De®cit irrigation and nitrogen effects on
maize in a Sahelian environment
II. Shoot growth, nitrogen uptake and
water extraction
R.K. Pandey
1, J.W. Maranville
*,2, M.M. Chetima
Institut National De Recherche Agronomique Du Niger (INRAN) B.P. 429, Niamey, NigerAccepted 14 December 1999
Abstract
Maize growth in arid and semiarid regions is often limited by variation in the amount and frequency of irrigation or rainfall. Sub-optimal supply of nitrogen (N) may further curtail growth and development of the crop. Simultaneous optimization of these two inputs provides optimum conditions for crop growth and productivity. A maize (Zea maysL.) crop was subjected to different periods of de®cit irrigation and rates of N in the ®eld on a medium-deep Tropudalf clay loam soil. Water de®cit effects on shoot growth, N uptake and water extraction with varying level of N supply were analyzed to determine their inter-relationships. Water de®cit was created by withholding irrigation at different stages of crop development. Increasing moisture stress resulted in progressively less leaf area, crop growth rate (CGR), plant height, shoot dry matter and harvest index. Mean increase in above ground biomass was 7.7 and 8.7 kg per mm of water used in the 1996/1997 and 1997/1998 seasons, respectively. De®cit irrigation stress indices (DISI) for above ground biomass when the crop was subjected to a 2 week stress was 11.0 and 20.1 compared to 4 week stress values of 3.2 and 16.5 in the 1996/1997 and 1997/1998, respectively, indicating greater stress the ®rst season during vegetative growth. When de®cit irrigation was increased to 8 weeks, DISI values were 34.1 and 39.8 for the respective seasons. Biomass production response to N in both years was quadratic; however, N response differed with irrigation level in both seasons. Highest biomass yield with no irrigation de®cit was obtained at 120 kg N in 1996/1997 and at 160 kg N haÿ1in 1997. Nitrogen uptake was more dependent
on applied N than water supply although N uptake decreased with greater water and N de®cits. Water extraction was highest at the 120 and 160 kg N haÿ1rates with soil water de®cit.
*Corresponding author. Present address: Department of Agronomy, 102 KCR Lab, University of Nebraska,
Lincoln, NE 68583-0817 USA. Tel.:1-402-472-3057; fax:1-402-472-3654.
E-mail address: jmaranvillel@unl.edu (J.W. Maranville).
1Agronomist, Program Leader and technician, PNRA/INRAN, PB 429, Niamey. 2Professor, University of Nebraska, Lincoln, NE.
This study showed that a maize crop differs in its ability to maintain LAI, CGR and above ground dry matter production at different levels of water de®cit and N supply. The adaptive strategy of maize plants under vegetative water stress appears to be extended rooting depth and water extraction from the deeper soil pro®le, and simultaneous reduction in leaf area to decrease transpiration. Optimizing the inputs of water and N at the farm level would maximize biomass production and harvest index. This information can be useful to guide crop management strategy to enhance maize production in the irrigated perimeter of a Sahelian environment.#2000 Elsevier Science B.V. All rights reserved.
Keywords:De®cit irrigation; Crop growth rate; Leaf area index; Water extraction pattern; Shoot and root growth
1. Introduction
There has been an increasing interest in scheduling deficit irrigation in order to conserve water and maintain crop productivity in the Sahel of West Africa. In these areas, total biomass production is often as important to farmers as grain since animals are a significant part of their livelihood. We reported on the effect of seasonal water deficit and nitrogen (N) rate on grain yield and yield components (Pandey et al., 2000). This research suggested that irrigation deficit during the vegetative stages of maize (Zea mays L.) growth was possible without sacrificing significant grain yield. This irrigation response was dependent on the N application rate. Research has shown the importance of water and N interactions in optimizing maize productivity (Eck, 1984; Eghball and Maranville, 1993a). Most studies suggest that water shortage during vegetative growth reduces leaf area (Boyer, 1970; Acevedo et al., 1971; NeSmith and Ritchie, 1992; McCullough et al., 1994), internode elongation (Novoa and Loomis, 1981), and leaf and stem weight (Denmead and Shaw, 1960; Eck, 1984). In a field study conducted under semi arid environment of Texas, water deficits imposed 41 days after planting reduced leaf, stalk and ear yields of maize, while those imposed 55 days after planting reduced only stalk and ear yields. Water deficit during grain filling did not affect leaf and stalk yields (Eck, 1984). Management of water shortage through frequency and quantity of irrigaiton water during vegetative growth and/or reproductive growth merits attention in high evaporative environments to minimize curtailment of crop growth and yield and achieve higher water use efficiency (Eck, 1985; Chapman and Barreto, 1997).
Nitrogen requirement by maize compared with other nutrients is large in Sahelian soils for optimum vegetative and reproductive growth. Many physiological processes associated with maize growth are enhanced by N supply (Eck, 1984). Nitrogen has dramatic effects on maize growth, development and grain yield on soils that are limiting in N supply. Numerous studies have shown the effects of reduced N supply on leaf area index (LAI), plant height, shoot weight and plant N uptake (Eck, 1984; Pandey et al., 1984; Muchow, 1988; McCullough et al., 1994). In most maize producing areas, increasing rate of N supply results in greater LAI and leaf N (McCullough et al., 1994). Variation in N supply affects crop growth, development and potential kernel set and grain yield (Greenwood, 1976; Pandey et al., 2000). Leaf area index, leaf area duration, crop photosynthetic rate, radiation interception and radiation use efficiency are increased by N
supply (Muchow, 1988). Reduced N supply decreases crop growth (Cox et al., 1993); however, N response is modified by water supply under field condition (Greenwood, 1976; Pandey et al., 2000). Variable water supply either due to shortage of water or failure of the irrigation system to supply water during vegetative and/or reproductive growth stages in many irrigated areas of sub-Saharan Africa, particularly in the Sahel, is common and often results in deficit irrigation.
Information on frequency and quantity of irrigation water and effect of deficit irrigation on shoot and root growth of maize and the subsequent water and N extraction patterns have not been well elucidated under the stressful Sahelian environment, more so with differential rates of N supply. Further, information on crop growth responses and morphological/physiological yield determinants are needed to analyze crop productivity that relate to crop water production function and economic returns. The objectives of this study were to (i) determine the effects of timing and duration of deficit irrigation on maize shoot growth and water extraction, (ii) evaluate water deficit and N rate effects on N uptake, and (iii) analyze relationships between grain yield, plant growth and deficit irrigation and N supply.
2. Materials and methods
A field experiment was conducted for 2 years (1996±1997 and 1997±1998) at INRAN (Institut National de Recherche Agrononmique du Niger) research station near Konni (lat. 118N, long. 128E) on a Tropudalph clay loam soil (fine kaolintic thermic kanolic tropudalph). The site description, crop culture, and weather conditions were previously described (Pandey et al., 2000). Water deficits were imposed as previously described (Pandey et al., 2000). Irrigation was withheld at various crop growth stages (Ritchie and Hanway, 1982) at five N levels.
2.1. Plant sampling and data collection
Plant samples from 1 m row length (five plants) were harvested for above ground biomass at the tasseling (VT) crop growth stage and at physiological maturity. At grain maturity, plant height was recorded. Crop growth rate (CGR) was computed by sampling from between VT and physiological maturity. To monitor the greenness of the crop in response to N status, chlorophyll meter readings were collected each season using the Minolta SPAD 502 meter at the silking stage and converted to chlorophyll content (Markwell et al., 1995). Measurements were taken midway between the margin and the mid-rib of the ear-leaf from 10 representative plants from the center two rows from each plot (Chapman and Barreto, 1997). At 2 days past VT, data on LAI using the leaf canopy analyzer Li-Cor 2000 from the center rows of each treatment were collected in 1997/1998 according to Welles and Norman (1991). Soil moisture extraction patterns were monitored to a depth of 1.5 m using a neutron probe from three irrigation regimes at the R1 stage as previously described (Pandey et al., 2000).
At harvest, number of ears were recorded from each plot. Above ground vegetative biomass and ear yield were recorded after 4 days of air drying. Grain yield was determined from shelled ears and adjusted to 14% after a small sample of stover and grain
were taken and dried at 608C for 48 h. Total biomass included grain and stover. Harvest index was calculated by dividing grain per plot by total biomass after adjusting for moisture content. Grain and stover N was determined on composite samples from all replications from every treatment combination using the Kjeldahl procedure. Deficit irrigation stress index (DISI) was calculated as yield for unstressed treatment minus stressed treatment divided by unstressed treatment times 100 (Pandey et al., 1984). A N stress index (NSI) was calculated in a similar manner (Greenwood, 1976).
2.2. Statistical analysis
All plant data collected were statistically analyzed as a split plot factorial with four replications except water use where three treatments and three replications were used. Analysis of variance (ANOVA) was made to determine crop parameter response to irrigation and N rates. Means among treatments were compared using least significant difference atP0.05 probability. Regression analysis was performed on the relationship between measured crop parameters and water applied and N rates. Best fit regression equations were calculated.
3. Results and discussion
3.1. Aboveground biomass
Aboveground biomass production was different each season and presented separately in Table 1. There was less differential in biomass production between zero N and 160 kg haÿ1 in 1996/1997 than in 1997/1998. The greater rates of N in 1997/1998 resulted in markedly more biomass production than those rates in 1996/1997 and markedly less biomass at the low N rates for the second season. Response to applied N was quadratic each year. There was little difference, however, between the two seasons for response to deficit irrigation which was linear each year.
Deficit irrigation adversely affected total biomass yield at all N levels. The greatest reduction was observed at the 160 kg N haÿ1 of 10.7 and 14.8 kg per mm of deficit irrigation over the control in the respective seasons. The quadratic response to N peaked at 120 kg haÿ1in 1996/1997 and at 160 kg haÿ1in 1997/1998. Total biomass response was also reflected in the plant height (data not shown).
Irrigation deficit during any reproductive stage of maize resulted in marked loss of biomass production compared to deficits only during vegetative growth (Table 1). The DISI was approximately 3±10 times greater when water was withheld during reproductive stage (I-5) than at the vegetative stage (I-2). Even withholding water once during the reproductive phase (I-3) plus vegetative stages resulted in a considerable increase in DISI from that of I-2. The NSI was twice as great in the 1997/1998 season as the 1996/1997 season at the lower N rates (Table 1). This was mostly due to the higher yields in the 1997/1998 season with high N rates making the yield response to N steeper than in 1996/1997, although still quadratic. Grain yield and grain yield components were found to react in a similar manner in these experiments (Pandey et al., 1999).
3.2. Harvest index, crop growth rate (CGR) and leaf area index (LAI)
Harvest index (Table 2) was reduced by both water and N deficits each year as was CGR (Table 3). The greatest reduction for each of these calculated parameters was in the 1997/1998 season. Although total biomass yield was reduced by the deficits imposed, grain production was obviously more affected than stover production, and reflects these stress affects on partitioning efficiency of the crop. The influence of water deficit on these parameters was linear while N deficit effects were quadratic. Maize response to N application for CGR was modified by water stress, with the least response occurring on the most severely stressed irrigation regime (I-5).
The DISI and NSI values were generally less for harvest index compared to CGR which were comparable to the LAI response in 1997/1998 measured at tasseling (Table 4). Maximum LAI values were found at higher N rates under full irrigation as would be expected. The very low LAI values of plants under stress may be underestimates of the procedure since water stress causes leaf wilting (Hicks and Lascano, 1995). Leaf area index values generally range from 2±6 in maize during grain fill (Tollenaar, 1986). Nonetheless, intercepted radiation determines, in part, the CGR. Williams et al. (1965) achieved growth rates of 32.5 g mÿ2dayÿ1 in hydroponically grown corn which compares to a maximum of 22.3 g mÿ2dayÿ1 in the current experiment in 1997/1998.
Table 1
Aboveground biomass yield of maize at physiological maturaity as affected by irrigation (I) and nitrogen (N) de®cits imposed at different growth stages at Konni, Niger in the 1996/1997 and 1997/1998 growing seasons. De®cit irrigation stress indices (DISI) and N stress indices (NSI) are shown for individual irrigation and N treatments
Irrigation regime N applied (kg haÿ1)
0 40 80 120 160 Mean DISI
1996/1997a
I-1 6791 8886 9410 9800 9473 8872 0.0
I-2 6187 6953 8119 9446 8756 7892 11.0
I-3 5938 6927 7069 8476 7038 7090 20.1
I-4 5762 6377 6661 6748 6044 6318 28.8
I-5 5402 6100 5972 6297 5461 5846 34.1
Mean 6016 7049 7446 8153 7454
NSI 26.2 13.5 8.7 0.0 8.6
1997/1998b
I-1 5008 6706 8471 10810 11723 8544 0.0
I-2 4557 6231 8378 10553 11632 8270 3.2
I-3 4040 5991 8240 8743 8640 7131 16.5
I-4 3827 5218 6111 6399 8024 5916 30.8
I-5 3222 4397 5952 6130 6026 5145 39.8
Mean 4131 5709 7430 8527 9209
NSI 55.1 38.0 19.3 7.4 0.0
aLSD
(0.05)I591; N402; IN694; I linear(0.05)is signi®cant; N quadratic(0.05)is signi®cant. bLSD
It is interesting that the lowest value of CGR in that season (I-5; zero N) was 19.5% of the maximum CGR value achieved at the I-1, 160 kg haÿ1treatment (Table 3) corresponded to the difference in LAI (23%) for the same two treatment comparisons (Table 4). Patterns of CGR and LAI response to irrigation and N deficits also corresponded to biomass production in these experiments.
3.3. SPAD chlorophyll content and nitrogen uptake
Leaf chlorophyll content was not altered by irrigation deficit (Table 5) but decreased significantly and linearly with decreased N supply both seasons. Wolfe et al. (1988) showed that chlorophyll concentration (leaf greenness) in maize was positively correlated to leaf N concentration. This would indicate that leaf N content decreased with lower N treatments and leaves became progressively yellow with greater N deficits. Blackmer and Schepers (1995) found that the SPAD 502 m was able to distinguish between fertilizer N treatments for maize which resulted in differential yields. Chapman and Barreto (1997) found that SPAD meter readings were positively correlated with both leaf N concentration and specific leaf N in tropical maize and suggested that SPAD meters provide an inexpensive method to estimate these parameters. In the current experiment, leaf chlorophyll corresponded to biomass yield across N treatments, but was not related to yield with irrigation treatment. The estimate of leaf chlorophyll may provide producers
Table 2
Harvest index of maize as affected by irrigation (I) and nitrogen (N) de®cits imposed at different growth stages at Konni, Niger in the 1996/1997 and 1997/1998 growing seasons. De®cit irrigation stress indices (DISI) and N stress indices (NSI) as shown for individual irrigation and N treatments
Irrigation regime N applied (kg haÿ1)
0 40 80 120 160 Mean DISI
Mean 33.3 34.7 37.3 38.8 39.3
NSI 15.3 11.7 5.1 1.3 0.0
Mean 22.2 32.6 35.9 39.0 38.9
NSI 43.1 16.4 7.9 0.0 0.0
aLSD
(0.05)I2.1; N2.1; INns; I linear(0.05)is signi®cant; N quadratic(0.05)is signi®cant. bLSD
with a management tool to detect a need for N in the sorghum crop. Generally, farmers would not have the resources or expertise to effectively use a SPAD meter, but an extension agent or research specialist may find these more useful to recommend N application rates to meet certain yield goals.
Table 3
Crop growth rate of maize between tasseling and physiological maturity as affected by irrigation (I) and nitrogen (N) de®cits imposed at different growth stages at Konni, Niger in the 1996/1997 and 1997/1998 growing seasons. De®cit irrigation stress indices (DISI) and N stress indices (NSI) as shown for individual irrigation and N treatments
Irrigation regime N applied (kg haÿ1)
0 40 80 120 160 Mean DISI
1996/1997a
I-1 12.74 15.09 15.89 18.38 17.98 16.01 0.0
I-2 11.32 12.67 13.03 17.26 15.95 14.05 12.2
I-3 10.53 11.05 12.35 15.47 14.62 12.80 20.0
I-4 10.09 10.92 11.80 13.26 12.30 11.67 27.1
I-5 9.75 10.33 10.71 11.36 9.94 10.42 34.9
Mean 10.89 12.01 12.76 15.15 14.16
NSI 28.1 20.7 15.8 0.0 6.5
1997/1998b
I-1 9.06 12.44 14.92 17.71 22.27 15.28 0.0
I-2 7.07 11.05 15.21 17.32 20.36 14.20 7.1
I-3 6.69 9.37 13.43 16.91 16.80 12.64 17.3
I-4 6.02 9.26 12.92 13.65 13.91 11.15 27.0
I-5 4.35 6.72 8.62 9.80 9.62 7.82 48.8
Mean 6.64 9.76 13.02 15.08 16.59
NSI 60.0 41.2 21.5 9.1 0.0
Leaf area index of maize as affected by irrigation (I) and nitrogen (N) de®cits imposed at the tasseling growth stage at Konni, Niger in the 1997/1998 growing season. De®cit irrigation stress indices (DISI) and N stress indices (NSI) as shown for individual irrigation and N treatments
Irrigation regime N applied (kg haÿ1)
0 40 80 120 160 Mean DISI
Mean 0.96 1.30 1.84 2.47 2.66
NSI 63.9 51.1 30.8 7.1 0.0
aLSD
Nitrogen uptake (Fig. 1) decreased as irrigation deficits were imposed, and total uptake was dependent on amount of applied N. Uptake was linear (P0.05) over irrigation regimes both seasons and the response slopes were similar each season. The decrease in response to irrigation deficit was greater at the zero N treatment than the 120 or 160 kg N haÿ1 treatments. The decrease in plant N corresponded to similar decreases in leaf chlorophyll and biomass production when N deficits were imposed. However, the decreased N uptake due to irrigation deficit corresponded to only biomass production with irrigation treatment. Zweifel et al. (1987) found that N utilization efficiency of sorghum was most influenced by N level and not altered due to irrigation regime.
3.4. Water extraction pattern
Water extraction from 1.5 m of soil profile differed more with irrigation regime than with N level (Fig. 2). However, the total amount of water extracted was greater at 160 kg N haÿ1 than at zero kg N haÿ1 suggesting that more N may have been conducive to greater root development. However, Champigny and Talouizte (1981) found that under N deprivation, translocation of photoassimilates from shoot to root increased as roots became a stronger sink compared to other sinks. Conversely, Eghball et al. (1993b) found that N stress resulted in less root branching in maize which would imply an impaired ability to take up nutrients. Mackay and Barber (1986) also found that N supply affected
Table 5
Chlorophyll content of maize leaves as affected by irrigation (I) and nitrogen (N) de®cits imposed at different growth stages at Konni, Niger in the 1996/1997 and 1997/1998 growing season. Nitrogen stress indices (NSI) are shown for individual irrigation and N treatments
Irrigation
Mean 243 384 426 553 623
NSI 61.0 38.4 31.6 11.2 0.0
Mean 198 303 434 585 614
NSI 67.8 50.7 29.3 4.7 0.0
aLSD
(0.05)Ins; N41.0; INns; N linear(0.05)is signi®cant. bLSD
(0.05)Ins; N44.6; INns; N linear(0.05)is signi®cant.
maize root morphology, but the effect was genotype dependent. The highest water extraction occurred at the fully irrigated 160 kg N haÿ1 treatment. When water was withheld during both vegetative and reproductive stages of growth (I-5), markedly less extraction occurred to 1.5 m in comparison to the fully irrigated regime (I-1). Extraction
Fig. 1. Nitrogen uptake in maize at ®ve different N rates over ®ve irrigation regimes in the 1996/1997 and 1997/ 1998 growing seasons at Konni, Niger.
Fig. 2. Water extraction of maize from 1 Sm of soil pro®le measured at the R1 growth stage in the 1996/1997 and 1997/1998 seasons at Konni, Niger. Soil moisture was measured from the depleted soil (~) and after irrigation (&) at 0 and 160 kg N haÿ1and at three irrigation de®cit regimes. Horizontal bars represent the
standard deviation.
of water at the 80 kg N haÿ1rate was between the zero and 160 kg N haÿ1rates (data not shown) indicating a linear response to treatment.
Water extraction patterns within the soil profile are generally indicative of root activity. Eghball and Maranville (1993a) showed that greater root depth and water extraction from the lower profile was associated with greater corn yield and varied with cultivar. They found that severe water or N stress appeared to damage root systems while moderate stresses enhanced them. It would appear in the current experiment that severe stresses were detrimental to root system development in the deeper profile which was most likely one factor in reducing yield.
Since more N resulted in more water extracted, the crop would require a higher amount of irrigation water to meet the demand of more biomass since evapotranspiration (ET) is directly correlated to biomass production (Stewart, 1989). Currently, the cost of irrigation water to farmers in this region is constant regardless of irrigation frequency. Should policies change so that producers are required to pay per unit of water used, then each producer must balance cost inputs against returns due to irrigation. Our research has shown that yields can be maximized by choosing the proper stage of growth to irrigate. When water supplies are limited, it is much more efficient to use less irrigation on more land than to fully irrigate less land.
4. Conclusions
Maize is commonly grown in humid and subhumid tropics, but in Sahelian countries, it is grown either in high rainfall zones or with supplementary irrigation in low rainfall zones. Water limitation is a major constraint to maize production. High evaporative demand and warm temperatures expose the crop to water stress if irrigation is not adequately provided. Efficient production of maize where soils are extremely poor in N and where rainfall is highly variable, requires simultaneous attention to these two most important production inputs. Results of our study clearly demonstrate that these two inputs must be judiciously optimized to maximize productivity. Biomass yields were linearly related to water application each season. Water use efficiency was increased slightly with deficit irrigation. Applied N enhanced biomass yields and WUE both years, but to the greatest magnitude in the 1997/1998 season. Deficit irrigation during early vegetative growth modestly reduced LAI, plant height, CGR, N uptake and total biomass production. Deficit irrigation during late vegetative and reproductive growth stages severely reduced these growth parameters.
Nitrogen deficits reduced leaf chlorophyll content, thus, leaf N concentration. Nitrogen uptake was increased linearly as irrigation increased, but total uptake was more influenced by the amount of applied N. Total water extracted from a 1.5 m profile depended on both soil moisture and soil N supply, but more on the latter. It was speculated that root production was stimulated in the deeper soil profile when greater amounts of water and N were supplied. Very little extraction occurred when DISI and NSI values were high.
This study demonstrated that maize production is determined by optimizing both water supply and N application. Reducing irrigation during vegetative growth had less impact
on biomass production and N uptake than when irrigation deficits occur during reproductive stages. These reductions are a direct result of diminished LAI, CGR and root activity. The adaptive strategy of maize under moderate vegetative stress appears to relate to an extension of rooting depth and extraction of water deeper in the profile while simultaneously reducing LAI to decrease transpiration. Severe stresses may be detrimental to root development. Producers need to carefully weigh input costs against market income when considering the economic maximum they choose for irrigation and N use in Sahelian production areas.
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