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a Dep. of Agronomy, Iowa State Univ., Ames, IA 50011
b USDA-ARS National Soil Tilth Lab., Ames, IA 50011
* Corresponding author (blaserb{at}iastate.edu)
Received for publication December 16, 2005.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: DM, dry matter • GDD, growing degree days • LSD, least significant difference • PAR, photosynthetically active radiation
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Introducing a winter cereal grain/red clover intercrop into a corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotation can provide producers with crop alternatives that can diversify income (Exner and Cruse, 2001), improve yields of subsequent crops with reduced inputs (Singer and Cox, 1998), improve soil quality (Reicosky and Forcella, 1998), and disrupt pest cycles (Cook, 1988). Growing a winter cereal grain will provide producers a cash grain crop while minimizing soil erosion (Kaspar et al., 2001) and weed competition (Van Heemst, 1985) during legume establishment. A legume crop growing during the period following winter cereal grain harvest could provide weed suppression (Mutch et al., 2003), forage for livestock (Scott et al., 1987), N for subsequent crops (Singer and Cox, 1998), and reduce erosion (Scott et al., 1987).
Frost-seeding (Mutch et al., 2003; Singer et al., 2006) is a simple, low-cost method for establishing forage legumes in winter cereal grains. The freeze-thaw movement of the soil and precipitation in early spring establishes good seed-to-soil contact for germination and seedling establishment (Barnhart, 2002).
Competition for resources occurs when establishing forage legumes with companion crops and is greater when frost-seeding red clover in a winter cereal compared to a spring-sown cereal (Smith et al., 1986; Tesar and Marble, 1988). Light was identified as the most limiting resource for DM production of forage legumes grown under cereal companion crops in the North Central USA (Pritchett and Nelson, 1951; Klebesadel and Smith, 1959). Additionally, soil water content can be an important determinant of forage legume survival under some conditions (Singer and Cox, 1998) and can be influenced by the stand density of the companion crop (Smith et al., 1954). Competition for nutrients does not appear to be a limiting factor for forage establishment with cereal grains in highly fertile soils (Pritchett and Nelson, 1951). Decreasing cereal grain stand density to the lower end of the optimum range for grain yield can reduce light and soil moisture competition with an intercropped forage legume because the cereal grain stand has fewer plants per area (Bula et al., 1954; Smith et al., 1954).
Optimal management practices must be developed for a winter cereal grain/forage legume cropping system before it will be adopted by North Central USA grain and livestock producers. Currently, there are no established seeding rate guidelines for a winter cereal grain/red clover intercrop. The objectives of this study were (i) to determine the optimum cereal grain and red clover seeding rates for maximum returns using partial budget analyses and (ii) to evaluate resource availability to red clover frost-seeded into winter cereal grains.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Recently harvested soybean fields with Clarion loam soil (fine-loamy, mixed, mesic Typic Hapludoll) in 2002–2003 and Canisteo silty clay loam soil (fine-loamy, mixed (calcareous) mesic Typic Haplaquoll) in 2003–2004 were prepared for planting with one pass of a tandem disk followed by one pass of a culti-packer roller. ‘Kaskaskia’ soft red winter wheat (39700 seeds kg–1, 98% germination, 100% purity) and ‘DANKO Presto’ triticale (55100 seeds kg–1, 92% germination, 100% purity) were planted at 100, 200, 300, and 400 seeds m–2 on 11 Oct. 2002 for 2003 harvest and 1 Oct. 2003 for 2004 harvest using a tractor-mounted 7.6 m wide Marliss grain drill (Marliss Industries, Jonesboro, AR) with 19-cm row widths. Both species were replanted at the targeted rates on 15 Oct. 2003 because of inadequate stands resulting from planter equipment malfunction. While quantification of the exact number of seeds dropped during the first planting was difficult, visual observation of the plots and actual stand densities achieved suggested it was <10% of the intended seeding rate. The planted area for each cereal grain seeding rate was 7.6 by 22.9 m in 2002–2003 and 7.6 by 27.4 m in 2003–2004.
‘Cherokee’ red clover (536 100 seeds kg–1, 94% germination, 100% purity, 2% hard seed) was frost-seeded into each cereal grain seeding rate plot at 0, 300, 600, 900, 1200, and 1500 seeds m–2 using a tractor-mounted, 3.66 m wide Gandy Model 1012T-TBM drop spreader (Gandy Co., Owatonna, MN) on 26 Mar. 2003 and 12 Mar. 2004. Each red clover seeding rate treatment occupied 3.8 by 7.6 m in 2002–2003 and 3.8 by 9.1 m in 2003–2004. To adapt to a limited plot area in 2002–2003, the 0, 1200, and 1500 seeds m–2 clover seeding rates were not included in the 100 seeds m–2 cereal grain seeding rate plots, which were reduced in size to 3.8 by 22.9 m. All plots were broadcast fertilized with 45 kg N ha–1 in the form of NH4NO3 on 25 Mar. 2003 and 12 Mar. 2004.
Soil Water and Light Interception
Volumetric soil water content of the upper 6 cm of the soil profile was measured with a portable Delta-T Thetaprobe ML2 moisture sensor attached to a Delta-T HH2 handheld data logger (Delta-T Devices Ltd., Cambridge, UK). The measurements were collected on 23 Apr., 3 May, 19 May, 26 May, 5 June, 13 June, 30 June, and 14 July 2003 and 10 April, 3 May, 8 May, and 30 June 2004. Three measurements were collected within the nontrafficked area of each red clover seeding rate plot and averaged to determine soil water content.
Photosynthetically active radiation (PAR) interception by the cereal grain canopy was determined every 7 to 10 d beginning on 23 Apr. 2003 and 3 Apr. 2004 using an AccuPAR Linear PAR Ceptometer, Model PAR-80 light measuring instrument (Decagon Devices, Pullman, WA). Measurements were obtained by placing the ceptometer diagonally across three cereal grain rows. The instrument was positioned below the cereal grain canopy, but above the red clover plants to measure the quantity of PAR that reached the top of the red clover canopy. Measurements were collected under full sunlight between 1130 and 1400 h. Percent light transmittance was calculated by dividing the average of six below canopy PAR readings by one above canopy reading and multiplying by 100.
Cereal Grain Density, Growth, and Yield
Cereal grain plant density counts were made following green-up in the spring (14 Apr. 2003, 2 Apr. 2004). Emerged plants were counted in 1 m of three adjacent rows from 12 random areas within each cereal grain seeding rate plot. Cereal grain phenology was recorded every 2 wk during active growth using the mean growth stage of 12 randomly sampled plants from each subplot and Zadoks scale for cereal grain staging (Zadoks et al., 1974). All stand density and phenological measurements were obtained from plants within nontrafficked rows.
Spikes m–2 for each cereal grain seeding rate were counted from samples collected from 12, 1-m lengths of row before grain harvest. Cereal grains were machine harvested using a Massey Ferguson Model 25 combine (Sampo Rosenlew Ltd., Pori, Finland). In 2003, wheat subplots were harvested on 16 July and triticale subplots were harvested on 22 July. In 2004, both wheat and triticale subplots were harvested on 15 July. Grain yield for each cereal grain seeding rate was determined using an electronic scale integrated in the combine. Final grain yields were adjusted to a 135 g kg–1 moisture basis. Thousand-kernel weight for each cereal grain seeding rate treatment was determined by weighing two 1000-kernel subsamples obtained from a 1-kg sample collected during harvest. Kernels spike–1 for each cereal grain seeding rate was calculated from the total yield, spikes m–2 and 1000-kernel weight data. Stubble height following harvest was approximately 30 cm. The straw was baled and removed the day after grain harvest both years.
Red Clover Density and Biomass
Red clover plant density was measured on 13 May 2003 and 5 May 2004 by counting the number of plants in two 0.38 m2 quadrats per red clover seeding rate. Red clover density following cereal grain harvest was determined by counting two 0.25 m2 quadrats per red clover seeding rate on 23 July 2003 and 21 July 2004. Weed density was collected from two 0.25 m2 quadrats in each red clover seeding rate plot following cereal grain harvest on 24 July 2003 and 13 Aug. 2004. All density counts were collected from nontrafficked areas.
Red clover shoot biomass accumulation was determined at approximately 40 d after cereal grain harvest. Red clover biomass from two 0.25 m2 quadrats in each red clover seeding rate was harvested 6 cm above the soil surface on 22 Aug. 2003 and 23 Aug. 2004 and oven dried at 60°C until a constant weight was achieved. All red clover biomass was mechanically removed on 22 Aug. 2003 and 23 Aug. 2004 by harvesting with a Green Chopper Lacerator (Gruett's, Potter, WI) leaving a 6-cm stubble height. An additional red clover harvest occurred approximately 40 d following the machine harvest using the previously described hand clipping method. These harvests were completed on 1 Oct. 2003 and 4 Oct. 2004. All red clover growth was removed with the lacerator on 1 Oct. 2003 and 4 Oct. 2004. Red clover biomass harvests were obtained from nontrafficked areas from three replications.
Partial Budget Analysis
A partial budget analysis was conducted to determine the seeding rates that optimized cereal grain and red clover DM return. Winter triticale and wheat varieties evaluated in Iowa contain on average about 33 075 seeds kg–1. Seed costs for triticale and wheat, determined by contacting local agribusinesses, average about $0.44 and $0.35 kg–1. Because triticale prices are difficult to obtain and triticale is grown as a corn feed substitute in Iowa, we used the average 10-yr National Agricultural Statistics Service (NASS) corn price ($0.0811 kg–1) to calculate triticale value. The 10-yr average wheat price (NASS) was used to determine wheat grain value ($0.1062 kg–1). Because red clover has similar feed value as alfalfa (Broderick et al., 2001), we used the Iowa 10-yr average alfalfa price ($97.91 Mg–1 dry alfalfa) to value red clover. Red clover seed costs average about $5.51 kg–1. We also assumed that all other inputs would remain similar for the different cereal grain and red clover seeding rates.
Statistical Design and Analysis
The experiment had an incomplete randomized block design in 2003 because the 0, 1200, and 1500 seeds m–2 red clover seeding rates were not planted in the 100 seeds m–2 cereal grain seeding rate plots. The experimental design for 2004 was a randomized complete block. Statistical analysis was performed using the PROC MIXED (method = type 3) of the Statistical Analysis System Version 9.1 (SAS Institute, 2002). A Fisher's protected LSD (
= 0.05) was used to test significant differences between treatment means. Light transmittance and soil moisture data were analyzed using a repeated measures model with first order autoregressive correlation.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Cereal Grain Stand Density
A range of stand densities were obtained from the four seeding rates in 2003 and 2004 (Table 1). The stand establishment success as a proportion of the seeding rate decreased from 95 to 54% in 2003 and 103 to 74% in 2004 as seeding rates increased from 100 to 400 seeds m–2. Similar reductions have been reported previously and have been attributed to greater plant competition at higher densities (Whaley et al., 2000). A cereal grain species by seeding rate interaction was detected both years. In 2003, the 100 and 200 seeds m–2 seeding rates of wheat had 24 to 30% fewer plants than triticale, respectively, whereas the densities of the 300 and 400 seeds m–2 seeding rates were similar for both species. In 2004, wheat had 24% fewer plants than triticale at 100 seeds m–2, while stand densities were similar for both species at 200, 300, and 400 seeds m–2 (Table 2).
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In 2003, partial budget returns were greatest at the 200 seeds m–2 seeding rate in triticale ($30.83 ha–1) and 400 seeds m–2 seeding rate in wheat ($19.12 ha–1). In 2003, the maximum yield and the greatest return were obtained in the 400 seeds m–2 seeding rate in wheat. Triticale grain yield did not respond in 2003 to increasing plant density probably because the separation in plant density between each incremental seeding rate was marginal. If the actual triticale plant density had exhibited greater separation to increasing seeding rate (Table 1), we would expect the greatest return to occur at higher seeding rates. In 2004, wheat had the greatest yield and partial budget return at the 400 seeds m–2 seeding rate ($23.37 ha–1), while triticale yielded greater at the 400 seeds m–2 rate but had the greatest return at the 300 seeds m–2 rate ($21.90 ha–1).
Spike number per area has been identified as the main grain yield component affected by changes in stand density in wheat (Smid and Jenkinson, 1979; Blue et al., 1990; Dahlke et al., 1993). This was evident in both wheat and triticale in both years of this study. In 2003, cereal grain stand densities of 82 to 217 plants m–2 produced a range in spikes m–2 of 325 to 406 (Tables 1 and 2). In 2004, cereal grain stand densities of 89 to 301 plants m–2 produced a range in spikes m–2 of 413 to 636. The relative increase in spikes m–2 was similar to the increase in grain yield across the range of stand densities in both years.
While kernels spike–1 and kernel weight were affected much less than kernels m–2 by seeding rate, there were instances where both species and seeding rate had some influence on these yield components. There was a cereal grain species by seeding rate interaction for kernel weight in 2003 because 100 seeds m–2 resulted in wheat kernels that weighed 5% less than kernels from the 200, 300, and 400 seeds m–2 seeding rates, which had similar weights (Table 1). Triticale kernel weight did not change with seeding rate in 2003. In 2004, Septoria leaf blotch infection beginning early in reproductive growth reduced triticale kernel number and weight (Table 2). This resulted in 29% fewer kernels spike–1 and 18% lower kernel weight in triticale than wheat, which had good resistance to Septoria. There was a species by cereal grain seeding rate interaction for both kernels spike–1 and 1000-kernel weight in 2004. The number of triticale kernels spike–1 increased as seeding rate increased, whereas wheat kernels spike–1 decreased. Triticale 1000-kernel weight was similar across all seeding rates. However, wheat 1000-kernel weights increased as seeding rate increased.
Red Clover Density and Dry Matter
Red clover was successfully established under the triticale and wheat cultivars used in this study, yet density differences between species were observed (Table 3). There were more red clover plants in wheat than in triticale after the 2003 grain harvest, but there were no differences in red clover DM at 40 or 80 d after grain harvest. In 2004, there was an interaction of cereal grain species and cereal grain seeding rate on red clover density 7 wk after frost seeding. This interaction occurred because red clover densities in the 100 and 200 seeds m–2 cereal grain rates were 23 and 11% greater for wheat than triticale, respectively, whereas the red clover densities from 300 and 400 seeds m–2 were 8 and 12% lower for wheat than triticale (data not shown). This interaction was still present when red clover plants were counted after cereal grain harvest. Red clover stands in the 100 and 200 seeds m–2 cereal grain rates were 19 and 14% greater for wheat than triticale, respectively. Red clover densities were similar in triticale and wheat at 300 seeds m–2 and 13% greater in triticale than wheat at 400 seeds m–2 (data not shown).
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On average, 8 to 11% of the red clover seed resulted in plants at 7 wk after seeding (Table 3). Red clover stand densities increased by 33 to 51% between 7 wk after planting and post-cereal grain harvest, which resulted in an overall establishment success of 10 to 22%. The increase in red clover plants from the first count to the second suggested red clover plants continue to emerge and become established throughout the spring and summer. Our red clover stand establishment with winter cereal grains was similar or slightly better than the 7 to 11% reported with pasture seeding of red clover in Wisconsin (Casler et al., 1999).
In 2003, red clover DM was not affected by cereal grain seeding rate when harvested 40 d after cereal grain harvest or at a second harvest 40 d later (Table 3). Precipitation during the red clover DM production period was 65 mm below average in 2003 (Fig. 1), which reduced red clover DM production (Table 3). In 2004, red clover DM production in the first harvest decreased 25% as cereal grain seeding rate increased from 200 to 300 seeds m–2. However, no difference in DM for these two rates was detected at the second red clover harvest.
The two lower cereal grain rates produced greater amounts of red clover DM in the harvest at 40 d after cereal grain harvest than the two higher seeding rates (Table 3). But, regrowth harvested 40 d later (80 d after cereal harvest) increased with seeding rate. The greater amount of red clover biomass initially produced after establishment with lower cereal grain rates may have resulted in more soil water use during the first 40 d red clover growth period. This may have limited soil water available to red clover established with lower cereal grain rates during the second 40 d growth period. Seasonal red clover DM (both harvests combined) was not influenced by cereal grain seeding rate (data not shown).
Red clover DM response to increasing red clover seeding rate varied by harvest (Table 3). In 2003, DM at 40 d after cereal grain harvest was similar for the five red clover seeding rates. The DM of red clover regrowth in the second harvest, 40 d after the first, was affected by red clover seeding rate. Dry matter production from the 900 seeds m–2 seeding rate was >300 and 600 seeds m–2 and similar to 1200 and 1500 seeds m–2. The 300 and 600 seeds m–2 seed rates produced 26 and 13% less DM than 900 seeds m–2, respectively.
In 2004, red clover DM for both the 40 and 80 d harvests increased with each red clover seeding rate increase up to 1200 seeds m–2. At 40 d after cereal grain harvest, red clover DM was 9, 14, and 29% less for the 900, 600, and 300 seeds m–2 red clover rates than 1200 seeds m–2, respectively. At 80 d after cereal grain harvest, red clover DM was 10, 15, and 24% less for the 900, 600, and 300 seeds m–2 red clover rates than 1200 seeds m–2.
Red clover DM production and partial budget returns from the 2 yr suggested 900 to 1200 seeds m–2 as optimum red clover seeding rates when frost seeding into winter cereal grains. In 2003, returns for the 900 seeds m–2 rate were $3.43 ha–1 greater than the 600 seeds m–2 rate and in 2004, the 1200 seeds m–2 rate returned $29.86 ha–1 greater than the 900 seeds m–2 rate. Red clover DM production reached its maximum when red clover stand density after cereal grain harvest was 123 to 140 plants m–2. This is a lower density for maximum yield than the 172 plants m–2 reported for alfalfa (Volenec et al., 1987).
Weed Density
The triticale/red clover intercrop was better at suppressing weeds than the wheat/red clover intercrop in 2003, but not in 2004 (Table 3). In 2003, red clover seeding rates of 1200 and 1500 seeds m–2 suppressed weeds below the no red clover control. In 2004, the presence of red clover at all densities reduced weed densities and weed density generally decreased as red clover density increased. Similar results have been reported by Mutch et al. (2003), who found frost-seeded red clover suppressed common ragweed (Ambrosia artemisiifolia L.) growth after winter wheat harvest.
Resource Competition
The quantity of PAR transmitted through the cereal grain canopies to the underseeded red clover (Fig. 2
) decreased from greater than 90% before mid-April (300 GDD; growth stage (GS) 20; Table 4) to a minimum between 7 and 12% in late May and early June (600–1100 GDD; GS 50–GS 70) in both years. The light compensation point for red clover is about 6% of daylight (Taylor and Smith, 1995). Light available to red clover in our study was greater than this critical level in both cereal grain species and all cereal grain seeding rates throughout the growing season in both study years.
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In 2004, the 100 seeds m–2 cereal grain seeding rate provided more light to red clover than 200, 300, and 400 seeds m–2 until 640 GDD were accumulated (GS 40; Fig. 2). From 640 to 1710 GDD, the 100 seeds m–2 rate generally increased light transmittance more than 300 and 400 seeds m–2. The 200 seeds m–2 rate increased light transmittance to the red clover more than 300 and 400 seeds m–2 during the majority of cereal growth. Light transmittance was similar for 300 and 400 seeds m–2 until 1550 GDD, when more light was available to the red clover in the 300 seeds m–2 rate. The lower amount of light available to seedlings did not decrease red clover density (Table 3). However, it did reduce red clover DM production during the first 40 d after cereal grain harvest by as much as 30% (Table 3).
Soil water content measurements were timed to coincide with periods of decreased rainfall and increased probability of plant water stress. Average soil water content in 2003 was 0.17, 0.21, 0.19, 0.12, 0.20, 0.22, 0.29, and 0.22 m3 m–3 on 23 April, 3 May, 19 May, 26 May, 5 June, 13 June, 30 June, and 14 July, respectively. Water content was 6% lower with wheat than triticale on 5 June. Soil water content on 14 July was 10 to 13% greater for 300 and 400 seeds m–2 cereal grain rates than 100 seeds m–2 and 8% greater for 400 than 200 seeds m–2. Average soil water content in 2004 was 0.19, 0.17, 0.15, and 0.25 on 10 April, 3 May, 8 May, and 30 June, respectively. Water content was 4 to 7% lower in triticale than wheat on 10 April, 3 May, and 30 June. Early season soil water content decreased with each increase in cereal grain seeding rate in 2004 (Fig. 4 ). However, as the season progressed this effect shifted and, during ripening of the cereal grains, there was actually 4 to 9% more soil water measured in the 300 and 400 seeds m–2 seeding rates than the 100 and 200 seeds m–2 rates. Red clover seeding rate had no effect on soil water content in either year.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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{[(daily max. temp. + daily min. temp.)/2] – base temp.} > 0 with base temperature = 0°C.
