatment applied early season. SPAD readings reflected trends similar to the yield response (Table 4).

 

Table 1. Rainfall in St. Joseph, 2011.

Month	Rainfall
	inches
March	8.3
April	3.0
May	0.9
June	4.0
July	4.4
August	1.3

 

Table 2. Influence of N fertility treatments on corn yield and yield components on Sharkey clay, 2011.

						N rate
ESN1 rate		ESN source2			12-leaf		Early silk		Late N source		Total N applied		Yield		Ears		Kernel weight		Kernels
lb/a					———lb/a——						lb/a		bu/a		no/a		g/100		no/ear
0		–			–		–		–		0		15.9		33,350		30.4		42
150		Urea			–		–		–		150		114.3		32,700		33.9		270
150		Urea + Ag			–		–		–		150		124.8		32,050		33.4		295
150		UAN			–		–		–		150		164.8		28,780		36.6		414
150		UAN			60		–		Urea		210		187.8		28,780		37.2		441
150		UAN			60		–		UAN		210		184.4		30,080		38.2		416
Average													186.1		29,430		37.7		429
150	UAN				–		60		Urea		210		191.1		28,780		37.2		446
150	UAN				–		60		UAN		210		186.1		34,010		37.6		402
Average													188.6		31,400		37.4		424
210	UAN				–		–		–		210		183.9		32,050		37.6		400
240	UAN				–		–		–		240		197.5		33,350		34.8		471
LSD (0.10):													10.9		NS3		NS		80

¹ESN, early-season N injected at about 4-leaf growth stage.

²Ag = Agrotain; UAN = 30-0-0-2;

³NS = Non-significant at the 0.10 probability level

Table 3. Influence of N fertility treatments on N nutrition on Sharkey clay, 2011.

						N rate
ESN1 rate		ESN source2			12-leaf		Early silk		Late N source		Total N applied		Leaf N		Seed N		Seed N uptake		NFUE3
lb/a					———lb/a——						lb/a		%		%		lb N/a
0		–			–		–		–		0		0.98		1.29		11.5		–
150		Urea			–		–		–		150		1.47		1.24		79.8		0.46
150		Urea + Ag			–		–		–		150		1.60		1.20		84.1		0.48
150		UAN			–		–		–		150		2.50		1.23		114.1		0.68
150		UAN			60		–		Urea		210		2.60		1.27		133.7		0.58
150		UAN			60		–		UAN		210		2.60		1.32		136.0		0.59
Average													2.60		1.30		134.9		0.59
150	UAN				–		60		Urea		210		–		1.31		140.3		0.62
150	UAN				–		60		UAN		210		–		1.34		139.5		0.61
Average													–		1.33		139.9		0.62
210	UAN				–		–		–		210		2.57		1.30		134.3		0.59
240	UAN				–		–		–		240		2.65		1.36		149.9		0.58
LSD (0.10):													0.17		0.05		9.1		0.05

¹ESN, early-season N injected at about 4-leaf growth stage.

²Ag = Agrotain; UAN = 30-0-0-2;

³NFUE = N fertilizer use efficiency

Table 4. Influence of N fertility treatments on SPAD readings at the early season and

12-leaf growth stages on Sharkey clay, 2011.

				N Fertilizer Source
N rate1	Total N	Urea				Urea + Agrotain		UAN
lb/a	lb/a	———————–SPAD readings—————————
					Early-Season N Application
150	150	39.6				40.3			54.6
210	210								59.1
240	240								56.1
			12-leaf Growth Stage N Application
60	210	54.2							56.7
LSD (0.10):						3.8