Agronomy Journal 93:1169-1173 (2001)
© 2001 American Society of Agronomy
WASTE MANAGEMENT
Dairy Compost, Variety, and Stand Age Effects on Kenaf Forage Yield, Nitrogen and Phosphorus Concentration, and Uptake
James P. Muir*
Texas Agric. Exp. Stn., 1229 U.S. Hwy 281, Stephenville, TX 76401-9698
* Corresponding author (j-muir{at}tamu.edu)
Received for publication October 4, 2000.
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ABSTRACT
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The dairy and beef industries are looking for alternative forages with high yields that are capable of efficient manure-P recycling when soil moisture is limiting. Three varieties of kenaf (Hibiscus cannabinus L.) were grown under dryland conditions and harvested at 60, 90, and 120 days after planting (DAP) with yearly applications of 0, 10 (56 kg P), and 20 (112 kg P) Mg DM ha-1 of dairy manure compost. Kenaf variety ‘India’ was less productive but had as high or higher N and P concentrations than either ‘Guatemala 4’ or ‘Everglades 41’ during the second year of the trial. Yields tended to increase and N or P concentrations decrease with DAP, but were affected by rainfall patterns. In 1998, when rainfall was concentrated during the first 60 DAP and the last 30 DAP, the 120 DAP harvest averaged 5.28 Mg DM ha-1 yr-1, 2.2 times the 90 DAP average. In 1999, when rainfall fell exclusively during the first 90 DAP, the 90 DAP harvest averaged 5.06 Mg DM ha-1 yr-1, 1.6 times the 120 DAP average. Plant P concentration tended to increase and N concentration to decrease with compost application the second year. The cumulative effect of compost application increased forage DM yields as well as P and N uptake during the second year. Average P removal by kenaf in the 10 and 20 Mg compost DM ha-1 plots was 10.4 and 6.8%, respectively, of the P equivalent added by the compost. This indicates that the average forage P concentration of 2.11 g kg-1 from the plots that received compost would result in insufficient forage P uptake to avoid excessive soil P buildup with sustained yearly compost application.
Abbreviations: LSD, least significant difference • DAP, days after planting
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INTRODUCTION
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ALTHOUGH kenaf has been traditionally grown as a fiber crop (Taylor and Kugler, 1992), it has also been utilized as a ruminant animal feed (Swingle et al., 1978; Wildeus et al., 1995). The selection of photosensitive varieties (short-day flowering) has extended the vegetative growth season, thereby favoring their use as forage in temperate zones (Webber, 1996). This crop can also ratoon, allowing for multiple harvests under favorable rainfall regimes (Hollowell, 1997). It has the ability to tolerate periodic drought, as does its close relative, cotton (Gossypium hirsutum L.; Webber, 1996). Its ability to recover from moisture stress in dryland conditions, however, needs further study.
Forage P uptake from soils is highly variable and is a direct function of soil P content, soil physical properties, forage biomass removed, and forage species (Pierzynski and Logan, 1993). The annual amount of kilograms P removed by a forage crop can be as low as 14.6 kg ha-1 (as in annual bluegrass, Poa annua L.) and as high as 83 kg ha-1 [as in johnsongrass, Sorghum halepense (L.) Pers.], as shown in two separate studies (Pierzynsky and Logan, 1993). When large quantities of dairy manure were applied by Sanderson and Jones (1997), up to 20% of the manure P equivalent was removed by a bermudagrass–wheat [Cynodon dactylon (L.) Pers.–Triticum aestivum (L.)] cropping system. Because there is a strong interest in zero-excess P recycling within dairies (Alocilja, 1998), utilizing high-quality forages that concentrate soil P may contribute to optimized P recycling within dairies. Composting dairy manure has gained wider usage in recent years because transport costs are reduced (Rynk, 1994).
The objectives of this trial were threefold: (i) determine forage DM yields as well as N and P concentrations and uptake of three kenaf varieties; (ii) determine the effect of dairy compost application (at a constant N rate) at three rates on kenaf production, P concentration, P uptake, and soil P levels; and (iii) determine the effect of plant maturity on kenaf DM yields, P and N forage concentration, and P and N uptake.
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MATERIALS AND METHODS
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The factors studied in this experiment consisted of 2 yr, three kenaf varieties, three dairy compost rates, and three harvest regimes. The experimental design used was a randomized complete block with a split-strip plot (harvest regimes in strips; Gomez and Gomez, 1984) arrangement of treatments in four blocks. Forage DM yields, forage P and N concentrations, forage P and N uptake, as well as residual soil P were submitted to analysis of variance. The least significant differences (LSD0.05) were determined for mean separation for interactions or, if no interactions were significant, for simple effects.
In April of 1998 and 1999, ‘Everglades 41’ (EV41; photosensitive), ‘Guatemala 4’ (G4; photo-insensitive) and ‘India’ kenaf were seeded in 9 by 12 m plots at the Texas Agricultural Research Station located in Stephenville, TX. Soil in the plots was a Windthorst fine sandy loam (fine, mixed thermic Udic Paleustalfs), pH was 7.6, and Texas A&M extract P and K were 13 and 126 mg kg-1, respectively. Seeding rate was 240000 plants ha-1 on 50-cm row spacing at 1.5- to 2.5-cm seed depth. Metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] was applied at 1.12 kg a.i. ha-1 before seeding the first year and clethodin [(E,E)-(±)-2[1[[(3-chloro-2-propenyl)oxy]imino][propyl]-5-[2(ethylthio) propyl]-3-hydroxy-2-cyclohexen-1-one] was applied at 0.15 kg a.i. ha-1 postemergence the second year to control weeds. The plots were hand-weeded each year at 30 days after planting (DAP).
Composted dairy manure from the same batch (5.6 g kg-1 P, 11.2 g kg-1 N, 42.0 g kg-1 organic matter) was incorporated in March of each year to the same 9 m by 4 m subplots at 0, 10, and 20 Mg DM ha-1. Nitrogen (NH4NO3) was added to all plots to adjust every plot up to a 150 kg N ha-1 rate. Initial soil preparation was done with a disk and compost incorporation and seedbed preparation both years consisted of rototilling to a 150-cm depth and then compacting soil with a roller, taking care not to move soil between plots. The inner 2.0 by 2.0 m of each sub-subplot was harvested at a 12-cm stubble height 60, 90, and 120 DAP or days after previous harvest until the first frost. In 1998, the 60 and 90 DAP treatments had some regrowth after the initial harvest and these were harvested at 120 and 180 DAP (at 60 and 90 d after the previous harvest), respectively, to be added to total DM yield.
This experiment was not irrigated and rainfall was not uniform between years or among months within years (Table 1). Total rainfall for the 1998 growing season, including 1 mo before seeding (March–August), was 353.1 and 254.0 mm (38% less) for the 1999 growing season. Rainfall was very low at seeding in 1998, resulting in variable initial germination. Precipitation was negligible in the last 45 d of the 1999 trial, essentially stopping plant growth for the last 30 d of the 120 DAP harvest treatment.
The entire sub-subplots were harvested with a sickle bar harvester to measure yield but subsamples, consisting of four representative plants, were dried at 55°C for 48 h to determine DM and then ground through a shear mill equipped with a 2-mm screen. Variables measured for each sub-subplot were DM yield while subsamples were analyzed for N and P concentration. To determine P and N, samples were digested using a modification of the aluminum block digestion procedure of Gallaher et al. (1975). Sample weight was 1.0 g, digested in 5 g of 33:1:1 K2SO4/CuSO4/TiO2 for 2 h at 400°C using 17 mL of H2SO4. Phosphorus and N in the digestate were determined by semiautomated colorimetry (Hambleton, 1977) using a Technicon Autoanalyzer II. Forage P and N uptake were estimated based on forage yields and mineral concentrations.
Soil in the experimental area was sampled to a 150-mm depth before the experiment. Soil pH was 6.5 and Texas A&M extractant indicated an average 13 mg P kg-1, 24 mg NO3 kg-1, 130 mg K kg-1, 855 mg Ca kg-1, and 124 mg Mg kg-1. Each sub-subplot was sampled at the end of the second season to determine treatment effects on soil P. Soil P was determined by NH4OAc–EDTA–extractable P procedure of Hons et al. (1990).
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RESULTS AND DISCUSSION
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Since monthly rainfall patterns were very different during 1998 and 1999 and since the effect of compost application was cumulative, there were significant (P < 0.05) interactions between years and all factors studied. Forage yield and nutrient concentrations or uptake are therefore discussed by year.
Forage Dry Matter Yields
1998
Due to low and sporadic rainfall early in the 1998 growing season, seedling emergence, based on visual observations, was uneven (see Table 1 for monthly rainfall totals). One heavy rain in mid-July allowed for full stand development, but regular rainfall did not resume until early September. Despite lack of growth, kenaf plants showed strong drought tolerance by exhibiting no wilting or leaf loss during these periods of suboptimal soil moisture.
Yields were not affected by compost application (P > 0.05) during the first year (Table 2). Yields of each variety indicated differing responses to harvest regimes (P = 0.01 for variety x harvest interaction). There were no differences for any variety between 60 and 90 DAP harvests, while all varieties had greater yields at the 120 DAP harvest than at 60 or 90 DAP (Fig. 1). There were also no differences among varieties at the first two harvests. At 120 DAP, however, EV41, responding to late rainfalls, out-yielded (P < 0.05) G4 by 26% and India by 27%.
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Table 2. Effect of dairy compost application rates on kenaf characteristics averaged over three harvest regimes and three varieties during 1998 and 1999.
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Fig. 1. 1998 Yields of three kenaf varieties harvested at three maturity stages averaged over manure compost applications (variety x harvest interaction, P = 0.01; LSD0.05 = 1060).
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1999
The cumulative effect of two applications of dairy compost had an effect (P = 0.01) on overall kenaf yield the second year (Table 2). Annual applications of 10 Mg compost ha-1 did not increase yields (P > 0.05), whereas 20 Mg increased yields 25.7% over the control. Several explanations for this effect are possible. Increased cation exchange capacity resulting from greater soil organic material may have been important in retaining N for late season plant use or releasing organically complexed N (Mott, 1974). Improved soil moisture retention and changes in soil bacterial populations due to increased soil organic matter have also been reported in the literature (Boehm et al., 1993; Hoitink and Fahy, 1986).
Rainfall in 1999 was adequate for early plant development but not for late season growth (Table 1). All three varieties at 90 DAP out-yielded varieties at 60 DAP (variety x harvest interaction P = 0.03; Fig. 2). In contrast to shorter moisture stress periods earlier in the 1998 season, leaf loss and self-thinning were observed during the 52 d between the last significant rainfall in July (shortly after the 90 DAP harvest) and the 120 DAP harvest in 1999. For this reason, the 120 DAP treatment had lower yields than the 90 DAP. The G4 cultivar appears to be more drought tolerant than the other two entries, because its 120 DAP yield decreased less, relative to 90 DAP, than did EV41 or India. At 90 DAP, the peak production in 1999, EV41 out-yielded (P < 0.05) G4 by 14% and India by 39%.
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Fig. 2. 1999 Yields of three kenaf varieties harvested at three maturity stages averaged over manure compost applications (variety x harvest interaction, P = 0.03; LSD0.05 = 705).
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Nitrogen Concentration and Uptake
1998
Nitrogen concentrations and yields were similar (P > 0.05) among varieties in 1998 (Table 3). Average N concentration declined (P = 0.001), however, from 23.5 g kg-1 at 60 DAP to 20.6 g kg-1 at 120 DAP (Table 4). This supports data presented by Webber (1993), who showed that the leaf component, as principal N repository in kenaf, declined as a proportion of total plant with advanced maturity. As a result of this and the greater effects of higher yields at later harvests, N recovered in plants was 90% higher (P < 0.05) at 120 DAP than at 90 DAP (Table 4). Similarly, N concentration was 14% higher (P = 0.03) in the 20 Mg compost plots than either the 0 or the 10 Mg plots, which were similar (Table 2).
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Table 3. 1998 and 1999 kenaf variety performance averaged over three harvest regimes and three manure compost application rates.
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Table 4. 1998 and 1999 kenaf harvest regime performance averaged over three varieties and three manure compost application rates.
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1999
Nitrogen concentration was affected (P = 0.006) in 1999 by an interaction among varieties, harvests, and compost (Table 5). Concentrations were consistently higher at 60 DAP than at the 90 DAP harvest for all varieties, but tended to be similar between 90 and 120 DAP for all varieties; the exception was G4 at the 0 and 20 Mg ha-1 compost rates where the 120 DAP forage had lower concentrations. This phenomenon likely resulted from the lack of plant growth due to low rainfall after the 90 DAP harvest. Differences among varieties were not consistent, although India had the highest overall average (Table 3).
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Table 5. Effect of variety, manure compost application, and harvest regime on 1999 kenaf N concentration [P = 0.006; LSD(0.05) = 0.26].
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The 1999 total plant N uptake yields likewise showed a tendency (P = 0.06) toward the same variety, harvest, and compost interaction (Table 6). The highest N uptake tended to appear in the 90 DAP harvest (Table 4); India tended to be the least productive (Table 3); while manure application had variable effects on the harvest and variety combinations.
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Table 6. Effect of variety, manure compost application, and harvest date on 1999 kenaf N uptake [P = 0.06; LSD(0.05) = 19].
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Phosphorus Concentration and Uptake
1998
In 1998, forage P concentration was not affected (P > 0.05) by an interaction among varieties, harvests, and compost. There were also no differences (P > 0.05) among varieties (Table 3). Average P concentration declined (P = 0.001) from 2.50 g kg-1 at 60 DAP to 1.73 g kg-1 at 120 DAP (Table 4). These values are within the range reported for other forages. Examples are 2.0 to 4.0 g P kg-1 of ryegrass (Lolium multiflorum Lam.) (Tunney and Pommel, 1987) and 2.2 to 3.3 g P kg-1 of various legumes (Pierzynski and Logan, 1993). Plant P concentration increased (P = 0.001) 22.7% from the 0 to the 10 Mg compost ha-1 plots and 10.9% from the 10 to the 20 Mg compost ha-1 plots (Table 2). Plant uptake of P increased (P = 0.001) 97% from the 90 to the 120 DAP harvest (Table 4). This nearly doubling uptake of P was due more to DM yield differences than to P concentration in the plant material.
1999
The cumulative effect of the second application of 10 Mg compost in 1999 increased (P = 0.01) plant P concentration an average 7.7% over plants from plots with no manure, whereas the addition of 20 Mg manure increased P concentration a further 5.6% (Table 2). Varieties had varying P concentrations depending on harvest regime (variety x harvest interaction, P = 0.001), but overall, and as plants matured, P concentration decreased (Fig. 3). The EV41 cultivar had the highest concentration of P at 60 DAP, G4 had the highest at 90 DAP, and India had the highest at 120 DAP relative to the other varieties in the respective harvests. Phosphorus concentration did not decrease in India from 90 to 120 DAP, as it did in the other varieties.
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Fig. 3. 1999 P plant concentration of three kenaf varieties at three maturity stages averaged over manure compost applications (variety x harvest interaction, P = 0.001; LSD0.05 = 0.11).
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India had 33% lower (P = 0.01) P uptake on average than the other two varieties in 1999 (Table 3). Primarily as a result of greater forage yield at 90 DAP, P uptake was greatest for this harvest and increased with greater amounts of compost for all varieties (Fig. 4). The application of 20 Mg of compost also produced the greatest P uptake at the 60 DAP harvest but not at 120 DAP.
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Fig. 4. 1999 P recovered by kenaf grown at three rates of compost application and harvested at three maturity levels (compost x harvest interaction, P = 0.05; LSD0.05 = 0.92).
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Soil Phosphorus
There were no interactions among kenaf varieties, harvest regimes, or compost application rates for soil P at the end of the trial (P > 0.05). The application of composted dairy manure, however, did increase soil P levels (P = 0.001; LSD0.05 = 7.7). The control plots had 18.5 mg kg-1 P; the application of 10 Mg DM manure ha-1 increased soil P 49% to 27.6 mg kg-1 P; and 20 Mg DM manure ha-1 increased soil P a further 28.6% to 35.5 mg kg-1 P.
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CONCLUSIONS
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Despite a 27.2% drop in season-long 1999 rainfall, average kenaf forage production that year (3.5 Mg DM ha-1) was slightly higher compared with the 1998 season. This may be a refection of the April rainfall in 1999, at seeding, that allowed greater growth early in the season. Yields overall were low relative to those reported in the literature. Yields from one tropical trial (Cuba) with higher rainfall (786 mm rainfall during a 5-mo growing season) averaged 7.13 Mg ha-1 season-1 for the best variety (Vinent and Viera, 1988). Webber (1993), reporting results from a cooler climate (Texas, USA), indicated weak effects of seasonal rainfall on yields. In that experiment, a growing season with 847 mm rainfall resulted in average yields of 5.7 Mg ha-1 while a second year, with better weed control but only 357 mm rainfall during the growing season, resulted in an average 11.6 Mg ha-1 kenaf yield.
Apparent N recovery was 37.6% on average across all years and treatments. This is a considerably higher recovery rate than the 23 to 26% reported for bermudagrass fertilized at various dairy manure rates (Sanderson and Jones, 1997). Harvests at peak production each year recovered 50% of the N at 120 DAP the first year and 46% at 90 DAP the second year. The addition of 20 Mg compost ha-1 yr-1 increased N recovery an average 8% compared to the plots that received no compost.
During two seasons, the kenaf forage, under the conditions of the trial and averaged over variety and harvest regime, was able to remove, on average, only 11.7 kg P ha-1 on the plots receiving 10 Mg compost, and 15.3 kg P ha-1 from the plots receiving 20 Mg compost. The overall average 6.8 kg P removed ha-1 yr-1 is low compared with higher yielding crops grown under higher rainfall that remove >20 kg P ha-1 yr-1 (Sharpley and Withers, 1994). This was equivalent to only 10.4% removal of the P applied to the 10 Mg plots and 6.8% of the P applied to the 20 Mg plots. Other forage crops such as bermudagrass have been able to recover between 14 and 20% of P applied in manure (Sanderson and Jones, 1997). This contributed to an increase in P available in the soil, at the end of the trial, in plots that received compost. Kenaf, under the dryland growing conditions of the trial, was unable to remove sufficient P to sustain indefinite continuous dairy compost applications without, eventually, resulting in runoff water contamination.
Dairy compost at the heavier, 20 Mg DM ha-1 yr-1 rate did, however, increase kenaf yields 25.7% by the second year of this trial compared with plots without compost. Plant P concentration tended to increase and N concentration to decrease with compost application the second year. The N decrease was likely due to greater plant biomass accumulation in plots receiving compost with consequent N dilution in forage material. This, in combination with an increase in forage P concentration and P yields, indicates that the use of dairy compost benefits kenaf crops under dryland conditions. Increased rainfall, irrigation, or heavier N fertilizer applications to kenaf would all likely increase the effectiveness of dairy compost on kenaf fields.
Forage production was greater for EV41 and G4 than for India. The latter variety, however, showed more consistent N and P forage concentrations over the trial. This would indicate that, for forage production, N and P uptake, EV41, and G4 would be the better choices of the varieties studied. The low and sporadic rainfall and 120 maximum DAP to harvest resulted in short growing seasons that did not favor the photo-insensitive G4. The higher nutritive value of India indicates that this variety, despite its lower production, would be useful in a situation where forage nutritive quality was more important than quantity.
This same trade-off between quantity of forage produced and quality is apparent in DAP to harvest. Nitrogen and P concentration in the forage material was highest in the less mature forage harvested at 60 DAP. In contrast, higher forage yields were obtained at 120 DAP, after rain late in the growing season. When rain fell primarily in the early part of the season, higher yields were harvested at 90 DAP. Further trials with rainfall or irrigation throughout the season are needed to determine if production would continue up to and beyond 120 DAP when moisture does not limit growth up to 90 DAP.
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TOP
ABSTRACT
INTRODUCTION
