Published in Agron. J. 96:1453-1458 (2004).
© American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
Forages
Trends in Forage Yield and Nutritive Value of Hay-Type Sorghum spp.
J. L. Moyera,*,
J. O. Fritzb and
J. J. Higginsb
a Kansas State Univ., Southeast Agric. Res. Cent., P.O. Box 316, Parsons, KS 67357
b Dep. of Stat., Kansas State Univ., Manhattan, KS 66506
* Corresponding author (jmoyer{at}oznet.ksu.edu)
Received for publication November 26, 2003.
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ABSTRACT
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Sudangrass [Sorghum bicolor (L.) Moench], other Sorghum spp., and sorghum–sudangrass hybrids are used extensively for hay on U.S. cropland because of their production potential. Performance evaluations have emphasized yield and other agronomic characteristics but seldom emphasized forage quality. Available hay-type sorghum cultivars released over a period of four decades were grown in five trials over a 21-yr period to evaluate trends in forage yield and nutritive value. First-cut (boot stage to head emergence) forage was harvested for yield and subsampled for one or more quality components. The first trial in 1977 tested forage crude protein (CP), and subsequent trials included more quality traits. In 1993 and 1998, the traits measured included leaf/stem ratio and leaf and stem concentrations of CP, neutral-detergent fiber (NDF), and acid-detergent fiber (ADF). Pearson correlation coefficients by year and correlations pooled across trials by two methods indicated similar relationships between forage characteristics and year of cultivar release. Year of release was positively associated with forage yield in two of the five trials and for the pooled correlations. Forage CP and CP of leaf and stem fractions were negatively correlated with year of cultivar release for the pooled trials and in most individual trials. Leaf NDF and ADF were positively correlated with year of cultivar release in the pooled trials. Yield gains of about 0.5% per year were found, but concomitant relative reductions of 0.1% or more per year in forage CP and leaf NDF run counter to the perceived need for improved hay quality. These findings emphasize the need to locate and incorporate cost-effective sources of germplasm to enhance nutritive value of forage in hay-type Sorghum spp.
Abbreviations: ADF, acid-detergent fiber • CP, crude protein • NDF, neutral-detergent fiber
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INTRODUCTION
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SUDANGRASS, other Sorghum spp., and sorghum–sudangrass hybrids are grown on 60000 ha of U.S. cropland, some of which are for hay (Fribourg, 1995). They are warm-season annuals with forage production potential similar to that of corn (Zea mays L.) but generally of lower nutritional quality. Thus, cultivar performance evaluations have emphasized yield and agronomic characteristics such as height and bloom date but usually not forage quality, providing what may be a poor index of feed value (Kalton, 1988). However, quality in terms of high nutritive value and fiber of low concentration and/or high utility for ruminants is an important aspect of hay production.
Much has been done to improve the sudangrass germplasm introduced to the USA in the early 1900s. Since the 1950s, sudangrass has been hybridized with other Sorghum spp. to increase forage productivity. A survey of breeders of sudan-type Sorghum spp. by Kalton (1988) showed that selections made were primarily for total yield, leafiness, digestibility, regrowth capacity, disease resistance, and low prussic acid content. A few respondents mentioned selecting sudangrass types for higher seed yield in seed production fields. Realistically, seed production of any commercial cultivar must be high enough to enable competitive pricing of seed. Despite apparent variability among cultivars, the majority of sorghum–sudangrass lines used in the USA resulted from crosses of ‘Redlan’ x ‘Greenleaf’ (Harvey, 1977). This has limited the genetic variability of sorghum–sudangrass lines that currently exists in the marketplace.
Changes in yield potential over time have been studied in a number of crops. However, no studies have been found to date that dealt with changes in yield or nutritive value of hay-type sorghum forage. The change in grain yield of dryland Sorghum spp. in the southern Great Plains between 1957 and 1997 was estimated by Unger and Baumhardt (1999). By using historical yields in a 40-yr uniformly managed trial, they determined that 46 of the 139% yield increase was attributable to improved hybrids, or about 1.2% per year. Miller and Kebede (1984) demonstrated genetic gains in grain yield increases of 1 to 2% per year in Sorghum spp. grown in the same trials by comparing yield and yield components of three hybrids from the 1950s era with three from the 1980s. No reports were found of studies with forage or hay-type Sorghum spp. Corn forage as well as grain production trends were measured recently by Duvick (1996). When one variety was grown with 36 corn hybrids from different eras, only a slight positive trend (nonsignificant) toward increased fodder was found to accompany the increased grain yield potential. However, Lauer et al. (2001) found that yields of whole-plant corn forage and stover increased in cultivars that ranged in era from 1930 to 1998. Concomitantly, whole-plant forage nutritive traits were generally increased or unchanged, but with ears removed, nutritive traits of stover generally declined.
Retrospective studies of changes in crop cultivar characteristics have been performed in various ways (Slafer et al., 1994). In the studies cited previously, most used common entries grown in tests over different years and/or locations (Miller and Kebede, 1984; Duvick, 1996; Lauer et al., 2001). However, Unger and Baumhardt (1999) used a progression of different hybrids over time under constant management, factored out climatic effects, and attributed the residual gain to the use of improved hybrids. The use of pooled data from different experiments is another way to obtain estimates of genetic effects on changes in crop performance. Slafer et al. generalized that genetic gain (the slope of regression of a variable against the year of cultivar release) could be standardized in relation to the mean of the experiment to result in what they termed relative genetic gain. Using that approach, they found that relative genetic gains from 12 experiments by wheat (Triticum aestivum L.) breeding programs in nine countries approached 0.45% per year in all but two exceptional cases.
Pooling regressions from similar trials can be accomplished using metastatistical procedures such as one described in Hedges and Olkin (1985). More typically, some form of indexing is used wherein a variable is expressed relative to the mean of check cultivars common to each trial, such as one used on cotton (Gossypium hirsutum L.) trials by Meredith and Bridge (1984). They used three methods of adjusting strain performances in 15 advanced strain trials to those of included check cultivars, one method being adjustment to the average check yield, and all gave similar results. More recently, Kawano (2003) measured breeding progress in cassava (Manihot esculenta Crantz) by using the mean yield of three control cultivars in all advanced yield trials to express yields of all entries. He reasoned that comparisons relative to the mean of the controls should give an unbiased estimate of progress.
We studied the entries of hay-type Sorghum spp. submitted by commercial and public sources, which we assumed to be the best available, plus three checks and other cultivars of historical significance for forage yield and a few characteristics that measure nutritive value. The cultivars spanned more than four decades. The objective was to determine whether any trend in yield and nutritive value occurred during that time for hay-type Sorghum spp. vs. the null hypothesis that no change occurred.
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MATERIALS AND METHODS
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Cultivars of Sorghum spp. were tested at the Mound Valley Unit of the Southeast Agricultural Research Center for forage yield and nutritive value in five trials since 1977. Cultivars in each trial for which a release date was obtained were used in this study, along with three check cultivars: ‘Greenleaf’, ‘Piper’, and ‘NB 280-S’. The source, type, and year of release for each cultivar are listed in Table 1. Soil at the trial site is a Parsons silt loam (fine, mixed, thermic Mollic Albaqualf). Cultural practices used for each trial were listed previously (Moyer et al., 2003), as well as precipitation and temperature data for the test years and averages for the site.
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Table 1. Cultivars of Sorghum spp. tested, years of release, type, source, and years tested at the Mound Valley Unit, Kansas State University Southeast Agricultural Research Center.
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Only the first harvest was used in these analyses because it produced the greatest and most consistent yields, was most similar in growth stage, and its yield and forage nutritive traits were least likely affected by drought. Except in 1993, 0.9-m-width flail harvesters were used to cut a 3.7- to 6.1-m length of plot, depending on the amount of fresh mass produced. Subsamples of the chopped material were dried at 50 to 60°C to determine moisture content and saved for further assay. In 1993, a sickle mower was used to harvest a 4.6- by 0.5-m 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 CP, NDF, and ADF analyses.
Subsamples of whole plants or leaf and stem components were ground in a Wiley mill to pass a 1-mm 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 (Linder and Harley, 1942) and assayed for N by the colorimetric technique of Crooke and Simpson (1971), and the N concentration was multiplied by 6.25. Fiber analyses of components were performed beginning in 1986 by the method of Goering and Van Soest (1970).
In each trial, plots were arranged in a randomized complete block design with four replications, except in 1981 when there were three replications. Data were analyzed using SAS procedures (SAS Inst., 1988). The GLM procedure was used to partition variance into block and cultivar effects, and significance was tested using the residual error term. Means were compared with Fisher's protected LSD. Pearson product-moment correlation coefficients between cultivar means and their year of release were calculated for each test using CORR. Pooled correlations were obtained by (i) the metastatistical procedure of Hedges and Olkin (1985) wherein Fisher's z-transformation and reciprocal variances were used to compute a weighted average that is known to be statistically optimal and (ii) indexing cultivar means relative to the average of the check means of each test, pooling the indexed means across trials (Meredith and Bridge, 1984), and then calculating the correlation with year of release. The REG procedure was used to calculate regression coefficients of variables against year of cultivar release for those variables that showed significant correlations with year of release.
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RESULTS AND DISCUSSION
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Pearson product-moment correlation coefficients (r) between year of release and first-cut forage yield and nutritive traits of Sorghum spp. cultivars are in Table 2. First-cut forage yield was positively related to year of cultivar release in the 1986 and 1993 tests although positive coefficients were obtained in each test. The pooled correlation by the metastatistical method of Hedges and Olkin (1985) was highly significant (P < 0.001), as was the correlation coefficient obtained using yield means indexed to the yields of check cultivars. This indicates a positive trend of yield over the years of cultivar release included in the tests. Corn trials also showed positive relationships between forage and stover yield and mean era of cultivar use (Lauer et al., 2001).
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Table 2. Pearson correlation coefficients (r) between year of release and first-cut forage yield and quality traits of Sorghum spp. cultivars in five tests and correlations pooled by a metastatistical method (M-S; Hedges and Olkin, 1985) or by indexing means to check means (indexed), Mound Valley Unit, Kansas State University Southeast Agricultural Research Center.
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Leaf/stem ratio was determined in four of the five tests, but no relationship was found with year of release in any test nor in the pooled correlations (Table 2). Sanderson et al. (1994) found that leaf proportion was associated somewhat positively with ADF but concluded that morphological composition is a poor predictor of laboratory-measured quality traits. We did not find leaf/stem ratio related to ADF, but it was positively associated with stem CP in the 1998 trial, total CP in the 1993 and 1998 trial, and leaf NDF in one of the two trials in which it was determined (Moyer et al., 2003). Only one of four trials (1998) showed association of leaf/stem ratio with yield (negative), so we did not find leaf/stem ratio to be a reliable indicator of yield or laboratory quality.
Crude protein concentration of forage was negatively related to year of cultivar release in most instances (five of nine, Table 2). Both the pooled correlations and the correlations obtained from indexed means pooled across trials were highly significantly negative for total forage CP as well as for leaf and stem fractions in the 1993 and 1998 tests; the most negative correlation was with leaf CP. This was consistent with the findings that yield and CP were usually negatively related (Moyer et al., 2003; Sanderson et al., 1994). In corn forage and stover, CP concentration has not changed significantly since 1930 (Lauer et al., 2001). However, grain protein percentage showed a linear reduction in cultivars released since 1960 (Duvick, 1996). Newer, high-yielding hybrids may have relatively more capacity for dry matter accumulation compared with their capacity for N uptake and assimilation, their efficiency of N use, or both. Wheat grain protein reductions have historically occurred as yields increased, but Slafer et al. (1994) suggested that the negative relationship of the two characteristics is weak, so genotypes with both high yield and high grain protein content might be selected.
Leaf NDF concentrations were positively related to the year of cultivar release in both 1993 and 1998 trials, in the pooled correlation, and the correlation from indexed data pooled across trials (Table 2). Total NDF was positively correlated to year of release in the 1986 test but not in the later tests, the pooled correlation, or the correlation of indexed means. Stem NDF was not related to year of release. Leaf ADF concentration was positively related to the year of cultivar release in the 1998 trial, the pooled correlation, and the correlation of indexed means, but neither total nor stem ADF were related to year of release (Table 2). Lauer et al. (2001) found that corn forage ADF and NDF of cultivars have decreased over time while in vitro true digestibility increased. At the same time, however, corn stover ADF increased and digestibility decreased. These antithetical trends were because the corn forage included the highly digestible, low-fiber grain, which accounted for much of the increase in whole-plant yield.
These trends in forage CP and fiber run counter to the perceived need for improved nutritive value of hay. Kalton (1988) observed from silage trials that the highest-yielding Sorghum spp. tended to be lowest in digestibility and protein content and highest in fiber content despite tremendous diversity in these traits among entries. He noted that breeding research to date has centered primarily on yielding ability and related traits with only limited attention to feeding value. Lauer et al. (2001) indicated the same situation in corn breeding where little change in stover quality over time has resulted from lack of attention by breeders. They say that "it is easier for breeders to select for yield or quality improvement, but difficult to breed for both."
Regressions from pooled indexed data were plotted for traits that were correlated (P < 0.01) to year of cultivar release (Fig. 1 and 2). As indicated in the regression coefficients, yield gains of 0.7% per year were recorded (Fig. 1A). By comparison, yield gains for corn forage reported by Lauer et al. (2001) amounted to about 1.0%, with gain in stover yield about 0.6%. Conversely, Duvick (1996) found no trend in corn fodder weight per plant.
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Fig. 1. Relationship between year of cultivar release and (A) yield or (B) crude protein (CP) concentration relative to the mean of the control cultivars in each trial. Fit of the regression equations was highly significant (see correlation coefficients in Table 2).
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Fig. 2. Relationships between year of cultivar release and leaf neutral-detergent fiber (NDF), leaf crude protein (CP) concentration, and stem CP concentration relative to the mean of the control cultivars in each trial. Fit of the regression equations was highly significant (see correlation coefficients in Table 2).
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Those gains found for forage yield were comparable to those reported for some grain crops. Average wheat yields of varieties released between approximately 1874 and 1987 showed a genetic improvement rate of 0.7% per year (Cox et al., 1988). Soybean cultivars increased by 0.7% per year in the southern USA between 1942 and 1973 (Boerma, 1979) and by 0.5% per year in short-season cultivars between 1934 and 1992 (Voldeng et al., 1997). However, corn grain yields of lines released from 1930 to 1991 increased by an average of approximately 1.3% per year (Duvick, 1996). Sorghum grain yield increases of hybrids in the 24 yr before 1981 averaged about 1.6% per year (Miller and Kebede, 1984). Grain yield increases for dryland sorghum in the Texas high plains due to genetics were calculated to be 1.2% per year from a uniformly managed study conducted from 1956 through 1997 (Unger and Baumhardt, 1999). Thus, our estimates of yield gain for sorghum hay production and corn forage are about half those of gains in grain yield of corn and sorghum.
Concomitantly, losses in factors associated with nutritive value were recorded, as illustrated by regression coefficients for traits that related to year of release: CP, leaf NDF, and leaf ADF. For CP, relative decreases ranged from 0.2% per year in leaf CP (Fig. 2) to 0.3% per year in total forage CP (Fig. 1B). Over the 44 yr of cultivar releases represented in these trials, actual reductions ranged from 9 to 44 g CP kg–1 of forage, amounting to relative reductions of 15 to 25%. However, the data illustrated (Fig. 1A and 1B) indicate that recent, high-yielding cultivars exist that are above average in CP concentration.
Concentrations of leaf NDF (Fig. 2) and ADF (data not shown) increased by a relative 0.1% per year. These rates represent absolute increases ranging from 18 to 30 g fiber kg–1 of forage or a relative increase of 4.2 to 6.1% over the years of cultivar release. Lauer et al. (2001) found that corn stover NDF in the early maturing trials increased at the rate of 0.05% per year and in vitro true digestibility in the late-maturing trials decreased by 0.02% per year.
Changes of corn grain quality factors in hybrids released from 1930 to 1991 were found by Duvick (1996). Grain protein declined by 0.3 g kg–1 yr–1, but grain starch increased at the same rate, and there was no change in grain oil. Seed protein levels of short-season soybean cultivars released between 1934 and 1992 declined 0.4 g kg–1 yr–1, but seed oil level increased by the same amount, indicating that the commercially important sum of protein plus oil content was increased concurrently with seed yield (Voldeng et al., 1997).
During the four-decade period of cultivar release represented by the sorghum hay trials conducted here, some progress in forage yield was indicated, but forage nutritive value appeared to deteriorate. The general decline in forage CP and increase in NDF with year of cultivar release indicate that factors other than forage nutritive value were considered when decisions were made regarding hay-type sorghum germplasm development, release, and/or purchase. Gains in traits that indicate nutritive value must be dramatic and cost effective for adoption. Whether the increase be from traits such as the brown midrib mutation (Porter et al., 1978; Kalton, 1988), the use of biotechnology (Kalton et al., 1989), or conventional breeding efforts, traits should be exploited to promote gains in nutritive value along with yield.
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ACKNOWLEDGMENTS
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Partial support of the experiment was obtained from testing fees paid by the seed companies listed in Table 1. 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 and dates for cultivars. The authors acknowledge advice and comments of the late Dr. R.R. Kalton.
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NOTES
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Contribution no. 04-307-J of the Kansas Agric. Exp. Stn., Manhattan, KS 66506.
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ABSTRACT
INTRODUCTION
