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Published in Agron. J. 96:1443-1452 (2004).
© American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA

Tillage and Cropping Systems

Long-Term Tillage, Cover Crop, and Nitrogen Rate Effects on Cotton

Plant Growth and Yield Components

Donald J. Boqueta,*, Robert L. Hutchinsona and Gary A. Breitenbeckb

a Louisiana State Univ. Agric. Cent., 212 Macon Ridge Rd., Winnsboro, LA 71295
b Louisiana State Univ. Agric. Cent., Agron. Dep., 104 M.B. Sturgis Hall, Baton Rouge, LA 70803

* Corresponding author (dboquet{at}agcenter.lsu.edu)

Received for publication October 27, 2003.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Cotton (Gossypium hirsutum L.) yield is influenced by tillage, cover crop, and N fertility, but the plant growth and yield component responses related to these yield responses have not been well defined. A field study was conducted from 1991 through 2001 on Gigger silt loam (fine-silty, mixed, thermic Typic Fragidaulf) to determine the long-term effects of tillage practices, cover crops, and N fertilization rates on cotton growth and yield components. Cotton was grown continuously without tillage (no-till) or with surface tillage (surface till) following annual winter cover crops of wheat (Triticum aestivum L.), hairy vetch (Vicia villosa Roth), and volunteer winter vegetation in plots receiving fertilizer N rates of 0, 39, 78, 118, or 159 kg ha–1. Tillage practice, cover crop, and N rate significantly affected cotton plant height, main-stem node number, number of nodes above white flower (NAWF), main-stem internode length, lint fraction, percentage first harvest, individual boll weight, and boll number per square meter. Increases in lint yields were associated with increases in plant height (r = 0.73 to 0.95), node number (r = 0.71 to 0.83), internode length (r = 0.44 to 0.91), NAWF (r = 0.65 to 0.90), boll weight (r = 0.12 to 0.86), and boll number per square meter (r = 0.91 to 0.93). Lint fraction showed no association or, in some years, a negative association with lint yield (r = –0.12 to –0.70). No-till management and optimal N rate improved the environment for plant growth, which enhanced several growth parameters and yield components that were associated with increases in yield.

Abbreviations: DAP, days after planting • NAWF, number of main stem nodes above the uppermost white flower at Fruiting Position 1


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
A NUMBER OF STUDIES have assessed the impact of tillage and cover crops on the growth of cotton crops and their yield components influencing lint yield. Most of these studies have focused primarily on the effects of these conservation practices on seedling emergence and stand establishment. Some studies found that both no-till and winter cover crops can reduce cotton stands, but no consistent pattern emerges from all reported studies, and there is little evidence that the modest stand reductions commonly observed adversely affect lint yields. For example, Brown et al. (1985) reported that cotton seedling mortality in the southeastern USA was higher in no-till than in disk-tilled systems in only 1 of 3 yr and seedling mortality did not significantly influence yields; whereas Nyakatawa and Reddy (2000) and Nyakatawa et al. (2000) found that both no-till and a winter rye (Secale cereale L.) cover crop improved seedling emergence in 2 yr and lint yield in 1 yr of a 2-yr study. In those studies, soil moisture during emergence was considered an important factor affecting plant growth and related yield parameters, suggesting that no-till and cover crops are useful for soil moisture conservation in the southeastern USA.

Undoubtedly, some observed differences of the impact of cover crops and no-till on seedling emergence and survival are related to differences in local climatic and soil conditions near time of planting and throughout the growing season. Pettigrew and Jones (2001) found that seedling emergence of no-till cotton planted into wheat residue delayed germination and reduced stands when compared with cotton planted after incorporation of wheat residue by tillage on a Mississippi Delta alluvial soil of the Midsouth, USA. A field study conducted on a sandy loam soil (Aridic Paleustolls) in West Texas, USA (Wheeler et al., 1997), showed inconsistent effects of tillage and cover crops during the 4-yr study. In 2 yr, seedling survival was higher with conventional till and no cover crop system than with no-till and a wheat cover crop. In the other years, seedling survival was similar in the two systems. They concluded that seed quality was more important than tillage practice in seedling establishment.

The factors responsible for occasional reductions in cotton stands caused by no-till or cover crops are not well understood. Hicks et al. (1989) found that when wheat straw remained on the soil surface (no-till), seedling emergence was not affected, but incorporation of wheat residues by tillage led to allelopathic effects that reduced cotton emergence by 9% (resistant cultivars) to 21% (susceptible cultivar) in West Texas, USA. Allelopathic effects may occur with other winter cover crops as well. Stevens et al. (1992) noted that no-till cotton stands were lower following a hairy vetch cover crop but not following a wheat cover crop. The possibility that surface debris left by no-till harbors pests and thereby increases seedling disease in cotton has also been investigated. Coyler and Vernon (1993) found that although no-till consistently reduced plant stands in the Red River Valley, USA, disease indices were higher in no-till in only 1 of 3 yr. They concluded that factors other than disease affected seedling emergence in no-till and that the small stand reductions observed did not significantly influence seed cotton yield.

Field studies have reported both significant increases and decreases in cotton yields due to conservation tillage and cover crops (Bauer and Busscher, 1996; Boquet et al., 1994; Dabney et al., 2001; Keeling et al., 1989; Scott et al., 1990). To better understand the effects of no-till, Triplett et al. (1996) examined yield components as well as seed cotton yields. They found that no-till cotton produced more nodes, fruiting sites, and bolls and matured earlier than conventional-till cotton. These findings were not supported by Pettigrew and Jones (2001), however, who reported that no-till delayed both vegetative and reproductive development of cotton and resulted in fewer bolls and delayed maturity. The effects of no-till management and type of cover crops on cotton response to applied N are also inconsistent among reported field studies (Boquet and Coco, 1993; Bronson et al., 2001; Varco et al., 1999). The specific causes for these variable responses of cotton to no-till remain largely unidentified, but it is likely that changes in soil conditions and refinement of equipment and management techniques contribute to variability when fields are initially converted from conventional to no-till.

In the study reported here, the effects of tillage regime, winter cover crops, and a range of N fertilization rates on selected yield components and growth characteristics of cotton were determined in Years 5 through 11 of a long-term field study. Relationships between measured plant parameters and yield responses were used to interpret the effects of tillage, cover crops, and fertilizer rates.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
A field study was conducted from 1991 through 2001 at the Louisiana State University AgCenter Macon Ridge Research Station near Winnsboro on Gigger silt loam (fine-silty, mixed, thermic Typic Fragidaulf). In this paper, we report the results from 1995 through 2001, Years 5 through 11 of the experiment. Two tillage systems, three cover crops, and five N rates were evaluated for cotton production. The tillage systems were no-tillage (no-till) and surface tillage (surface till). The cover crops were winter wheat, hairy vetch, and volunteer winter (native) vegetation. Each of the tillage x cover crop treatments was evaluated with fertilizer N rates of 0, 39, 78, 118, and 159 kg ha–1.

Field Methods
Cover Crops
Wheat and hairy vetch cover crops were drill-seeded at 78 and 22 kg ha–1, respectively, late October of each year after cotton harvest. Following cover crop planting, cotton stalks were shredded with a rotary mower. No fertilizer N was applied to the cover crops.

Seedbed Preparation
The surface-till treatments were disked twice in early April each year and, in late April, were formed into 1-m-wide beds using disk hippers. A reel-and-harrow bed conditioner was used for final seedbed preparation. The wheat and native vegetation no-till treatments received an application of glyphosate, N-(phosphonomethyl) glycine in the form of its isopropylamine salt, 1.12 kg ha–1, in early April. The hairy vetch managed with no-till received two applications of paraquat dichloride (1,1'-dimethyl-4, 4'-bipyridinium dichloride), 0.56 kg ha–1, one in early April and one in late April.

Cotton Planting
Cotton was planted in early May each year at a seeding rate of 16 seed m–1 row using a planter equipped with bubble coulters and double-disk openers. The cotton cultivars changed during the course of the study as seed companies discontinued old cultivars and new cultivars were developed and released. ‘Stoneville LA887’ was planted in 1995, ‘Stoneville 474’ was planted from 1996 through 1999, and ‘Stoneville 4892BR’ was planted in 2000 and 2001. Fertilizer N treatments [32% urea ammonium nitrate (UAN) solution] were injected at crop emergence 8 cm deep and 25 cm from each row center.

Weed Control
A tank mixture of fluometuron [1,1-dimethyl-3-({alpha},{alpha},{alpha},-triflouro-m-tolyl) urea], 1.35 kg ha–1, and pendimethalin [N-(-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine], 1.12 kg ha–1, was broadcast for pre-emergence weed control. Early-season weed control consisted of directed applications of fluometuron, 0.68 kg ha–1, plus monosodium acid methanearsonate (MSMA), 1.12 kg ha–1. Midseason weed control consisted of application of either prometryn [2, 4-bis (isopropylamino)-(methylthio)-s-triazine], 0.34 kg ha–1, plus MSMA, 1.12 kg ha–1, or cyanazine {2[[4-cholor-6-(ethylamino)-s-triazin-2-yl]-amino]-2-methylpropionitrile}. Beginning in 2000, glyphosate-resistant cotton varieties were planted, and early-season weed control was accomplished with a single application of glyphosate, 1.12 kg ha–1. No cultivation was used for weed control in either of the tillage regimes.

Irrigation
Water was applied with a linear-move sprinkler system whenever the accrued water deficit was about 38 mm based on rainfall, irrigation, and daily pan evaporation. Twenty millimeters of water was applied at each irrigation. The average amount of water applied each year was 140 mm.

Harvest
Plots were defoliated with a tank mix of chemical defoliates (S,S,S-tributyl phosphorotrithioate), 0.84 kg ha–1, and thidiazuron (N-phenyl-N'-1,2,3-thiadiazol-5-ylurea), 0.12 kg ha–1, in mid-September each year. The plots were harvested with a spindle picker in late September or early October.

Data Collection
Cotton
Seedling emergence and survival were determined by counting emerged plants on two center rows of each plot about 30 d after planting (DAP). Plant height, the number of main-stem nodes, and the NAWF were determined by taking measurements on 10 random plants per plot. Average internode length was calculated by dividing plant height by node number. The NAWF was determined during flowering at 6- to 8-d intervals by counting the number of nodes on the main stem above the sympodial branch with a white flower at the first fruiting position (Bourland et al., 1992). To determine seed cotton and lint yield, four rows of the eight-row plots were harvested two times at a 2-wk interval. (Cotton yields from this study were reported in an earlier paper by Boquet et al., 2004.) Lint yield from the first picking divided by total harvested lint in two pickings was the percentage first pick. A representative sample of 50 bolls plot–1 was randomly hand-harvested from different fruiting positions in the bottom, middle, and top of plants, avoiding plants near plot ends. The 50-boll seed cotton samples were ginned on a 20-saw laboratory gin to determine lint percentage. Boll number per square meter was calculated by dividing the seed cotton yield per square meter by average boll weight for each plot.

Experiment Design and Analyses
The experiment design was a split-plot arrangement in a randomized complete block with three replications. Tillage regimes were main plots, and a factorial arrangement of cover crops and N rates were subplots. The experimental units consisted of eight 1-m-wide rows 15 m in length. Data for all years for each measured variable were analyzed by analysis of variance (SAS Inst., 1988). In the ANOVA, years were considered main plots, tillage practices were on subplots, and cover crop–N rates treatments were on sub-subplots nested within tillage regimes. The protected LSD at P = 0.05 was calculated for mean separation. Pearson correlation coefficients were calculated within years to identify associations among lint yield, yield components, and plant growth variables.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Plant Growth Parameters
Cotton Stand Establishment
The ideal plant population for cotton is about 10 plants m–2, but no significant reductions in yield are generally observed with stand densities between 3 and 15 plants m–2 (Bednarz et al., 2000). In the present study, stands differed significantly among treatments, but average stands were sufficient in all tillage–cover crop combinations, ranging from 8.4 to 11.1 plants m–2 (Table 1). Tillage, type of cover crop, and N rate each significantly influenced plant density. Cover crop x N rate and tillage x cover crop interactions were also significant.


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  		Table 1. Cotton plant density per square meter 30 d after planting in no-tillage and surface-tillage regimes following winter fallow native vegetation, hairy vetch, and winter wheat cover crops, each with five fertilizer N rates, 7-yr average from 1995 through 2001.{dagger}

 
Stands of cotton following a wheat cover crop tended to decrease as fertilizer N rate increased. Since soil water was not limiting during early growth, reduced stands were most likely caused by the greater amounts of wheat residue present in plots receiving greater amounts of N. With increase in N, the amounts of residual N scavenged by the wheat cover crop increased and resulted in substantial increases in wheat biomass (1680 kg ha–1 with 0 N; 4800 kg ha–1 with 159 kg N ha–1). The effects of wheat and vetch covers on seedling establishment were more evident in no-till than when incorporated. This finding contrasts with that Wheeler et al. (1997), who reported that stands of cotton were reduced more by incorporation of wheat straw than by no-till possibly because of allelopathic effects of the residues. It was not possible to determine whether the stand reductions reported in Table 1 were the result of residue impairment of planting equipment, allelopathic compounds, or enhanced root disease or pests.

Even so, the effects of wheat and vetch cover crops on stand establishment were small, and all stands were within the acceptable range for optimal yields. Correlation analyses confirmed that no consistent relationship existed between lint yields and the narrow range of plant densities in this study. These observations support the general conclusion of others (Brown et al., 1985; Stevens et al., 1992; Bauer et al., 1993; Coyler and Vernon, 1993; Nyakatawa et al., 2000) that use of no-till and winter cover crops does not significantly reduce cotton yields because of reduced stands.

Plant Height
Immediately before bloom initiation (55 DAP), tillage, cover crop, and N rate each influenced plant height. Tillage x cover crop and cover crop x N rate interactions were also significant. The type of cover crop exerted the largest influence on prebloom plant height, especially with no-till systems (Fig. 1). Following native cover, cotton was generally taller in surface till than in no-till, whereas cotton following wheat was taller in no-till than surface till, except were no fertilizer N was applied. In all treatments following vetch, cotton was taller in no-till than in surface till. Interestingly, plant height was greatest where no fertilizer N had been applied following vetch. The incorporation of large quantities of wheat or vetch residue by surface till reduced early plant growth. Where the amount of residue was low, as with native vegetation, plant growth was not slowed by incorporation. These findings are consistent with those of Wheeler et al. (1997) and support the suggestion that incorporation of cover crop residues results in allelopathic effects not evident in no-till. These findings, however, do not agree with those of Pettigrew and Jones (2001), who found that the growth rate of cotton in no-till wheat was slower than in tilled wheat plots. It should be noted, however, that cotton in the present study was irrigated, whereas that in the study of Pettigrew and Jones (2001) was rainfed, and this may have contributed to differences in results. Moreover, differences in response to tillage and cover crops across soil types undoubtedly contribute to inconsistencies among locations.



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  		Fig. 1. Response of cotton plant height at bloom initiation (55 d after planting) to tillage, winter cover crop, and fertilizer N rate.

 
Prebloom plant height of cotton following native and wheat cover crops was maximized with the application of 118 kg N ha–1 in no-till and 78 kg N ha–1 in surface-till treatments. Plant growth between emergence and blooming was greatest following a vetch cover crop with no applied N or following wheat with 118 kg N ha–1. Highly significant correlations in all years between plant height at bloom initiation and lint yield (r = 0.82 to 0.95, p < 0.0001) indicated that early-season plant growth was a reliable indicator of eventual yields. They also suggest that vigorous early growth of cotton in conservation systems may contribute to higher cotton yields.

At the beginning of boll opening (110 DAP), the influences of cover crop, tillage system, and N fertilization on plant height were more distinct than at bloom initiation (Fig. 2). Growth responses following native and wheat cover crops were similar to those observed at prebloom though the impact of N rate on growth was much greater at 110 DAP. At lower N rates, plant heights at boll opening were consistently greater following a vetch cover crop than following wheat or native cover. While much of the benefit of a vetch cover on plant growth was due to the contribution of legume N, the slightly lower plant densities obtained following vetch may have contributed to increased plant height by reducing competition. Plant heights at boll opening were highly correlated in all years with lint yields (r = 0.73 to 0.89, p < 0.0001). This is in marked contrast to the findings of Pettigrew and Jones (2001), who found no association between plant height and cotton yield.



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  		Fig. 2. Response of cotton plant height at boll opening (110 d after planting) to tillage, winter cover crop, and fertilizer N rate.

 
Main-Stem Node Number
Node number is important to yield because nodes produce the fruiting branches, each of which can support from one to three bolls. The type of cover crop and N fertilization rate significantly influenced the number of nodes developed by prebloom (55 DAP), whereas tillage practices did not have measurable effects (Fig. 3). Cover crop x N and tillage x cover crop interactions were significant. Consistent with plant height, maximum node number at prebloom was obtained without supplemental fertilizer following vetch. Following native or wheat cover crops, maximum node number was obtained by applying fertilizer N at 39 kg ha–1 with surface till and 78 kg ha–1 with no-till (Fig. 3). In most respects, prebloom responses in node number paralleled plant height responses as expected because of the interdependence of these two plant characteristics.



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  		Fig. 3. Response of cotton main-stem node number at bloom initiation (55 d after planting) to tillage, winter cover crop, and fertilizer N rate.

 
At boll opening (110 DAP), node numbers were markedly affected by tillage, cover crop, and especially, N rate. The tillage x cover crop, tillage x N rate, and cover crop x N rate interactions were all significant at boll opening. Cotton in no-till produced more nodes than cotton in tilled treatments except for cotton following a native cover crop and receiving deficient levels of fertilizer N (Fig. 4). Under all N rates in both tillage regimes, cotton following vetch produced more nodes than cotton following native cover. Cotton following vetch also produced more nodes than cotton following wheat, except where inhibitory amounts of N fertilizer were applied. Because of the relationship between nodes and fruiting branches, it is not surprising that strong relationships were evident in all years between lint yields and the numbers of nodes developed by bloom initiation (r = 0.71 to 0.82, p < 0.0001) and by boll opening (0.77 to 0.83, p < 0.0001).



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  		Fig. 4. Response of cotton main-stem node number at boll opening (110 d after planting) to tillage, winter cover crop, and fertilizer N rate.

 
Main-Stem Internode Length
Main-stem internode length reflects the rate of plant growth in response to ambient conditions. Combined with plant height and number of nodes, internode length can be used to gauge the effects of management on crop development. At prebloom, average internode length was significantly (p < 0.05) affected by cover crop, N rate, tillage x cover crop, and cover crop x N rate interactions. Internode length in cotton following native cover was 5% shorter in no-till than in tilled systems (Fig. 5). Following vetch and wheat cover crops, however, internode length of cotton plants was 5 to 8% longer in no-till than in surface till. Unfertilized cotton following wheat was an exception in that internodes were 10% shorter in no-till. Overall, no-till systems with planted cover crops provided an environment for plant growth that was superior to no-till with native vegetation and to all surface-till systems.



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  		Fig. 5. Response of cotton main-stem length of internodes developed before bloom initiation (55 d after planting) to tillage, winter cover crop, and fertilizer N rate.

 
Application of fertilizer N had a greater effect on prebloom internode length in no-till native and wheat systems (increases of 21 to 24%) than in the corresponding surface-till systems (increases of 5 to 15%). Maximum internode length was attained with 118 kg N ha–1 following native and wheat cover crops. Following a vetch cover crop, application of fertilizer N did not increase internode length. Although internode length itself does not contribute to yield, there was a positive correlation between prebloom internode length and lint yield (r = 0.70 to 0.91, p < 0001), suggesting that environments that favor internode expansion also contribute to increased yield.

Postbloom effects of tillage, cover crop, and N rate on internode length were similar to the prebloom responses except that differences were more pronounced and tillage regime became a significant factor (Fig. 6). In general, internode lengths were greater at postbloom than before bloom. Differences in internode lengths observed between these developmental stages were enhanced by increased N availability. At boll opening, longer internode lengths occurred in no-till than in corresponding surface-till systems, suggesting greater N availability with no-till. Internode length in cotton following a vetch cover crop was greater than following native or wheat cover crops under both tillage regimes at 0 and 39 kg ha–1 N rates. These observations support the conclusion that quantity of N supplied by continuous use of a hairy vetch cover crop is sufficient to fully meet the demands of a cotton crop throughout the entire boll-fill period. Following native and wheat covers, maximum postbloom internode length was attained with application of 78 kg N ha–1 with both no-till and surface till except for the surface-tilled wheat cover system where internode length continued to respond to incremental increase in N. As with prebloom internode length, increased postbloom internode length was associated with increased lint yield (r = 0.44 to 0.90, p < 0.01).



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  		Fig. 6. Response of cotton main-stem length of internodes developed between bloom initiation (55 d after planting) and boll opening (110 d after planting) to tillage, winter cover crop, and fertilizer N rate.

 
Nodes above White Flower
Number of nodes above white flower, like internode length, reflects plant growth rate and is used to determine developmental progress during the boll set period (Bourland et al., 1992). Statistical analysis showed that whereas tillage, cover crop, N rate, and year were all significant, interactions between year and tillage, year and cover crop, and year and N rate were negligible. The rate of N fertilization consistently exerted the largest effect on NAWF (Fig. 7 and 8). Differences in NAWF due to tillage regime and type of cover crop appear to have been related to the effects of these practices on the availability of N supplied either by supplemental fertilizer or biologically fixed N. When no fertilizer N was applied, NAWF values were similar throughout the bloom period in cotton following native or wheat cover. When fertilizer N was applied to cotton following these cover crops, NAWF values were consistently higher in no-till than in surface till, suggesting greater fertilizer N availability under no-till management. The apparent magnification in response to applied N as the season progressed was due, at least in part, to the fact that N-deficient cotton was subject to early cutout (NAWF = 4) before completing the third week of flowering. At lower N rates (0 and 39 kg ha–1), NAWF values were greater in no-till cotton following vetch than following wheat or native cover crops. Differences between vetch and native or wheat cover crops were not as evident in surface till as in no-till, possibly because surface till caused more rapid mineralization of N in cover residues that benefited early-season growth after all cover crops (Wilson and Hargrove, 1986; Varco et al., 1999; Utomo et al., 1990). In all years, there were strong correlations between lint yields and NAWF at bloom initiation (r = 0.0.73 to 0.90, p < 0.0001) and at 21 d after bloom initiation (r = 0.65 to 0.83, p < 0.0001). These observations support the use of NAWF as a reliable guide for describing overall crop development and yield potential of cotton in conservation systems. Bourland et al. (1992) reported that decrease in NAWF was associated with decrease in canopy photosynthesis and NAWF was reduced when plants were under increased stress. Reduced yields in the present study were associated with NAWF values <7 during the week of first bloom, <6 during the second week, <5 during the third week, and <3 during the fourth week. Number of nodes above white flower remained above these critical values where adequate N was supplied as supplemental fertilizer or by use of a vetch cover crop. Previous work by Pettigrew and Jones (2001) also showed that NAWF was greater in no-till than in tilled wheat. Although, as was true in the present study, greater NAWF typically represents increased growth rate, the increase in NAWF in no-till in their study was attributed to slower rather than faster growth, which delayed flowering.



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  		Fig. 7. Response of nodes above white flower in surface tillage (ST) cotton receiving different fertilizer N rates following three winter cover crops. LSD (0.05) = 0.4.

 


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  		Fig. 8. Response of nodes above white flower in no-tillage (NT) cotton receiving different fertilizer N rates following three winter cover crops. LSD (0.05) = 0.4.

 
Cotton Yield Components
Individual Boll Weight and Boll Number
Boll weight and number of bolls were significantly influenced by tillage, cover crop, and N rate. Tillage x cover crop and cover crop x N rate interactions both were significant. Cotton boll number per square meter was maximum with application of 78 kg N ha–1 in no-till native, 39 kg N ha–1 in surface-till native, 118 kg N ha–1 in no-till and surface-till wheat, and 0 kg N ha–1 in no-till and surface-till vetch. Where no fertilizer N was applied, bolls of cotton plants following native or wheat covers were larger in tilled than in no-till regimes, whereas bolls were larger in no-till than in surface till following a vetch cover crop (Fig. 9). When optimal amounts of fertilizer N were applied to cotton following wheat or native covers, bolls were larger with no-till than with surface-till management. The relationship between boll weight and lint yield was inconsistent over the years of this study. Relationships ranged from negligible (r = 0.12, ns in 2000) to strong (r = 0.86 in 1997, p < 0.0001). This variability was not surprising since boll weight usually represents a much smaller contribution to yield than boll number (Jenkins et al., 1990). In fact, lint yields were more closely and consistently related to numbers of bolls (bolls m–2) than to average boll weights. Correlations between boll number and lint yield were highly significant each year (r = 0.93 to 0.98, p < 0.0001). Yield declines that were the result of the high rate of fertilizer N (159 kg ha–1) following vetch (but not wheat) were associated with smaller bolls in no-till regimes and with both smaller and fewer bolls in surface till (Fig. 9 and 10). Excessive N fertilization of cotton following a wheat cover did not decrease boll weight or boll number, and as a result, there was no yield decline from the excess 159 kg ha–1 N rate. These results confirm that changes in management do not change the importance of using the correct fertilizer N rate and adjusting for specific field situations whether soil type differences or, in this case, tillage and cover cropping practices. The present results for boll number agree with those of Triplett et al. (1996), who reported higher boll numbers in no-till than in conventional-till cotton. Contrary to the present findings and to those of Triplett et al. (1996), Pettigrew and Jones (2001) found higher boll number per square meter and larger boll mass for cotton planted conventional till into wheat than no-till into wheat—as a result of overall better plant growth in conventional till.



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  		Fig. 9. Response of cotton individual boll weight to tillage, winter cover crop, and fertilizer N rate.

 


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  		Fig. 10. Response of cotton boll number per square meter to tillage, winter cover crop, and fertilizer N rate.

 
Lint Percentage
Lint yields are ultimately determined not only by the number and weight of bolls, but also by the fraction of boll weight contributed by lint, i.e., the lint percentage. In this study, tillage, cover crop, and N rate all significantly influenced lint percentage. The cover crop x N rate interaction was also significant. Type of cover crop and N rate exerted greater influence on lint percentage than did tillage regime, but differences among years were greater than those due to any other variable. The differences observed in lint percentage among years appeared to be due largely to genetic differences in the cotton cultivars used during the course of the study. No significant interactions were found between average annual yields and any experiment variable. Therefore, only lint percentages averaged across years are presented in Table 2. Differences among treatments, although significant, usually amounted to <1%. In general, lint percentage tended to decrease as N availability increased. For example, cotton receiving little or no N fertilizer following a wheat cover crop displayed the highest lint percentages. It is noteworthy that lint percentage was inversely related to lint yield in 3 of 6 yr (r = –0.44 to –0.70, p < 0.01). In those years, lint percentage decreased as boll weight and plant height increased (r = –0.40 to –0.81, p < 0.01). These observations suggest that environments that produce vigorous plants and higher yields produce bolls with somewhat lower lint percentages. The lint percentage results of Pettigrew and Jones (2001) were inconsistent but demonstrated a lower lint percentage in no-till cotton that had less vigorous growth than tilled cotton in 1 of 2 yr.


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  		Table 2. Lint percentage of cotton in no-tillage and surface tillage regimes following winter fallow native vegetation, hairy vetch, and winter wheat cover crops, each with five N fertilization rates, 7-yr average from 1995 through 2001.{dagger}

 
First Harvest Percentage
The percentage of the lint harvested in the first picking is a gauge of relative differences in maturity (earliness) among treatments. Tillage had a significant but small effect on earliness, affecting first harvest by {approx}1% (Table 3). The effects of cover crop and N rate were somewhat larger, but their influence was limited to a range of 3% in the fraction of total yield harvested at first picking. As with other plant growth variables, N availability was the dominant factor influencing earliness. Low N availability caused by either low fertilizer input or by the presence of wheat residue contributed to earlier maturity. There was no clear relationship between first harvest percentage (earliness) and lint yield in the present study. Pettigrew and Jones (2001) found that no-till cotton was consistently earlier than tilled cotton by a wide margin. Stevens et al. (1992) also noted delayed maturity in no-till cotton planted into wheat residue, but no-till cotton planted into winter fallow seedbeds was similar in earliness to conventional-till plots.


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  		Table 3. First harvest percentage of cotton in no-tillage and surface-tillage regimes following winter fallow native vegetation, hairy vetch, and winter wheat cover crops, each with five fertilizer N rates, 7-yr average from 1995 through 2001.{dagger}

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
In Years 5 through 11 of this long-term irrigated study, plant growth and yield components were beneficially affected by no-till and cover crops. General improvement in plant height, main-stem node number, NAWF, individual boll weight, and boll number demonstrated that no-till and cover crop practices can provide a better environment for cotton production than conventional practices. Seedling survival, although an area of emphasis in earlier studies, was not adversely affected by the cropping practices evaluated here and did not affect cotton yield. Nitrogen availability, either through the release of biologically fixed N during the decomposition of a vetch cover crop or from application of fertilizer N, exerted a dominant influence on crop characteristics leading to increased lint yields. In general, cotton under no-till management required greater amounts of fertilizer N than cotton under surface till to produce optimal growth and yield characteristics following native or wheat cover crops. Although growth variables were usually associated with lint yield in the present study, this has not been the case in other studies. Because the present study was irrigated, water availability was not a factor limiting growth responses, and the results for tillage and cover crops were more consistent across years than in previous rainfed studies, which had inconsistent and variable results within and across locations. Other weather variables, soil types, and differences in specific treatment variables among locations also have had a major impact on the specific responses to tillage and cover crops. Furthermore, the present study has been in place for 11 yr, and other studies were 2 to 3 yr in duration. Short-term effects may not be the same or as consistent as long-term effects.


   ACKNOWLEDGMENTS
 
The authors express their appreciation to Cotton Incorporated and the Louisiana Cotton State Support Committee for providing financial support for this project. Special thanks are extended to W. James Thomas, Allen Brown, and Jay Caylor for their assistance in plot maintenance and data collection.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
This research was funded in part by Cotton Incorporated under Grant no. 93-909LA.


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




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