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Agronomy Journal 92:16-24 (2000)
© 2000 American Society of Agronomy

SOIL FERTILITY

Soybean Yield and Nutrient Composition as Affected by Early Season Foliar Fertilization

Mazhar U. Haqa and Antonio P. Mallarinoa

a Dep. of Agronomy, Iowa State Univ., Ames, IA 50011 USA

apmallar{at}iastate.edu


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
The response of soybean [Glycine max (L.) Merr.] to foliar fertilization during early growth stages has received little attention. Recent Iowa research showed a 15% probability of positive yield response to a 3–15–8 (N–P–K) fertilizer applied at the V5 growth stage. This study evaluated the effects of fertilizers varying in N–P–K ratio on soybean grain yield and tissue nutrient composition. Twenty-seven field trials were conducted in soils that tested at or above optimum soil P and K levels for soybean. Six treatments included a control and nonfactorial combinations of rates and application frequency of 28 to 56 L ha-1 of 3–8–15, 10–4–8, and 8–0–7 fertilizers sprayed at the V5 stage. Differences between treatments were inconsistent across sites. Some or all treatments increased or decreased yields significantly at six sites. The mean yield increase or decrease for responsive sites was 400 kg ha-1. The 3–8–15 fertilizer caused no leaf damage and other fertilizers caused little or no damage, although the damage was not clearly related with yield decreases. Analyses by site showed that fertilization seldom increased tissue N–P–K composition, nutrient uptake, photosynthesis, or plant weight measured at the R2 growth stage. Multivariate analyses across sites showed that 27% of the variation in yield response was explained by a combination of N, P, and K availability, vegetative growth, and rainfall. Positive yield responses tended to occur when soil or weather conditions reduced plant growth and nutrient availability. Foliar fertilization across all conditions will not offset the application costs.

Abbreviations: CEC, cation exchange capacity • CER, leaf CO2 exchange rate


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
FOLIAR FERTILIZATION of soybean at reproductive stages has resulted in inconsistent grain yield increases. Garcia and Hanway (1976) reported yield increases of 27 to 31% when a liquid N–P–K–S fertilizer was sprayed at late reproductive stages. These authors suggested that root activity decreases during the period of pod-fill and that nutrient uptake is not enough to meet the seed demands for nutrients. Other reports did not replicate these results and showed that most often foliar fertilization did not influence soybean yield, leaf nutrient concentration, or photosynthesis, and sometimes decreased yield (Boote et al., 1978; Parker and Boswell, 1980; Syverud et al., 1980; Poole et al., 1980; Seasy and Shibles, 1980). In some instances leaf damage due to foliar fertilization was sufficiently severe to be the probable cause of the yield reduction. Also, leaf injury and yield depressions tended to be more frequent when fertilizer was applied during midday rather than in early morning or late afternoon hours.

Little research has been conducted on foliar fertilization of soybean at early vegetative stages. Foliar fertilization at early stages could increase P and K supplies at the time when the root system is not well developed. Also, a small amount of foliar-applied N probably will not inhibit N2 fixation and could boost plant development and grain yield. Roselum et al. (1982) showed no yield differences when two formulations of macronutrients (5–15–15 and 14–4–7) were sprayed at 45 and 60 d after seedling emergence. The soil tested lower than optimum in both P and K according to local interpretations and received 37 kg P ha-1 and 19 kg K ha-1 at planting. Recent research in Iowa (Haq and Mallarino, 1998) showed, however, that foliar fertilization with various rates of a commercial 3–8–15 fertilizer applied at the V5 growth stage (Fehr et al., 1971) increased soybean grain yields in seven of 48 trials, reduced yields slightly at two trials, and increased yield slightly (54 kg ha-1) across all trials. Although no simple relationship between yield response and any single site-variable was found, the responsive sites had higher soil cation exchange capacity (CEC) and higher Ca and Mg content, lower plant P concentration in soybean tissue at the V5 growth stage, and lower rainfall in late spring to mid-summer. Their results showed that responses as high as 700 kg ha-1 are possible, even in soils that test optimum or higher in P and K.

The objective of this study was to evaluate the soybean response to foliar fertilization at early vegetative stages with three commercially available fertilizers varying in N–P–K ratio. The responses measured in 27 farmers' fields included grain yield and nutrient composition of vegetative tissues at the R2 to R3 growth stage. In addition, leaf CO2 exchange rate (CER) at full bloom also was measured in eight fields.


   Materials and methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Conclusions
 REFERENCES
 
Twenty-seven foliar fertilization trials were established during 1995 and 1996 on farmers' fields in three major agricultural regions of Iowa (east, west, and north-central). The management practices, except the foliar fertilization, were those commonly used by the farmers. There were wide ranges of soybean varieties, soil types, tillage systems, planting dates, and other management practices (Table 1) . The row spacing was 19 cm in no-till fields and 76 to 97 cm in chisel-plow or ridge-till fields. Soil fertilization was not used in most sites (most Corn Belt farmers apply fertilizer only before corn in the corn–soybean rotation). Fields 3, 11, 20, 26, and 27 received, however, 17 to 45 kg P ha-1 and 56 to 112 kg K ha-1 4 to 5 mo before planting (in the fall after harvesting the previous corn crop). Thus, the soil analyses for these nutrients on samples collected at planting time should reflect these applications.


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  		Table 1 Summarized information about the conditions of 27 foliar fertilization trials

 
The treatments were a control, a single 28 L ha-1 application of 3–8–15 (N–P–K), 38 L ha-1 of 3–8–15 split into two equal applications, single applications of 28 and 56 L ha-1 of 10–4–8, and 42 L ha-1 of 8–0–7. The single applications and the first of the split 3–8–15 treatments were at the V5 growth stage (Fehr et al., 1971). The second application was 8 to 10 d later. The 28 L ha-1 rate of 3–8–15 corresponds to 1.2, 3.1 and 5.9 kg ha-1 of N, P, and K. The 56 L ha-1 of 10–4–8 corresponds to 7.1, 3.1, and 5.9 kg ha-1 of N, P, and K and 42 L ha-1 of 8–0–7 corresponds to 3.8, and 3.2 kg ha-1 of N and K. The fertilizers are commercially available. The 3–8–15 and 10–4–8 are fluid fertilizers manufactured by reacting H3PO4 with aqueous ammonia and KOH and by adding urea. The 8–0–7 fertilizer is manufactured by dissolving KNO3 into water. The experimental layout was a randomized complete-block design with four replications. Each plot measured 12 m in length and 4.5 to 5.5 m in width, depending on the row spacing. The foliar fertilizer was applied using a hand-held CO2-powered sprayer adjusted to a constant pressure of 0.17 MPa diluted into 100 L ha-1 of water (no other additives were used). The plots were sprayed during late afternoon or evening hours when wind speed was less than 15 km hr-1 and air temperature was less than 27°C.

Numerous soil, plant, and climate measurements were collected in an attempt to explain the results of the study. Data for the most relevant measurements are shown. Some (such as soil micronutrients or temperature and rainfall during various periods) are not included either because they were not related to yield response or to reduce the number or size of tables. Weather information was obtained from meteorological stations located within 10 km from the trials. Soil samples were collected before the first foliar spraying from each replication at each site (10 cores per composite sample, 0- to 15-cm deep). Samples were analyzed for pH, organic matter, and several nutrients (Table 1). Phosphorus was extracted by the Bray P1 method; K, Ca, and Mg by the ammonium acetate method; and organic matter by loss of weight on ignition (LOI). Cation exchange capacity was estimated by the sum of K, Ca, Mg (from the ammonium acetate extraction), and exchangeable H+ as estimated from measurements of pH and buffer pH by the Shoemaker, McLean, and Pratt (SMP) method. The procedures followed for all methods were those described and recommended for soils of the north-central region (Brown, 1998).

Plant tissue samples were collected at two growth stages. A composite sample of the aerial part of 10 plants was randomly collected from each replication at each site on the day of the first spraying (V5 growth stage) to study the initial N, P, and K concentration of the tissues. The samples were dried in an air-forced oven at 65°C and ground to pass a 2-mm screen. For total P and K determinations, the samples were dry-ashed and dissolved in 0.1M HCl. Phosphorus was determined colorimetrically and K was determined by flame emission. Total N was determined with a Carlo Erba analyzer (Carlo Erba, Milan, Italy). Samples of trifoliolate leaves (including petioles) consisting of 30 uppermost fully expanded leaves were randomly collected at the R2 growth stage from plots of four selected treatments, which were the control, the single 28-L and split 38-L rates of 3–8–15, and the single 56-L rate of 10–4–8. The aerial part of eight plants were collected from all replications of the same treatments at the same growth stage. In this case the samples were not collected from the two center rows that would be used for grain harvest. The nutrient concentrations of trifoliolate leaves and whole plants were measured with the methods described for the small plants. These concentrations were used together with dry plant weight to calculate total N, P, and K uptake in the aerial part of the plants by the R2 growth stage.

Visual ratings of leaf injury due to the first fertilizer application were collected from all trials by two independent observers at the time of second application. Leaf injury was expressed as the percentage of leaf area damaged. Potential treatment effects on duration of green leaf area were estimated by visual ratings of the proportion of green and yellow leaves before leaf drop began. Grain yield was measured by cutting a 9-m length of the center two rows (or a 1-m swath in no-till fields) of each plot. Stems were cut with a sickle-bar mower when the row spacing was 19 cm (in no-till fields) and with a hand-held rotary mower when the row spacing was wider. The grain was threshed with a stationary thresher and weighed at the field. A grain sample was collected from each plot to measure grain moisture and weight of individual grains, and grain yield was adjusted to 130 g kg-1 moisture.

The CER measurements, which estimate the apparent photosynthetic rate, were collected at the R2 growth stage from four selected treatments of eight arbitrarily selected trials (four in 1995 and four in 1996). The treatments selected were the control, the single 28-L rate of 3–8–15 fertilizer, and the single 28- and 56-L rates of 10–4–8 fertilizer. The CER measurements were made at the field with a portable photosynthesis analyzer (LI-6200, LI-COR, Inc., Lincoln, NE 68504) on the most recently fully expanded terminal leaflet of eight plants from each plot between 10:00 and 14:00 hr under clear sky conditions. The leaflets were taken to the laboratory for determination of dry weight and surface area with an LI-COR 3100 leaf area meter.

Analyses of variance (SAS Institute, 1996) were conducted for each site, across sites within each year, and across all sites. The treatment sum of squares of randomized complete-block designs was partitioned into an orthogonal contrast of the control vs. fertilized treatments. Means were further compared by LSD (P <= 0.1) when the treatment main effect or the contrast were significant. Simple correlation and multivariate factor analyses were used to study the relationship between yield response and continuous site variables across all sites. The yield increase was calculated for each trial by subtracting the mean yield of the control from the mean yield of the fertilizer treatments, excluding the 56-L rate of 10–4–8. This treatment was excluded because it never increased yield. Groups of correlated site-variables across sites were defined using factor analysis (Johnson and Wichern, 1992). Factor analysis has been used before to study relationships between yields and correlated site variables (Mallarino et al., 1996; Haq and Mallarino, 1998). Factors were extracted with the principal factor procedure and the Promax rotation method (SAS Inc., 1996). New variables (called latent variables) were created for each site by standardizing and averaging selected variables from each factor for which the eigenvalues of the correlation matrix were one or greater. The basis for selecting variables from each factor was the size of partial correlation coefficients, often referred to as factor loadings. Simple and multiple regression equations were fit to relationships between yield increases and the latent variables across sites.


   Results and discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Conclusions
 REFERENCES
 
Grain Yields
Study of responses at each trial showed that foliar fertilization effects on grain yields were infrequent and usually small (Table 2) . Analyses of variance for each trial showed significant responses at five sites in 1995 (Sites 1, 7, 8, 9, and 17). At Site 1, most treatments increased yield (except the 56-L rate of 10–4–8 fertilizer) and there was an average positive response of 403 kg ha-1. At Site 7, there was a negative overall response and differences among fertilized treatments were not significant. At Site 8, there was a negative response to most treatments except to the split 38-L rate of 3–8–15, which did not differ from the control. At Site 9, there were positive responses to the three treatments that applied low nutrient rates (28 L of 3–8–15, 28 L of 10–4–8, and 42 L of 8–0–7). Yields for the two treatments applying the highest rates (the split 38-L rate of 3–8–15 and the 56-L rate of 10–4–8) did not differ from the control. The average response was about 250 kg ha-1 when these two treatments were not considered. At Site 17, the 28-L rate of 3–8–15 and 42-L rate of 8–0–7 decreased yield slightly, whereas other treatments did not differ from the control. The mean yield response across all sites conducted in 1995 was not statistically significant and was essentially zero. At Site 24 (the only responsive site in 1996) the 28-L rate of 10–4–8 fertilizer (the low rate) and the 42-L rate of 8–0–7 fertilizer increased yields and other treatments did not differ from the control. The average response to these two treatments was about 400 kg ha-1. The yield response across the 1996 sites was not statistically significant and the average yield increase due to fertilization was 70 kg ha-1. Foliar fertilization did not affect soybean yield significantly across all the 27 sites.


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  		Table 2 Effects of foliar fertilization on grain yields of soybean at 27 sites conducted in 1995 and 1996

 
The infrequent positive yield responses could be partly explained by the mostly optimum or above-optimum P and K supplies of the soils. Only six sites tested below optimum in P (Sites 1, 2, 4, 18, 19, and 25) and only two (Sites 4 and 17) tested below optimum in K according to Iowa State University soil test interpretations (Table 1). These are the only sites that should have received higher than maintenance P or K fertilization according to current recommendations for soybean. Soil tests varied at sites where positive yield responses were observed. At Site 1, P was below optimum and K was high but even the treatment with 8–0–7 that applied no P increased yield. At Site 9, P was very high and K was optimum and at Site 24 both P and K were high. The predominance of high-testing soils in this study coincides with the distribution of soil-test values of major agricultural areas of Iowa.

Leaf Injury, Plant Maturity, and Grain Size
The foliar fertilization treatments produced little leaf injury and, therefore, results are not shown. The 3–8–15 fertilizer and the low rate (28 L) of the 10–4–8 fertilizer produced no visual injury at any trial. The high rate (56 L) of 10–4–8 fertilizer produced moderate leaf injury at several trials probably because of the higher N application. The percentage leaf area affected was only 5% or less, however, except at two sites. At Site 3, 6% of the leaf area was injured and at Site 20, 10% of the leaf area was injured. Although this level of leaf injury could possibly explain a lack of positive yield response or a small yield decrease at these and other sites, it did not result in a statistically significant yield decrease. Treatments that did not produce significant foliage injury sometimes decreased yields, however (for example, at Site 8). Foliar fertilization did not affect the maturity of the soybean plants and the size of individual grains at any site and, therefore, data are not shown.

Tissue Nutrient Concentration, Plant Weight, and Nutrient Uptake
The foliar fertilization treatments had little and infrequent influence on the N, P, and K concentration of the top mature trifoliolate leaves at the R2 growth stage (Table 3) , which is the tissue most frequently used to diagnose nutrient status of soybean. Nitrogen concentration of trifoliolate leaves was increased by all treatments at Sites 6 and 8. Some treatments increased, decreased, or did not affect N concentration at Sites 7, 9, 18, 20, and 25. The 56-L rate of 10–4–8 reduced (Sites 7 and 9) or did not increase N concentration (Site 25). The N concentration of the control treatment ranged from 34 to 49.7 g N kg-1 across sites. Results of many studies summarized by deMooy et al. (1973) showed that N concentrations lower than 41 g kg-1 are usually deficient and that concentrations 42 to 55 g kg-1 are sufficient for optimum soybean yield. Nine sites (Sites 1, 2, 4, 5, 6, 8, 15, 18, and 25) tested low according to this criterion, but foliar fertilization seldom increased grain yields or leaf N concentrations at these sites. The responses in grain yield were positive only at Site 1 and the responses in leaf N were positive only at Sites 6, 8, and 18.


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  		Table 3 Total N, P, and K concentrations of trifoliolate leaves for selected treatments of 27 trials

 
All treatments increased P concentration at Site 18 and decreased it at Site 14 and 22. All treatments decreased K concentration at Site 4 and apparent differences at other sites were not statistically significant. Leaf P concentrations for the control treatment ranged from 2.2 to 4.3 g P kg-1 across sites. Data summarized by deMooy et al. (1973) suggest that leaf P concentrations lower than 2.5 g kg-1 usually are deficient and that concentrations of 2.6 to 5 g kg-1 usually are sufficient for optimum soybean yields. Only four sites (Sites 4, 18, 24, and 25) tested low according to this standard. Foliar fertilization increased leaf P concentration at Site 18 but had no effect on grain yield at any of these sites. The leaf K concentration for the control ranged from 10 to 36 g kg-1 across sites. Sufficient levels of leaf K cited by deMooy et al. (1973) range from 17 to 25 g kg-1. Only five sites (Sites 1, 4, 9, 18, and 24) tested low according to this standard. Foliar fertilization increased grain yields at only one of these five sites (Site 9) and did not increase leaf K concentrations at any of these sites.

The treatments seldom influenced the dry weight and N, P, or K uptake of whole plants sampled from selected treatments at the R2 growth stage (Table 4) . Fertilization effects on whole-plant nutrient concentrations are not shown or discussed because the general trends were similar to those found for trifoliolate leaves and they could be calculated from the weight and uptake data. Analyses of variance for each site showed that the split application of 38 L ha-1 of 3–8–15 increased plant weight at Site 20 and the 56-L rate of 10–4–8 decreased plant weight at Sites 2 and 27. No effect was statistically significant across all sites. Foliar fertilization effects on nutrient uptake were consistent with effects on plant weight at Sites 2, 20, and 27. Treatments that increased or reduced growth also increased or reduced nutrient uptake. Isolated significant effects on nutrient uptake at other sites were not consistent with effects on plant growth and there is no obvious explanation for the responses. No site in which foliar fertilization influenced growth or nutrient uptake coincided with sites in which it increased or decreased yields.


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  		Table 4 Plant dry weight and total N, P, and K uptake at the R2 growth stage for selected treatments

 
The result that foliar fertilization seldom influenced tissue N, P, and K concentrations is in agreement with the general lack of effect of foliar fertilization with 3–8–15 fertilizer on P and K leaf concentration shown by Haq and Mallarino (1998). Research of fertilization at late reproductive stages also showed small or no effect of foliar fertilization on plant P and K concentrations (Boote et al., 1978; Parker and Boswell, 1980). It was believed possible that foliar fertilization at early vegetative stages with the different NPK ratios used in this study could increase shoot and root growth as well as nutrient uptake. Although fertilization increased or decreased growth and nutrient uptake at a few sites, results of analyses of variance indicated that grain yields were not significantly affected at those sites. This observation suggests that yield responses could not be explained solely by effects in nutrient concentration or uptake.

Leaf Photosynthesis
Foliar fertilization enhanced leaf CER significantly at Site 26, one of the eight sites in which CER was measured (Table 5) . At this site, the 28-L of 3–8–15 increased CER rate while the two treatments with 4–10–8 did not differ from the control. Foliar fertilization did not influence leaflet surface area or dry weight at any site (data not shown). The small significant effect of the 3–8–15 fertilizer at Site 26 did not coincide with increased tissue nutrient concentration, whole-plant dry weight, or grain yield. The lack of consistent treatment effects on the measurements collected cannot be explained by site characteristics or leaf damage due to fertilization. For example, at Site 17 there was a negative effect of some treatments on grain yield, which was not observed for CER. Correlations for this set of eight trials (not presented) showed that grain yield was not correlated with CER or leaf nutrient composition.


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  		Table 5 Effect of foliar fertilization on leaf CO2 exchange rate{dagger}

 
Relationships Between Yield Response and Site Variables
Study of treatment effects on yield responses, plant growth, or nutrient content of plants by analyses of variance suggested that foliar fertilization effects were infrequent and inconsistent across sites. These analyses, although valid and useful, do not consider possible relationships between response and several measured site-variables across sites. Study of yield responses at each trial in relation to management practices (some of which are shown in Table 1) showed no obvious general pattern that could explain the responses across all sites. Many varieties, soil types, tillage systems, and other practices overlapped across sites. Soil-test values and other continuous variables (such as rainfall, plant nutrient content, etc.) varied greatly across sites (Table 1). The study of the correlations between yield responses and measured continuous site variables across sites provides an evaluation of treatment effects different from the conventional analysis of variance and direct observation of data.

Table 6 shows the simple correlations between absolute yield increase due to foliar fertilization and selected continuous site variables (data for several variables that showed no correlation or did not add meaningful information are not shown). Yield response sometimes was positively or negatively correlated with some measurements, but most often correlations were not statistically significant. Analyses based on relative yield increases produced similar correlation coefficients (not shown). Data in Table 6 also show that many site variables were intercorrelated. In some instances, these correlations seem logical but in others they are difficult to explain or seem absurd. Examples of expected intercorrelations are those between soil Ca, Mg, CEC, and organic matter. Examples of absurd or difficult to explain intercorrelations (probably random results for this set of data) are those between Ca, Mg, and CEC with air temperature in spring or summer. The simple correlations between yield response and any site variable or between site-variables should be interpreted with caution. Significant correlations could be the result of the correlation of a measured variable with a nonmeasured variable that actually affected yield response. Also, simple correlations do not address appropriately the likely intercorrelations between variables and multicollinearity complicates interpretation of the statistical significance of any multiple regression model. These problems can be partly overcome by using multivariate analysis. Factor analysis is a useful technique to group many variables into a reduced group of correlated variables (Johnson and Wichern, 1992). In turn, yield responses across sites can be correlated with new variables created on the basis of these groups (Mallarino et al., 1996, 1999).


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  		Table 6 Simple correlations between absolute yield increase due to fertilization and various site-variables across 27 trials.{dagger}

 
Six factors were identified by the factor analysis. Each factor included several strongly correlated site variables, which were used to create the latent variables showed in Table 7 . Two latent variables were not significantly correlated with yield responses across the 27 sites and will not be discussed. The other four latent variables were correlated with yield response. The study of these relationships helps in suggesting reasons for the yield responses, although some cannot be completely explained or understood. Latent Variable 1 was negatively correlated with yield response. Because the correlation of temperature and rainfall with soil organic matter, Ca, and CEC was negative (most likely a random result), it is not possible to speculate on reasons for the relationship between this latent variable and yield response. The negative correlations of Latent Variables 2, 3, and 6 with yield response suggests that responses were higher when P and K availability and plant growth were low. A multiple regression model of yield response on these three latent variables explained 27% of the variation in responses across sites. The fact that estimates of P or K availability and dry weight were in different latent variables suggests that growth factors other than P or K availability were also influencing plant growth. The negative correlation of Latent Variable 6 (which included leaf P concentration and July rainfall) with yield response suggests that increased rainfall may have resulted in increased P availability later in the season and reduced response to early fertilization. These relationships suggest that factors that reduce growth and nutrient availability increase the likelihood of positive responses to foliar fertilization. A similar conclusion was observed in previous research (Haq and Mallarino, 1998) conducted with various rates and frequencies of application of only the 3–8–15 fertilizer.


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  		Table 7 Factor analysis and regressions of yield response on identified latent variables across sites

 

   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
REFERENCES
 
Foliar fertilization of soybean with macronutrients at early growth stages increased yield at some sites, decreased yield at others, but did not affect yield at most sites. Although differences between treatments were inconsistent across sites and years, a single application of 28 L ha-1 of the 3–8–15 fertilizer was as successful as other fertilizer mixtures or rates in increasing yields and did not produce leaf injury. The infrequent positive yield response could possibly be explained by the mostly optimum or above-optimum soil test P and K of the soils. Multivariate and regression analyses across sites showed that 27% of the variation in yield responses was explained by a complex combination of plant growth, N, P, and K uptake, and rainfall in July. Yield responses are more likely when all these variables are low. Reduced growth and P or K uptake were not necessarily related to low soil-test P and K, however, because soils of two responsive sites tested within or above values considered optimum for soybean. The negative responses at some sites were not related to leaf injury and could not be explained. Use of foliar fertilization of soybean at early growth stages across all production conditions will seldom result in average yield responses that would offset application costs. The probability of response is increased by soil or weather conditions that reduce growth and soil nutrient availability early during the growing season.Poole Randall Ham 1983


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
Iowa Agric. Home Econ. Exp. Stn. Journal Paper no. J-18088. Project 3233.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 




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