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PRODUCTION PAPERS

Response of Irrigated Acala and Pima Cotton to Nitrogen Fertilization

Growth, Dry Matter Partitioning, and Yield

^a Dep. of Agron. and Range Sci., Univ. of California, One Shields Ave., Davis, CA 95616
^b Univ. of California Coop. Ext., 680 N. Campus Drive, Suite A, Hanford, CA 93230
^c Univ. of California, Shafter Res. and Ext. Cent., 17053 N. Shafter Ave., Shafter CA 93263

* Corresponding author (rltravis{at}ucdavis.edu)

Received for publication October 30, 2001.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
Acala (Gossypium hirsutum L.) and Pima (G. barbadense L.) cotton growth, lint yield, and fiber quality responses to N in the San Joaquin Valley, CA were evaluated. Numerous reasons, including adaptation of N fertilization guidelines to modern production practices, recent increases in energy costs, and growing concerns about NO^-₃ contamination of ground water, led to the initiation of this study. Acala was grown for 3 yr on a Panoche clay loam [fine-loamy, mixed (calcareous), thermic Typic Torriorthents] and a Wasco sandy loam (coarse-loamy, mixed, nonacid, thermic Typic Torriorthents). Pima was grown for 2 yr on the Panoche clay loam. Four N treatments were established in a randomized complete block design: 56, 112, 168, and 224 kg N ha^-1. Three-year average aboveground dry matter production of Acala was 7800 and 12 600 kg ha^-1 on the Panoche clay loam and 8500 and 11 900 kg ha^-1 on the Wasco sandy loam for the 56 and 168 kg N ha^-1 treatments, respectively. The equivalent 2-yr averages for Pima were 7600 (56 kg N ha^-1) and 10 800 kg ha^-1 (168 kg N ha^-1). Linear increases in lint yield with increased N fertility level occurred for Acala on Panoche clay loam in every year. Maximum lint yield averaged over 3 yr was 1842 kg ha^-1 in the 224 kg N ha^-1 treatment. The response of Acala lint yield to N management on the Wasco sandy loam was smaller than on Panoche clay loam, with a maximum lint yield of 1666 kg ha^-1 (224 kg N ha^-1, 3-yr average). Pima lint yield responded to N management in a quadratic fashion with maximum yields in the 168 kg N ha^-1 treatment in both years (1638 kg ha^-1, 2-yr average). Acala gin turnouts were greater at the Panoche than at the Wasco site. Decreases in gin turnout with increasing N were significant on the Panoche clay loam (Acala and Pima) but not on the Wasco sandy loam (Acala). There was a generally positive relationship between increasing N fertilization and yield; however, efficient N management should include an assessment of available soil residual N, soil type, and yearly climatic conditions.

Abbreviations: LAI, leaf area index

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
UNDER CALIFORNIA COTTON production systems, N is generally considered to be a limiting factor and is commonly added to meet crop demand. Managing N fertilization is of particular importance because many environmental and production factors influence cotton N demand. Nitrogen deficiency reduces vegetative and reproductive growth and induces premature senescence, thereby potentially reducing yields (Gerik et al., 1994; Tewolde and Fernandez, 1997). On the other hand, high N availability may shift the balance between vegetative and reproductive growth toward excessive vegetative development, thus delaying crop maturity and reducing lint yield (Gaylor et al., 1983; Howard et al., 2001; Kohli and Morrill, 1976). It may also increase the likelihood of aphid infestations, reduce the time to adult emergence for whiteflies (Bemisia argentifolii Bellows and Perring), and complicate cotton defoliation (Cisneros and Godfrey, 2001; Blua and Toscano, 1994; Roberts et al., 1996). Nitrogen application in excess of an optimum level is usually less apparent than N deficiency and may be counterbalanced with plant growth regulators and water management under irrigated production systems like those in California. Average N fertilizer applied by California cotton growers increased considerably since the late 1970s from about 120 to around 200 kg ha^-1 in the mid-1990s (USDA Econ. and Stat. Syst., 2001). Among other factors, improved understanding of cotton growth and development and increased employment of in-season plant mapping results as decision-making tools may have contributed to modifications in management strategies resulting in lower N applications since the mid-1990s (172 kg N ha^-1 in 1999) (USDA Econ. and Stat. Syst., 2001). At least some of the increase in N fertilizer application since the late 1970s may be linked to the development of new cotton varieties and the introduction of mepiquat chloride [1,1-dimethylpiperdinium chloride] in California in 1981. Meredith et al. (1997) and Wells and Meredith (1984a)(1984b, 1984c) documented the changes in cotton varieties over time. They found that modern cotton cultivars are more determinate and earlier maturing, set and fill bolls over a shorter period of time, and respond more strongly to N applications than obsolete varieties. Mepiquat chloride is a plant growth regulator that allows cotton growers to prevent excessive vegetative growth without imposing potentially yield-limiting stress on the cotton plant (Kerby et al., 1996). The availability of mepiquat chloride may contribute to a tendency of some cotton growers to err on the high end of N application in cotton. However, excess N application may aggravate ground- and surface-water pollution. Nitrate concentrations in San Joaquin Valley well water have increased significantly from the mid-1980s to the mid-1990s, and in some cases, exceed the upper limits for drinking-water standards, leading to increased public concern (Franco and Cady, 1997; Burow et al., 1998).

The majority of cotton grown in the San Joaquin Valley is the Acala Upland type (Gossypium hirsutum L.), but a considerable amount of American Pima (G. barbadense L.) is grown every year. Pima is an extra-long staple cotton but generally yields less than Upland cotton (Unruh and Silvertooth, 1996). Due to its more pronounced indeterminate growth habit, Pima cotton is limited to regions with long growing seasons. In 2000, more than 69 000 ha of Pima cotton were grown in the United States (Arizona, California, New Mexico, and Texas), more than 84% of which was produced in California (USDA Econ. and Stat. Syst., 2001). Approximately 16% of the 2000 cotton area in California was planted in Pima cotton (USDA Econ. and Stat. Syst., 2001). Although similar in many respects, Pima differs from Upland cotton in others. Pima is more sensitive to delays in planting and excessive N fertility, which can result in greater vegetative growth and delayed maturity (Tewolde et al., 1995; Kittock et al., 1981; Silvertooth et al., 1995). Because most of the cotton grown in the USA is G. hirsutum [98.9% in 2000 (USDA Econ. and Stat. Syst., 2001)], little research is done on G. barbadense. However, the significant percentage grown in California merits research attention to the production of Pima cotton.

Most of the recent information on the response of cotton to N is available for dryland production systems in the southeastern and midwestern parts of the U.S. Cotton Belt. However, differences in environment and management including cultivars exist between those areas and California. Much of the information applicable to California cotton production predates changes in production practices that have occurred in recent years. In addition, the majority of the studies conducted throughout the cotton-producing states did not take residual soil N into consideration when treatments were established. Thus, in some cases, conclusions drawn about plant responses were limited to applied N, not available N. In such cases, absence of an N response may be due to the presence of substantial amounts of residual soil N, above which applied fertilizer may not significantly affect cotton yield. In California, where cotton is commonly grown in rotation with highly fertilized crops such as corn (Zea mays L.) and tomato (Lycopersicon esculentum Mill.) or with N-fixing alfalfa (Medicago sativa L.), residual soil N levels may be considerable. Moreover, a number of distinct soil types are present in the cotton-producing areas of California. Differences in their characteristics may contribute to variation in residual N levels at planting and amounts of N mineralized throughout the growing season.

The main objectives of this study were to (i) evaluate the effects of different N fertility levels on Acala and Pima cotton growth, dry matter partitioning, and yield and (ii) compare the response of Acala cotton grown on a sandy loam and a clay loam.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
Experiments were conducted in the San Joaquin Valley of California during the growing seasons of 1998, 1999, and 2000 on a farmer's field in Kings County and at the University of California West Side Research and Extension Center in Fresno County. The soil at the Kings County location is classified as a Wasco sandy loam (coarse-loamy, mixed, nonacid, thermic Typic Torriorthents) and the soil at the Fresno County location as a Panoche clay loam [fine-loamy, mixed (calcareous), thermic Typic Torriorthents]. Selected physical and chemical properties of the two soils are presented in Table 1. Acala cotton (‘Maxxa’) was grown at both locations in all three growing seasons. Pima cotton (‘S-7’) was grown only at the Fresno County site and only in 1999 and 2000. Weather data were obtained from automated stations at or near the experimental sites. Heat unit accumulation was calculated according to the single sine method with a lower threshold of 15.55°C and no upper threshold (Zalom et al., 1983). Degree days represent the sum of heat units accumulated since planting. To establish N target treatments of 56, 112, 168, and 224 kg N ha^-1, soil samples (top 0.6 m) were collected every year approximately 1 wk after seedling emergence from each replication of every treatment and analyzed for NO₃–N. Average soil NO₃–N levels across years in the N-56, N-112, N-168, and N-224 treatments were 48, 51, 61, and 88 kg N ha^-1, respectively, for the Panoche clay loam and 50, 56, 62, and 85 kg N ha^-1, respectivley, for the Wasco sandy loam. At each location, the amount of fertilizer N to be applied was determined by subtracting the soil NO₃–N content determined in a treatment from the treatment target rate. Urea-N (32% N) was injected approximately 0.15 m deep in bands 0.2 m from the plants on either side of each row at the three to five true leaf stage each year. A single application of N was made because results from previous years indicated that no significant yield increases were obtained by split applications (unpublished results, 1996). Each N treatment was maintained in the same experimental unit for the duration of the study. Experimental units were between 80 and 170 m long and 4 to 12 rows wide with 0.96- or 1.01-m row spacing. Small plots, 3 to 12 m long and 4 to 12 rows wide, were maintained without N fertilization within each experimental unit to serve as check plots. Crop management practices for each location and year were consistent with typical agronomic practices of the region. Water was provided by furrow or flood irrigation. Both sites were irrigated once before planting. In every growing season, cotton was irrigated three times (approximately 0.8 m including preirrigation) on Panoche clay loam and six times (approximately 1.0 m including preirrigation) on Wasco sandy loam. Additions of P and K were made according to soil test results and University of California Cooperative Extension guidelines as needed. The experimental design was a randomized complete block design with four replications for Acala and three replications for Pima.

Dry weights of all plant fractions were determined by drying to constant weight at 65°C in forced-air ovens. Total vegetative and reproductive dry matter was obtained by summation of the corresponding fractions. Vegetative dry matter consists of stems (including branches, petioles, squares, and flowers) and leaves. Reproductive dry matter is the sum of lint, seed, and burs (including immature bolls) and is also referred to as fruit fraction in the text.

Lint yields were determined after defoliation by harvesting the center two or center four rows of each experimental unit with a spindle picker. Check plots were not machine-harvested; thus, no data on lint yield or lint characteristics from those plots are presented here. Subsamples were ginned to determine lint percentage and lint quality characteristics. Fiber length, strength, and micronaire analyses were conducted by high-volume instrument (HVI) testing at the USDA Visalia California Classing Office, Visalia, CA.

Data were analyzed statistically as a repeated-measures design, treating years and sampling within years as repeated measures, utilizing the SAS software package (SAS Inst., 1999). PROC MIXED (SAS Inst., 1999) was used to analyze lint yield, gin turnout, and fiber quality data combined over years. Chi-square tests, based on -2 log likelihood values, were used to choose a model to describe the repeated measure covariance with years. Linear and quadratic trends were used to describe treatment effects. Lack of fit, defined as failure of the trend to fit treatment means, was considered an additional component of error. Relationships between selected plant characteristics were examined using correlation analyses (SAS Inst., 1999).

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
Weather conditions in the San Joaquin Valley were generally considered to be good for cotton growth and yield during 1999 and 2000 (Fig. 1). Excessive rainfall and unusually cool temperatures through the first half of April delayed planting in 1998. Rainfall in the first 5 mo of 1998 was over 2.5 times that of the 30-yr average. Following a few warm days after planting at the end of April 1998, temperatures dropped below normal again for the entire month of May and almost half of June. Delayed planting in combination with cooler temperatures during early cotton development made 1998 a trying year for cotton growers and contributed to variation among years. This variation resulted in highly significant year effects for almost all parameters examined in this study. This in turn likely caused numerous significant interaction effects observed in this study. In particular, interactions involving samplings and year were not surprising when the climatic differences among years were considered. Therefore, many of these interaction effects are not discussed in detail below. However, climatic conditions generally did not influence the type (positive or negative) but did influence the extent of a response to the treatments.

View larger version (56K): [in this window] [in a new window]   Fig. 1. Weather data for 1998, 1999, 2000, and the long-term average (1961–1990), including minimum and maximum daily temperature, daily rainfall, and cumulative degree days. Arrows indicate planting and harvest. Plus signs indicate samplings conducted throughout the growing season.

For both locations and all years, differences in dry matter production between the N treatments became more apparent as the seasons progressed. At the first sampling, conducted at early square, differences in total dry matter accumulation between treatments were not significant. Nonetheless, a very slight trend to greater dry matter production due to increased N application was already evident at this early stage (Tables 3 and 4). By the second sampling, approximately 30 d later, differences between treatments were common and averaged >1500 kg ha^-1 between N-56 and N-168. By the end of the season, the N-168 treatment had produced 60% more dry matter than the N-56 treatment on the Panoche clay loam and 40% more on the Wasco sandy loam (3-yr averages).

Ten percent or less of the total dry matter was produced in the roughly 70 d from planting to first sampling. In the next 8 wk, between early square and peak bloom, about two-thirds of the total seasonal dry matter was produced. The production of leaf dry matter was particularly rapid between early square and early bloom. Averaged across locations, years, and treatments, leaf dry matter increased from 29% of its maximum amount at early square to 86% by early bloom. Maximum leaf dry matter was usually observed at peak bloom. Leaf dry matter just before defoliation was reduced presumably due to leaf senescence and drop. The strong effect of N on total dry matter production was the result of increases in both vegetative and reproductive fractions in response to increased N application. The increases in dry matter production were significant in every year, but the extent of additional dry matter produced with increasing N fertility level varied from year to year.

Average dry matter partitioning at the final sampling was similar to that reported by Bassett et al. (1970). However, interpretation of dry matter partitioning results was complicated by numerous significant interaction effects (Table 2). In light of the differences in weather among years and the influence of the climatic conditions on cotton development, such interactions were not unexpected. Location main effects were significant for relative fruit and stem dry matter but not for relative leaf dry matter. Relative fruit dry matter, averaged over years and N treatments, was lower at the Panoche location than at the Wasco location, averaging 21.0, 46.6, and 59.0% and 24.0, 50.9, and 61.5% for the respective locations at Samplings 2, 3, and 4. Correspondingly, relative stem dry matter was greater at Panoche site than at the Wasco site because no significant location main effect on relative leaf dry matter was observed. The N rate x location interaction observed for relative leaf dry matter (Table 2) was the result of a marginally significant N effect on the Panoche clay loam. However, separate analysis for each year revealed that this effect was inconsistent across years. Differences in partitioning between two soil types, a silt loam and a fine sandy loam, have previously been reported by Mullins and Burmester (1990). They found greater relative fruit dry matter on the fine sandy loam than on the silt loam. In this study, additional N increased relative stem dry matter and reduced relative fruit dry matter at both locations. Just before defoliation, the 3-yr means for relative fruit dry matter on the Panoche clay loam were 59.4% (N-56) and 56.9% (N-168). Those for relative stem dry matter were 24.7 and 27.8%, respectively. The 3-yr means on the Wasco sandy loam were 63.0% (N-56) and 59.4% (N-168) for relative fruit dry matter and 20.9% (N-56) and 24.7% (N-168) for relative stem dry matter.

Pima
Additional N substantially increased total dry matter production by Pima cotton (Tables 5 and 6). Accumulated dry matter was not different between the control treatment and the N-56 treatment, but differences were found between the N-168 treatment and the N-56 and control treatments. As for Acala, this was not unexpected due to the residual NO^-₃–N levels in the control. In 2000, a linear relationship between final dry matter accumulation and N treatment was observed. Differences in dry matter production between high- and low-N treatments became more apparent as the seasons progressed (Table 6). By the final sampling, dry matter produced in the N-168 treatments was 33% (1999) and 54% (2000) greater than in the N-56 treatments. Total dry matter production was clearly affected by year. Final dry matter averaged across the control, N-56, and N-168 treatments was 29% greater in 1999 than in 2000.

Addition of N increased leaf, stem, and fruit dry matter production (Tables 5 and 6). However, the extent of the response was not the same in both years. Maximal differences between treatments N-56 and N-168 were greater in 2000 than in 1999. As indicated by significant F ratios for relative leaf, stem, and fruit dry matter, N treatment affected partitioning between the plant fractions (Table 5). Averaged across the 2 yr, relative leaf dry matter was marginally greater for the N-168 than the N-56 treatment at early bloom and peak bloom but not different at early square and at defoliation. While relative stem dry matter tended to increase with increasing N, trends in relative fruit dry matter were reversed. Although these relationships were weak, collectively they indicate a trend toward greater partitioning into vegetative structures associated with reduced partitioning into reproductive structures with increasing N application. Previously, Tewolde and Fernandez (1997) reported that Pima dry weight accumulation in reproductive parts was the least affected by N deficiency. In addition, they observed that N deficiency suppressed dry matter accumulation of leaves more than that of stems. However, Boquet and Breitenbeck (2000) reported little difference in G. hirsutum (non-Acala Upland) vegetative dry matter partitioning at the end of effective bloom in response to N and that, at maturity, reproductive structures accounted for 49, 48, and 43% of the plant dry matter produced with N application rates of 0, 84, and 168 kg ha^-1 respectively.

Plant Height, Nodes, and Leaf Area Index
Acala
Production of cotton, a perennial with indeterminate growth habit, as an annual presents considerable management challenges. The number of nodes and the rate of their appearance as well as plant height are simple indicators that allow assessment of cotton development for management purposes (Bourland et al., 1992, Kerby and Hake, 1996). The location main effect for plant height was not significant (Table 2). Yet, year and sampling modulated plant height responses at the two locations. In contrast to plant height, the response in node number was significantly affected by the location main effect; however, interaction effects of year x location and sampling x location were also found. With final counts of 20.9 vs. 19.6 (3-yr means), the number of nodes was slightly greater at the Panoche site than at the Wasco site. Significant differences as a result of N treatment were evident for both plant height and node count (Tables 2, 7, and 8). At the last sampling, plants were 29.8 and 35.1% taller in the N-168 than the N-56 treatments on the Wasco sandy loam and the Panoche clay loam, respectively. The same two treatments compared for final node count differed by 10.2% at the Panoche site and 5.6% at the Wasco site. Averages across N treatments, years, and locations show that by the second sampling (early bloom), plants had attained >91% of their final height and >86% of their final node count. This is consistent with data by Kerby et al. (1990a), who reported that final plant height was reached by early August.

The importance of early-season leaf area for fruit initiation and development has been documented by many researchers (Jackson and Gerik, 1990; Kennedy and Hutchinson, 2001). Kerby et al. (1990b) found that maximum dry matter gains per day can be achieved at an LAI of 3.0 early in the season but that an LAI near 3.9 was necessary for maximum total dry matter production, possibly due to senescence of lower leaves as the season progresses. Correlation coefficients calculated for the Panoche location confirm the importance of early leaf area development for seasonal dry matter production and lint yield (Table 9). At the Wasco site, relationships calculated across both 1998 and 1999 were not as strong but improved when calculated individually by year. Greater variability at the Wasco location within and between years resulted in weaker relationships than at the Panoche location. It is possible that LAIs above those observed would not have been associated with greater yields, just as reported by Kerby et al. (1990b). However, regression analyses of yield vs. LAI revealed a linear trend in the absence of a quadratic response, possibly indicating that lint yields could have been increased if the plants were managed for greater LAI.

View this table: [in this window] [in a new window]   Table 9. Pearson correlation coefficients for the relationships of leaf area characteristics and total dry matter and lint yield produced by Acala and Pima cotton.

Lint Yield
Acala
Increased N fertility raised lint yields significantly (P < 0.001). However, as indicated by a significant location x N rate interaction (P < 0.001), the effect was not the same at the two locations. Highly significant year (P < 0.001), year x location (P < 0.001), year x N rate (P < 0.001), and year x location x N rate (P < 0.001) effects were found, indicating that the growing season had a strong influence on lint yield, modulating it differently and to varying degrees at the two locations and among N treatments.

Lint yield differences were highly significant between the Panoche clay loam and the Wasco sandy loam in 1998 and 2000 but not in 1999. Averaged across years and treatments, lint yield was 1454 kg ha^-1 for the Panoche and 1548 kg ha^-1 for Wasco location. In 1998, average lint yield across all treatments was 1465 kg ha^-1 on the Panoche clay loam compared with 1332 kg ha^-1 on the Wasco sandy loam (Fig. 2). In 2000, the situation was reversed: with 1647 vs. 1268 kg ha^-1, the average lint yield was higher at the Wasco than at the Panoche location. Separate analyses for the two locations revealed that, on the Panoche clay loam, lint yield increased linearly with N application, with the highest yields attained in the 224 kg N ha^-1 treatment in all 3 yr (Fig. 2). However, the amount of lint produced per amount of N varied among years. Differences between the lowest and the highest lint yields were 322, 1136, and 880 kg ha^-1 lint in 1998, 1999, and 2000, respectively. Although a linear response to N was observed in 1999 and 2000 at the Wasco location, the trend was not as strong as on the Panoche clay loam. Yield decreases sometimes found as a result of N application above an optimum level (Howard et al., 2001; Kohli and Morrill, 1976) were not observed here. On the Wasco sandy loam, differences between lowest and highest lint yields were 196 kg ha^-1 in 1998, 186 in 1999, and 374 in 2000. This corresponds to a 3-yr mean yield increase of 18% at the Wasco site in contrast to 80% at the Panoche site from the lowest- to the highest-yielding treatments.

View larger version (23K): [in this window] [in a new window]   Fig. 2. Effect of N management on lint yield of Acala grown on (A) Panoche clay loam and (B) Wasco sandy loam and (C) Pima grown on Panoche clay loam. Trends or slopes followed by the same letter are not significantly different from each other (P < 0.05). Vertical bars represent standard errors.

The comparatively low lint yields and small response to N in 1998 were not unexpected because the cool, wet spring delayed planting and early cotton development (Fig. 1). While average yields on the Wasco sandy loam were very similar in 1999 and 2000, a drop in yield was observed on the Panoche clay loam from 1999 to 2000. A number of factors could have contributed to the reduced lint yields in 2000 at the Panoche site. Because the fields were not deep ripped between consecutive years of cotton, lower yields in 2000 were possibly due to reduced water infiltration rates, resulting in greater water stress in the later parts of the season. The absence of deep ripping was likely to have a greater impact on the clay loam than on the sandy loam.

Application of optimal N rates has been reported to benefit cotton yield by producing larger bolls at a greater number of fruiting sites (Boquet et al., 1994). In fact, increased number of bolls as a result of increased N is commonly observed (Gerik et al., 1994; Moore, 1999). Boll counts conducted in this study suggest that, at the Panoche site, greater lint yields produced at elevated levels of N may have been due mostly to a greater number of harvestable bolls per plant (data not shown; Pearson correlation coefficients for lint yield vs. boll number per plant: 1998, 0.76; 1999, 0.87; 2000, 0.76). Analyses of final plant mapping data for Acala show that the number of bolls per plant was significantly greater at N-168 than at N-56 at the Panoche site. Although not significant, a similar trend was observed at the Wasco location in the first 2 yr but not in the third year (Pearson correlation coefficients for lint yield vs. boll number per plant: 1998, 0.34; 1999, 0.39; 2000, -0.30).

Pima
Nitrogen treatment had a significant quadratic effect on Pima lint yield in both 1999 and 2000 (Fig. 2). Pima lint yield of the N-56 treatment was 35 and 27% less than the average of the other three treatments in 1999 and 2000, respectively. The number of bolls per Pima plant was greater in the N-168 than the N-56 treatment (data not shown; Pearson correlation coefficient for lint yield vs. boll number per plant: 1999, 0.54; 2000, 0.72). Although Pima lint yield was affected by year, the trend in response to N was the same in both years. Our results are comparable with those reported by Tewolde et al. (1994). They found that N application caused a significant lint yield response in 2 out of 3 yr for Pima grown in Texas with only small differences among application rates of 67, 135, 202, and 269 kg N ha^-1. In fact, they reported a tendency to reduced lint yield in response to applied N beyond 67 or 135 kg N ha^-1 and suggested maturity-delaying effects of high N rates as possible reason. We observed a slight, but nonsignificant decrease in lint yield from the N-168 to the N-224 treatment. However, the quadratic response of Pima lint yield to N may indicate its greater sensitivity to excess N application compared with Acala cotton (Silvertooth et al., 1995) for which we commonly observed linear trends in this study (except on Wasco sandy loam in 1998).

Gin Turnout
Panoche and Wasco sites differed significantly in gin turnout in every year. Average Acala gin turnout was 36.9% on the Panoche clay loam and 34.9% on the Wasco sandy loam (Fig. 3). At the Panoche site, a highly significant decrease of Acala gin turnout with increasing N was observed. Gin turnout decreased linearly with increasing N application in each of the 3 yr. At the Wasco location, a tendency toward lower gin turnout with increasing N was observed for 1999 and 2000. While the influence of year on gin turnout was strong at both locations, the slopes describing the change in gin turnout with increasing N were not different among years at either location.

View larger version (39K): [in this window] [in a new window]   Fig. 3. Effect of N management on gin turnout and fiber strength for Acala grown on (A and D) Panoche clay loam and (B and E) Wasco sandy loam and (C and F) Pima grown on Panoche clay loam. Slopes followed by the same letter are not significantly different from each other (P < 0.05). Vertical bars represent standard errors.

Fiber Quality
Acala
According to repeated-measures analysis, fiber strength was not affected by location, but year (P < 0.001) and year x location (P < 0.05) effects were significant. Fibers were strongest in 1998 and weakest in 1999, with averages differing by >10% between these 2 yr. A positive linear relationship between fiber strength and N fertility level was observed for the Panoche location in 2000 (Fig. 3), with slopes for 1998 and 1999 suggesting a similar trend. However, at the Wasco site, N fertility level did not influence fiber strength (Fig. 3). Similarly, other researchers did not find a relationship between fiber strength and N treatment (Boman and Westerman, 1994; Sawan et al., 1997).

Significant location, year, treatment x location, and year x location effects on micronaire were found for Acala cotton. At the Panoche site, effects of N on micronaire were inconsistent across years (Fig. 4). In 1998, micronaire tended to decrease with increasing N application, but in 1999 and 2000, micronaire increased with increasing N application, with F tests being significant (P < 0.01) for 1999 and 2000 but not for 1998. At the Wasco site, micronaire values tended to decrease with increasing N application in all 3 yr, resulting in a significant effect when analyzed over all 3 yr (P < 0.01). The relationships between micronaire and N treatment were best described by quadratic functions at the Panoche site and linear functions at the Wasco location. Three-year mean micronaire readings were lower on the Panoche clay loam than on the Wasco sandy loam. Nitrogen management effects on micronaire are frequently inconsistent. For instance, increased N application rates were reported to have no effect at all on micronaire or to increase or decrease micronaire readings (Boman and Westerman, 1994; Boman et al., 1997; Sawan et al., 1997). Based on 11 yr of data, Boman et al. (1997) reported that micronaire readings were reduced by applied N in low-micronaire environments and increased by applied N in high-micronaire environments. Because micronaire readings were always within the base range (no discounts between 3.5 and 4.9), it is doubtful if the differences found in this study are of practical importance.

View larger version (38K): [in this window] [in a new window]   Fig. 4. Effect of N management on micronaire and fiber length for Acala grown on (A and D) Panoche clay loam and (B and E) Wasco sandy loam and (C and F) Pima grown on Panoche clay loam. Trends or slopes followed by the same letter are not significantly different from each other (P < 0.05). Vertical bars represent standard errors.

Pima
Except for a marginally significant (P < 0.05) N effect on micronaire revealed by repeated measures analysis, Pima fiber quality was not affected by treatment or growing season (Fig. 3 and 4). The low micronaire values observed in the N-224 treatment (Fig. 3) were possibly the result of maturity delaying effects of the highest N rate since incomplete development of bolls, i.e., immature lint, results in low micronaire readings (Hake et al., 1996).

	SUMMARY AND CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
Weather conditions varied considerably among years. Unusually high rainfall and cool temperatures delayed planting and cotton development in 1998 while 1999 and 2000 growing seasons were closer to normal (30-yr average). The different weather conditions between growing seasons resulted in highly significant year effects for most characteristics examined in this study. However, the response to N was similar from year to year in its direction while the extent of the response was modified.

Aside from the obvious effect of sampling time, analyzed growth characteristics were affected predominantly by N treatment and season. Significant location main effects were only found for fruit dry matter production, number of nodes, and for partitioning into stem and fruit dry matter. Acala and Pima dry matter production increased in response to N. Increases in leaf, stem, and fruit dry matter combined for greater total dry matter. In addition, partitioning was shifted slightly toward greater vegetative production as N increased. Dry matter production and lint yields were related to early-season leaf area development.

On the Panoche clay loam in Fresno County, Acala yields increased linearly with increasing N in every year. On the Wasco sandy loam in Kings County, lint yield also increased with elevated N fertility level, but the response per unit N was not as great as at the Panoche location. Pima lint yields responded quadratically to N. The greatest lint yields were observed in the N-168 treatment in both 1999 and 2000. The quadratic response of Pima lint yield to N may indicate a higher sensitivity to excess N application compared with Acala cotton.

Acala gin turnout differed between the two locations. Higher turnouts were recorded on the Panoche clay loam than the Wasco sandy loam. While significant linear decreases in gin turnout were observed with increasing N at the Panoche site, only a slight trend to reduced gin turnout was observed at the Wasco location as N fertility increased. Pima gin turnout also decreased with increasing N.

Acala micronaire and length differed in response to N at the two locations. Except for significant differences between years, effects on fiber strength were minimal. A slight differential seasonal effect between the two locations and, on the Panoche clay loam, a tendency toward increased strength with additional N was observed. Pima fiber quality was not affected by N treatment.

Although the experimental setup did not allow for direct comparisons between Acala and Pima, the different pattern in response to N treatment suggest that N fertilization of Pima should not be simply based on experience gained with Acala. Variation in N dynamics such as mineralization–immobilization and leaching is inherent among soil types and is influenced by management practices. The responses of lint yields to N application suggest that the rates currently recommended in California cotton production systems should be re-evaluated. Both Pima on the Panoche clay loam and Acala on the Wasco sandy loam showed relatively small increases in yield above 168 kg N ha^-1, yet rates as high as 212 kg N ha^-1 are not uncommon. To attain an accurate assessment of N needs, growers should have a thorough knowledge of available soil N, cropping history, and specific climatic conditions for their location. The resulting improved management should have a significant impact on reducing excess N in the San Joaquin Valley environment.

	ACKNOWLEDGMENTS

 
The authors thank Wayne and Doug Wisecarver and the staff of the West Side Research and Extension Center and the University of California Cooperative Extension at the Kings County office for their cooperation.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
This research was supported in part by grants from Cotton Incorporated, the California Department of Food and Agriculture Fertilizer Research and Education Program, and the California Crop Improvement Association.

	REFERENCES

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