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© 2003 Plant Management Network.
Accepted for publication 29 October 2003. Published 9 December 2003.

Relationships Among Forage Yield and Quality Factors of Hay-Type Sorghums

Joseph L. Moyer, Southeast Agricultural Research Center, P. O. Box 316, Parsons, KS 67357; John O. Fritz, Department of Agronomy, and James J. Higgins, Department of Statistics, Kansas State University, Manhattan, KS 66506

Corresponding author: Joseph L. Moyer. jmoyer@oznet.ksu.edu

Moyer, J. L., Fritz, J. O., and Higgins, J. J. 2003. Relationships among forage yield and quality factors of hay-type sorghums. Online. Crop Management doi:10.1094/CM-2003-1209-01-RS.

Abstract

Sudangrass [Sorghum bicolor (L.) Moench] and various sorghum lines are used for hay production because of their production potential. Most performance evaluations emphasize yield and agronomic characteristics, but seldom include forage quality. Hay-type sorghum cultivars were grown in five independent trials to test yield and forage quality. First-cut (boot stage to head emergence) forage was harvested for yield and subsampled for one or more quality components. The earliest trial tested forage crude protein (CP) and more recent trials included assays of leaf:stem ratio, and leaf, stem, and total forage concentrations of CP, neutral-detergent fiber (NDF), and acid-detergent fiber (ADF). Pearson correlation coefficients within trials and pooled correlations across trials indicated relationships of forage yield and quality factors. Forage CP was negatively correlated with yield in each trial, with a pooled value of -0.52 (P < 0.001). Leaf:stem ratio was not strongly related to yield or laboratory quality. Stem components’ concentrations of CP and fiber were closely related to their concentrations in the total forage. Negative associations found between forage yield and quality factors emphasized the need to obtain quality as well as yield information for sorghum lines for hay production.

Introduction

Sudangrass, other sorghums, and sorghum-sudangrass hybrids are grown on about 1.5 million acres of cropland in the United States, some of which are for hay (2). They are warm-season annuals with production potential similar to that of corn but generally of lower forage quality. Performance evaluations have emphasized yield and other agronomic characteristics but usually not forage quality, providing what may be a poor index of feed value (5). Emphasizing yield without considering quality doesn’t provide a complete picture of cultivar performance. Quality, in terms of high nutrient content and fiber of low concentration and/or high utility for ruminants, is an important aspect of hay production.

Pedersen et al. (11) measured ensilage yield and quality of 49 sorghum hybrids for 2 years and reported correlations among pairs of quality traits. They found relatively high negative correlations between forage sorghum in vitro digestibility and neutral-detergent fiber (NDF), acid-detergent fiber (ADF), and acid-detergent lignin. Highly positive relationships between NDF and ADF, and ADF and acid-detergent lignin were reported. Dry matter yield was measured in the study, but no correlations with yield were reported.

Relationships between yield and quality traits of 12 forage sorghum hybrids harvested for ensilage were reported by Sanderson et al. (12). Leaf, stalk, and panicle proportions were determined for 3 years and correlated with yield, as were crude protein, ADF, acid-detergent lignin, and in vitro true digestibility. Yield was negatively correlated with crude protein and digestibility in two of the three years, but was positively correlated with stalk proportion in the same two years, and with acid-detergent lignin in two other years.

Sanderson et al. (12) also found relationships among traits besides yield. In vitro digestibility and crude protein were positively correlated, but each was negatively correlated with ADF and acid-detergent lignin concentrations. Leaf proportion was positively correlated with ADF in each year, but no other morphological traits were consistently associated with forage quality traits.

Relationships among forage yield and quality factors are not well understood. Emphasizing yield without considering forage quality may have undesirable consequences on forage quality. We studied the relationships among yield and several quality characteristics using private and public cultivars of hay-type sorghums with known backgrounds that were released over a 40-year period. Data from the first cutting of five independent trials were used to indicate factors besides yield that might be appropriate to consider when developing or selecting sorghum lines for hay production.

Procedures and Conditions for Testing Forage Quality and Yield

Cultivars of sorghum spp. were tested at the Mound Valley Unit of the Southeast Agricultural Research Center, Parsons, KS for forage yield and quality in five trials, with the first established in 1977 and the last trial initiated in 1998. Soil at the trial site is a Parsons silt loam (Mollic Albaqualf, fine, mixed, thermic). Cultivars in each trial for which a release date could be obtained were used in this study. Cultural practices and test conditions for each trial are listed in Table 1. Precipitation and temperature data for the test years and averages for the Unit are in Table 2. In 1981, hard rains after seeding slowed emergence and developing pest problems necessitated using 2,4-dichlorophenoxyacetate (2,4-D; 1 lb/acre a.i.) for broadleaf weed control and dimethoate (Cygon; 0.5 lb/acre a.i.) for control of greenbugs (Schizaphis graminum (Rond.)). Preplant fertilizer rates were determined according to the soil test and yield potential of each site. In 1977, a Planet Junior was used to seed 30 lb/acre in five 12-inch rows that were 30 ft in length. After 1977, a belt-cone seeder was used to plant about 450,000 live seeds per acre at 0.3- to 1-inch depth in plots of six 10-inch rows, 30 ft in length.

Table 1. Cultural and test conditions used for each of five trials of Sorghum spp. hay production and quality conducted between 1977 and 1998.

¹ Number of hay-type sorghums entered/number with release information that could be used in the analysis.

² Rep 4 harvested July 5 after rainout; subsamples from Reps 1 and 2 were used for crude protein analysis.

Table 2. Precipitation and air temperature for 3 months (1977, 1981, 1986, 1993, 1998, and 30-year average) at the Mound Valley Unit, Kansas State University - Southeast Agricultural Research Center.

† 30-year average, 1971-2000.

The first harvest was cut to a 4-inch stubble height when all cultivars were between the boot stage and head emergence (Table 1). Some harvests were later than desired because of wet conditions in June (Table 2). Only the first harvest is reported here because it produced the greatest yields, was most similar in maturity, and varied most in quality. Except in 1993, 3-ft-width flail harvesters were used to cut a 12- to 20-ft length of plot, depending on the amount of fresh herbage produced. Subsamples of the chopped material were dried at 130 to 140°F to determine moisture content and saved for further assay. In 1993, a sickle mower was used to harvest a 15- by 1.7-ft area (two rows) because plant heights were excessive for the flail harvester. Beginning in 1981, subsamples of at least four plants per plot were hand-separated into leaf and stem+sheath components and dried for calculation of leaf:stem ratios. In 1993 and 1998, subsamples of each component were dried and ground for crude protein, NDF, and ADF analyses.

Subsamples of whole plants or leaf and stem components were ground in a Wiley mill to pass a 0.04-inch screen. Crude protein was determined for all plots in each trial, except that only two replications were assayed in 1977. Samples were digested in H₂SO₄-H₂O₂ (6), assayed for N by the colorimetric technique of Crooke and Simpson (1), and the N concentration multiplied by 6.25. Fiber analyses of components were performed beginning in 1986 by the method of Goering and Van Soest (3).

In each trial, plots were arranged in a randomized complete block design with four replications, except in 1981 when only three replications were used. Data were analyzed using SAS procedures (SAS Institute, Cary, NC). The general linear model procedure was used to analyze variances, and means were compared with Fisher’s protected LSD. Pearson product-moment correlation coefficients among variables were calculated using SAS’ correlation procedure. Pooled correlations were obtained by the procedure of Hedges and Olkin (4), wherein Fisher's z-transformation and reciprocal variances were used to compute a weighted average, which is known to be statistically optimal.

Association Between Forage Yield and Quality Factors

Data for sorghum cultivars for which release dates were available from 1977 and 1981 trials are in Tables 3 and 4. Data from the 1986 and 1993 trials are in Moyer (7,8). The 1998 trial’s forage yield, leaf:stem, and crude protein data are in (9), and forage fiber concentrations are in (10).

Table 3. First-cut yield and forage crude protein concentration of
sorghum spp. grown for hay in 1977, Mound Valley Unit, KSU-
Southeast Agricultural Research Center.

*  Abbreviations: SX, sorghum-sudangrass hybrid;
SS, sudangrass hybrid;
S, sudangrass variety;
DM, dry matter.

Table 4. First-cut yield, leaf:stem ratio, and forage crude protein
concentration of sorghum spp. grown for hay in 1981, Mound Valley
Unit, KSU-Southeast Agricultural Research Center.

 † Abbreviations: SX, sorghum-sudangrass hybrid;
SS, sudangrass hybrid;
S, sudangrass variety;
DM, dry matter.

The range in first-cut forage yields of entries included in the analyses compared to the experimental mean was 58% in 1977, 104% in 1981 (Tables 3 and 4), 37% in 1986, 51% in 1993, and 61% in 1998. Mean yields of included entries averaged 1.48, 3.64, and 2.67 tons dry matter per acre in 1986, 1993, and 1998, respectively. Sudangrass hybrids and especially sudangrass varieties, though too few to test, usually yielded numerically below the experimental mean. Pedersen et al. (11) found a yield range of 38% of the average for 49 forage sorghum hybrids over a 2-year period, and Sanderson et al. (12) found a 35% range in yield of 12 hybrids.

First-cut forage crude protein concentrations relative to the experimental mean ranged from 16% for 1993 and 1998 leaf crude protein to 42% for 1998 stem crude protein (8,9). Crude protein concentrations of the total forage ranged from 21% of the mean in the 1993 trial (8) to 35% in 1986 (7). Mean crude protein of total forage for included entries averaged 17.8, 9.6, and 9.8% for 1986, 1993, and 1998, respectively. Sudangrass hybrids and varieties sometimes had crude protein numerically above the experimental mean, but less consistently than when their yields were below the mean. Pedersen et al. (11) reported a range in crude protein of 38%, whereas Sanderson et al. (12) reported a range of 28% of the average, a value similar to the ranges found in our analyses. The ranges in leaf:stem ratio of entries for first-cut forage relative to the experimental mean were from 69% in the 1986 test (7) to 128% in 1981 (Table 4). Leaf:stem ratio was not assessed in 1977. Mean ratios of included entries averaged 1.43, 0.56, and 0.81 in 1986, 1993, and 1998, respectively. Sanderson et al. (12) also reported a range of 82% of the mean in leaf proportion, which is intermediate to the ranges obtained in these trials.

Total forage NDF concentration ranged from 7 to 9% of test averages and leaf and stem NDF ranged from 8 to 11% of the test mean (8,10). Mean NDF of included entries averaged 71.4, 67.6, and 56.9% for 1986, 1993, and 1998, respectively. The range in NDF of forage sorghum in the test reported by Pedersen et al. (11) was 22% of the test mean.

Total forage ADF concentration ranges were 7% of the test mean in 1993 (8) and 14% in 1998 (10). Mean ADF of included entries averaged 44.4% in 1993 and 31.7% in 1998. Stem ADF concentration ranges were 9% of the mean in 1993 and 17% in 1998. Variations in leaf ADF concentrations were intermediate to variability of total and stem ADF. In tests with forage sorghums, ADF concentrations relative to test means were 33% (11) and 28% (12).

In these trials, harvest occurred before reproductive structures were exerted. In the studies previously reported (11,12) forage sorghums were harvested during grain formation. The harvest maturity and the amount of grain in the sample would affect fiber and protein concentrations, and perhaps the test ranges in those studies. Grain would tend to reduce the fiber concentration of samples, and variations in the amount of grain produced by different cultivars could increase the variation in apparent fiber concentration among cultivars in the test.

It is important to evaluate relationships among first-cut yield and forage characteristics to provide a complete performance picture for cultivars. The relationship between yield and crude protein concentration is shown for each trial and for the pooled data in Fig. 1. In 1977, the correlation between first-cut yield and crude protein concentration was not significant (P > 0.10) . However, in 1981, 1986, 1993, and 1998 there were negative correlations between first-cut yield and crude protein concentration, such that the pooled correlation was highly significantly negative (Fig. 1). Pooled correlations of yield with crude protein of leaf and especially stem components (1993 and 1998 data, Table 5) were also negative.

Table 5. Pooled correlation coefficients (r) across trials
among first-cut yield, forage crude protein concentration
and other quality traits in different fractions of sorghum
cultivars, Mound Valley Unit, KSU - Southeast
Agricultural Research Center.

*,**,†,‡ Significant at the 0.10, 0.05, 0.01, and 0.001
probability levels, respectively.

Sanderson et al. (8) reported that yield in forage sorghums was negatively associated with crude protein in 2 of 3 years. This was similar to the most consistent association found in our study, where four of five trials produced a negative relationship between yield and crude protein, and the pooled negative correlation was highly significant (P < 0.01). Pedersen et al. (7) did not report any correlations with yield in forage sorghums that they tested, but inspection of their data indicates the possibility of a negative relationship between yield and crude protein. Their data also indicated low GCA effects for forage crude protein as compared to yield, suggesting that any genetic linkage between yield and crude protein may not be strong.

No association between yield and leaf:stem ratio in our pooled correlations was found (data not shown). Sanderson et al. (8) reported that the correlation of stalk proportion with yield was positive in two of the three years, but the relation of leaf proportion with yield was inconsistent, and no relationship was found between yield and panicle proportion.

Relationships among forage crude protein and other quality traits were observed (Table 5). A positive pooled correlation between leaf:stem ratio and stem and total crude protein was found, as well as expected positive correlations among crude protein of the components, particularly between stem and total crude protein. Leaf crude protein was negatively correlated with leaf NDF and leaf ADF, and total crude protein was negatively correlated with total NDF and ADF. Stem crude protein was less strongly negatively correlated (P < 0.10) with stem and total ADF, but not NDF. A slight (P < 0.10), inexplicable positive association between leaf crude protein and stem NDF also appeared (Table 5).

Positive correlations of yield with both total ADF and NDF were found (Table 6). Acid-detergent fiber concentration was positively correlated with yield in the first of a three-year study by Pederson et al. (11), acid-detergent lignin concentration was positively correlated with yield in two of the three years, and in vitro true digestibility was negatively correlated with yield in two of three years. Such negative relationships of yield and forage quality may negate much of any yield advantage for a particular cultivar.

Table 6. Pooled correlation coefficients (r) across trials among first-cut forage yield, leaf:stem ratio, and fiber concentrations in different fractions of sorghum cultivars, Mound Valley Unit, KSU - Southeast Agricultural Research Center.

*,†,‡ Significant at the 0.10, 0.05, and 0.001 probability levels, respectively.

Leaf:stem ratio was negatively correlated with total ADF, but was not significantly associated with concentration of any other fiber constituent (Table 6). Thus, we inferred that leaf:stem ratio was a weak and inconsistent indicator of fiber composition as well as crude protein. Pedersen et al (11) found that leaf proportion was associated only with ADF, so they concluded that morphological composition is a poor predictor of laboratory-measured quality traits.

Neutral- and acid-detergent fiber were closely related within plant components (Table 6). That is, leaf NDF and ADF were positively correlated as were stem NDF and ADF. Stem NDF and ADF were also positively correlated with total NDF and ADF. However, there was no positive relationship between leaf NDF or ADF with stem or total concentration of either NDF or ADF. Pedersen et al. (11) found relatively high correlations between forage sorghum NDF and ADF, which were negatively related to in vitro dry matter digestibility. Sanderson et al. (12) found similar relationships among digestibility, NDF, and ADF in forage sorghum, but they also indicated that acid-detergent lignin as well as ADF and digestibility could be reliable indicators of forage quality.

Kalton (5) observed from silage trials that the highest-yielding sorghums tended to be lowest in digestibility and protein content and highest in fiber content despite the fact that tremendous diversity existed in these traits among entries. Selection of lines for development or use in hay production should consider those with higher crude protein and/or lower fiber content, along with high yielding ability, to improve overall feeding value.

Acknowledgments

Partial support of the experiment was obtained from testing fees paid by seed companies. The authors acknowledge Dr. S. C. Fransen for conducting the 1977 test. Thanks are expressed to Dr. W. W. Hanna for supplying cultivars for testing. Technical assistance of Charles King, C. M. Cramer, K. R. McNickle, T. Erikson, and G. Couch was appreciated. Thanks are expressed to Dr. Bruce Maunder, Lee Leonard, and other commercial breeders for helping identify origins of cultivars. The authors acknowledge advice and comments of the late Dr. R. R. Kalton. (Contribution No. 03-73-J of the Kansas Agric. Exp. Stn., Manhattan, KS 66506.)

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