Published online 1 January 2007
Published in Agron J 99:195-202 (2007)
DOI: 10.2134/agronj2006.0047
© 2007 American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
Forages
Structural Composition, Growth Stage, and Cultivar Affects on Kentucky Bluegrass Forage Yield and Nutrient Composition
Johnathon D. Holmana,*,
Carl Huntb and
Donn Thillc
a Southwest Research and Extension Center, Kansas State Univ., Garden City, KS 67846
b Dep. of Animal and Veterinary Science, Agricultural Science Building, Univ. of Idaho, Moscow, ID 83844
c Dep. of Plant, Soil and Entomological Sciences, Agricultural Science Building, Univ. of Idaho, Moscow, ID 83844
* Corresponding author (jholman{at}ksu.edu)
Received for publication February 13, 2006.
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ABSTRACT
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Kentucky bluegrass (Poa pratensis L.) is an important turf and forage grass, yet there is little information on its forage yield and nutrient composition. This study evaluated the effect of Kentucky bluegrass cultivar (Ascot, Kenblue, Limousine, and Touchdown), growth stage (boot, anthesis, and seed ripening), and structural composition (percentage head, leaf, and stem) on forage yield and nutrient composition. Nutrient composition measurements included dry matter, crude protein (CP), acid detergent fiber (ADF), neutral detergent fiber (NDF), lignin, and 48-h in vitro true digestibility (IVTD). Tall cultivars had the greatest yield, and yield tended to be greatest at seed ripening. Structural composition varied among cultivars due to differences in cultivar height and yield. Nutrient composition was not different among cultivars despite differences in structural composition among cultivars and nutrient composition differences among structural components. Leaves were greatest in IVTD (636 g kg–1), stems were greatest in NDF (740 g kg–1) and ADF (430 g kg–1), and lowest in CP (30 g kg–1) and IVTD (455 g kg–1), and heads were lowest in ADF (269 g kg–1) and greatest in lignin (83 g kg–1). Nutrient level was greatest at boot and averaged 435 g kg–1 NDF, 197 g kg–1 ADF, 17 g kg–1 lignin, 165 g kg–1 CP, and 766 g kg–1 IVTD. On average between boot and anthesis, CP decreased 0.57% d–1 in 2003 and 0.21% d–1 in 2004, and IVTD declined 0.47% d–1 in 2003 and 0.49% d–1 in 2004. Nutrient level decreased as the stand aged due to an increase in structural carbohydrates and a decrease in IVTD.
Abbreviations: ADF, acid detergent fiber • CP, crude protein • CV, seed yield variability • IVTD, in vitro true digestibility • NDF, neutral detergent fiber
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INTRODUCTION
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KENTUCKY BLUEGRASS is a cool-season perennial grass primarily used in turf grass applications, however, it has also been used for erosion control practices, as a pasture grass, and harvested as forage (Wedin and Huff, 1996). Over 90% of the Kentucky bluegrass seed raised in the USA is produced in the Pacific Northwest (Idaho, Oregon, and Washington) (Holman and Thill, 2005a). This region is highly prone to soil erosion due to its steep topography and including a perennial crop like Kentucky bluegrass in the crop rotation helps reduce soil erosion. Kentucky bluegrass seed production has historically relied on postharvest field burning to sustain seed production. Stands usually remain productive for 6 to 10 yr in a burn system and only 3 to 4 yr in a nonburn system (Adams et al., 1976; Hinman and Schreiber, 2001; Holman and Thill, 2005b). Burning sustains seed production by removing thatch, and controlling insect, weed, and disease pests (Canode and Law, 1979; Holman and Thill, 2005a; Picha, 1976; Young et al., 1984). However, burning has been associated with negative air quality and human health issues, which has resulted in a ban on burning Kentucky bluegrass residue in Washington and burning restrictions imposed in Oregon and Idaho (Wulfhorst and Nielsen-Pincus, 2003).
Re-establishing a stand of Kentucky bluegrass costs approximately $568 ha–1 for dryland production and $988 ha–1 for irrigated production and increases the potential for soil erosion (Smathers, 2003a, 2003b). A significant cost of establishing Kentucky bluegrass is that it does not produce seed during the establishment year because it must complete fall floral induction and winter vernalization before producing seed (Sylvester and Reynolds, 1999). Growers need alternative reduced burn and nonburn methods that enable stands to be grown longer than the current 3 to 4 yr in a nonburn system to ensure Kentucky bluegrass production in the region is sustained (Hinman and Schreiber, 2001; Van Tassell, 2002).
Kentucky bluegrass is an important forage species grown on over 16.5 M ha of pasture in the northcentral and northeastern USA (Duell, 1985). Harvesting Kentucky bluegrass as a forage crop, rather than as a seed crop after seed yield has declined, would increase stand life one additional year. A preliminary economic analysis showed that harvesting Kentucky bluegrass as a forage crop might be profitable for some producers when forage prices are high and there is a local demand for the forage (Wolfley, 2006). The majority of cultivars raised in the Pacific Northwest are for turf grass seed and little information exists on the forage nutrient composition of these cultivars. The turf grass cultivars grown in the region vary considerably in height and growth habit, which might cause differences in forage nutrient composition among cultivars.
Forage yield and nutrient composition are affected by plant species, growth stage, cultivar, environment, and resource (light, nutrients, water, etc.) availability (Barnes et al., 2003; Bregard et al., 2001; Brown and Munsell, 1945; Kellems and Church, 2003; Mason and Lachance, 1983; Phillips et al., 1954). Forage yield and nutrient composition differences among cultivars were attributed to differences in plant maturity, biomass partitioning, fiber concentration, and structural composition (percentage head, leaf, and stem) (Bregard et al., 2001; Casler and Jung, 2006; Groot and Lantinga, 2004). Fiber concentration might be reduced by selectively breeding for cell walls with decreased lignin concentration and increased water-soluble carbohydrate concentration (Casler and Jung, 2006). Timothy (Phleum pratense L.) cultivars that partitioned a greater percentage of biomass into shoots rather than roots had increased forage yield, and cultivars with greater leaf/stem ratio, tiller density, and specific leaf area had greater CP concentration (Bregard et al., 2001). Differences in CP, NDF, and ADF concentration were reported among Kentucky bluegrass cultivars, however this was confounded by differences in cultivar maturity at the time of harvest (Durr et al., 2005). The IVTD and CP concentration of the Kentucky bluegrass cultivar Park was not affected by its stem and leaf composition, yet the forage nutrient composition of several other grass species were affected by their structural composition (Hockensmith et al., 1997). Information is needed on the effect of structural composition on forage yield and nutrient composition among Kentucky bluegrass cultivars harvested at the same growth stage.
Differences in height (short and tall) and growth habit (aggressive and nonaggressive) of Kentucky bluegrass cultivars might affect forage yield and nutrient composition (Bregard et al., 2001; NTEP, 2005). Aggressive cultivars have extensive rhizome growth that creates dense (sodded) stands enabling it to withstand traffic, but this growth habit might result in lower forage yield because of less resource allocation to aboveground biomass production. Taller Kentucky bluegrass cultivars might have a greater leaf/stem ratio, aboveground biomass production, and lignin concentration than shorter cultivars, resulting in lower forage nutrient level but greater forage yield. Knowing if the structural composition of Kentucky bluegrass cultivars affects forage yield and nutrient composition would help plant breeders and producers select superior forage cultivars.
The objective of this study was to determine the effect of four Kentucky bluegrass cultivars (two tall and two short), growth stage (R0, boot stage; R4, anther emergence/anthesis; and S5, endosperm dry/seed ripe) (Moore et al., 1991); and structural composition (percentage head, leaf, and stem) on Kentucky bluegrass forage yield and nutrient composition to provide grass seed producers information on selecting and harvesting Kentucky bluegrass as a forage crop.
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MATERIALS AND METHODS
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Experimental Procedures
A field experiment was established at the Washington State University Turfgrass Research Area at Pullman, WA, on 12 Sept. 2001. Seeds were mixed with rice (Oryza sativa L.) hulls at a ratio of 2:1 hulls to seed, and seeded 6 mm deep at 4.5 kg ha–1 with rows spaced 17.8 cm apart using a single-row planter. The soil is classified as a fine-silty, mixed, mesic, Pachic Ultic Haploxeroll (Soil Survey Staff, 1980). Each year, 120 kg N ha–1 was broadcast in mid-October as ammonium sulfate [(NH4)2SO4 (21–0–0, N–P–K)]. Postharvest residue was raked and removed from the plots each year following seed harvest. The experiment was a randomized complete block design with three replications. Individual plots were 2.4 m wide and 7 m long. The treatments included four Kentucky bluegrass cultivars (Ascot, Kenblue, Limousine, and Touchdown) harvested at three stages of maturity (R0, boot stage; R4, anther emergence/anthesis; and S5, endosperm dry/seed ripe) (Moore et al., 1991). The cultivars used in this study are classified as: short and moderately aggressive (Ascot), tall and nonaggressive (Kenblue), short and aggressive (Limousine), and tall and aggressive (Touchdown) (NTEP, 2005).
Field experiments were measured in 2003 and repeated in 2004. Plant height was measured with the plant fully extended from the base of the plant to the top of the inflorescence in four random locations per plot. Whole plants were harvested at 5 cm above the soil surface from four 0.063-m2 quadrats plot–1 during the boot and anthesis treatment timings, and from six 0.063-m2 quadrats plot–1 during the seed ripe treatment timing. Plant material was harvested at random within the plot, with the restriction that samples were collected at least 31 cm in from the plot edge to minimize edge effect. Plots were harvested when it was visually determined that 50% or more of the stand reached boot, anthesis, and seed ripe growth stages (Table 1). Harvesting at similar growth stages allowed for testing the effect of cultivar, while minimizing the effect of plant maturity, on forage yield and nutrient composition. Total biomass was dried in a forced-air oven at 60°C for 48 h and weighed. After weighing the dried plant material, three of the six samples collected at the seed ripe timing were selected at random and separated into head, leaf, stem, and seed, and weighed. The other three samples were used for whole plant nutrient composition analysis. Total plant material from the boot, anthesis, and seed ripe treatment timings, and the head, leaf, and stem material from the seed ripe treatment timing were analyzed for nutrient composition. The separated clean seed was used to determine seed yield. The affect of structural composition on nutrient composition was measured at the seed ripe timing only since individual plant components were only measured at seed ripe. Individual plant components were measured at seed ripe only since plant vegetative growth continues until the inflorescence is fully emerged (R3), which does not occur until after anthesis (R4) in Kentucky bluegrass. Measuring structural composition after the vegetative growth period allowed for testing the affect of structural composition on forage nutrient composition, while minimizing the effect of differences in plant growth rate.
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Table 1. Sampling dates of four Kentucky bluegrass cultivars (Ascot, Kenblue, Limousine, and Touchdown) at three growth stages (R0, boot; R4, anther emergence/anthesis; S5, endosperm dry/seed ripe) in 2003 and 2004 field experiments. Sampling dates were determined when 50% of the stand reached the above growth stages.
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Forage Nutrient Composition
Forage nutrient composition was determined using a 300-g subsample from a composite of the dried plant material collected from each plot. Nutrient composition measurements included dry matter, CP, ADF, NDF, lignin, and IVTD (48 h). Dry matter was used to correct forage yield to a dry matter basis.
Dried plant material was ground sequentially in Wiley and UDY mills (Thomas-Wiley Laboratory Mill, Model 4, Thomas Scientific U.S.A.; Cyclone Sample Mill, Model 310-030, Udy Corporation, Fort Collins, CO) using a 2-mm then 1-mm screen. Neutral detergent fiber and ADF were determined using an ANKOM Fiber Analyzer (Model No. ANKOM 200; Ankom Technology, Fairport, NY) (Goering and Van Soest, 1970; Vogel et al., 1999). In vitro true digestibility was determined using an ANKOM Rumen Fermenter (Model No: Daisy II; Ankom Technology, Fairport, NY) (Goering and Van Soest, 1970). Total N was measured using the macro Kjeldahl procedure (Association of Official Analytical Chemists, 1999) with a BUCHI Nitrogen Analyzer (Model No. B-324; Buchi Labortechnik AG, Switzerland). Total N was multiplied by 6.25 to estimate CP. Lignin was determined according to procedures described by Goering and Van Soest (1970).
Data were analyzed with a mixed model ANOVA using the general linear model procedure of SAS software (SAS Institute, 2001) with blocks as a random effect, and years and treatments as fixed effects. Main and interaction effects were tested for in the model. Mixed model MANOVA was used first to test for an overall effect of cultivar, growth stage, and structural composition on forage nutrient composition before testing treatment effects on individual nutrient components (dry matter, CP, ADF, NDF, lignin, and IVTD). Treatment effects were declared significant at P 
0.05, and when ANOVA indicated significance, significant effects means were compared using the protected Least Significant Difference test at P 0.05.
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RESULTS AND DISCUSSION
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Cultivar Characteristics
The taller cultivars, Kenblue and Touchdown, matured more quickly after boot than the shorter cultivars, Ascot and Limousine (Table 1). Harvesting cultivars at similar growth stages controlled for differences in plant growth rate and maturity. In this study plant height was not affected by year, but was affected by growth stage and cultivar. The reproductive growth stage of Kentucky bluegrass begins with the emergence of the inflorescence (R3) and continues through anthesis (R4) and fertilization (R5) (Moore et al., 1991). Thus, plant height increased between boot and anthesis, and between anthesis and seed ripe (Table 2). At boot, Kenblue was 53% taller than Ascot, but was not taller than Limousine or Touchdown. At anthesis, Kenblue was 48% taller than Ascot, 40% taller than Limousine, and 15% taller than Touchdown; and Touchdown was 29% taller than Ascot and 22% taller than Limousine (Table 2). At anthesis, Ascot and Limousine were the approximate same height. At seed ripening the height of the shorter varieties (Ascot and Limousine) was not different from each other, the height of the taller varieties (Kenblue and Touchdown) was not different from each other, and the taller varieties were 38% taller than the shorter varieties.
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Table 2. The effect of Kentucky bluegrass cultivar (Ascot, Kenblue, Limousine, and Touchdown) and growth stage on plant height and aboveground biomass production in 2003 and 2004 field experiments.
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Aboveground plant biomass (forage yield) was affected by year, stage, and cultivar. Overall, forage yield was 46% greater in 2003 than 2004 (P 0.05) (Table 2). Although seed production also averaged greater in 2003 than 2004 (P 0.05), other research suggests that Kentucky bluegrass seed yield is not correlated to aboveground biomass production (Holman and Van Tassell, unpublished data, 2006). It is uncertain what caused lower forage yield in 2004. Precipitation between the previous year's seed harvest and seed ripening was 45 cm in 2003 and 70 cm in 2004, and the 65-yr average precipitation between seed harvest and the following year seed ripening was 54 cm. Thus, explanations related to differences in resource availability between years are unlikely since precipitation was greater in 2004 and the same amount of fertilizer was applied both years. The decrease in Kentucky bluegrass forage yield was associated with increasing stand age. Information regarding stand age effects on forage yield and nutrient level are limited. Alfalfa (Medicago sativa L.) forage yield decreased with stand age due to changes in plant morphology (Suzuki, 1991). More research is needed to understand the effects of stand age on Kentucky bluegrass morphology and physiology. Forage yield increased with vegetative growth and plant maturity. Differences in forage yield among cultivars were consistent across years. On average, forage yield was similar across cultivars at boot, ranging from 216 to 287 g m–2 (Table 2). At anthesis, Ascot biomass was 28% less than Kenblue, 30% less than Limousine, and 37% less than Touchdown biomass, which were not different from each other. At seed ripening, Ascot biomass was 18% less than Kenblue and 30% less than Touchdown, and was not different from Limousine. Limousine biomass at seed ripening was 22% less than Touchdown, and was not different from Kenblue, and Kenblue biomass was 15% less than Touchdown. Overall, biomass consistently averaged about 25% more among the taller cultivars than the shorter cultivars at all growth stages. Biomass increased most between boot and anthesis compared to anthesis and seed ripening. However, the biomass of Limousine increased only between boot and anthesis.
A similar study reported that Kentucky bluegrass forage yield was greatest soon after inflorescence emergence (Smith, 1981). These results indicate that forage yield is maximized at anthesis for cultivars with similar growth characteristics and structural composition as Limousine, while the forage yield is not maximized until seed ripening for cultivars similar to Ascot, Kenblue, and Touchdown. Differences in the rate and amount of aboveground biomass accumulation affect forage yield and might affect the optimum time to harvest Kentucky bluegrass forage.
Seed yield was not affected by cultivar, although seed yield averaged 94% greater in 2003 (126 g m–2) than in 2004 (65 g m–2) despite more precipitation in 2004. Seed yield ranged from 106 to 155 g m–2 in 2003, and 27 to 120 g m–2 in 2004. The 2003 seed harvest was the second seed harvest and the 2004 seed harvest was the third seed harvest for this stand of Kentucky bluegrass. Seed yield can decline in nonthermal production stands during the third or fourth seed harvest year due to thatch accumulation and stand sodding (Canode and Law, 1979; Ensign et al., 1976; Holman and Thill, 2005b; Picha, 1976). Thus, the declining seed yield trend in 2004 likely was caused by thatch accumulation and not differences in resource availability between years.
Accurate determination of seed yield differences among Kentucky bluegrass treatments usually requires a large number and size of samples because stands often are nonuniform. A survey of Kentucky bluegrass seed producers found seed yield variability (CV) in nonburn stands ranged from 18.2 to 51.9% when analyzed across growers by stand age (Holman, unpublished data, 2005). Seed yield variability in this study was 16.7% in 2003 and 63.2% in 2004, indicating the seed yield variation in this study was similar to that found across growers. Although small plot studies might contribute to variability in forage yield, any small plot size affects on forage yield in this study were likely minimal because the differences in forage yield among cultivars and growth stages were consistent across years.
Structural composition of cultivars was not affected by year (Table 3). With the exception of Touchdown, minor differences occurred in structural composition (percentage head, leaf, and stem) among cultivars despite differences in cultivar height and aggressiveness. The percentage of heads relative to stems and leaves was least for Touchdown because it is tall and produces a large amount of aboveground biomass, while the percentage of heads for Ascot and Limousine were the greatest because of their short stature and low aboveground biomass (Table 2). Short cultivars like Ascot and Limousine are known to produce high seed yields and low aboveground biomass (Lamb and Murray, 1999). Ascot had 66% more stems than Touchdown and Limousine had 68% more stems than Touchdown, while the percentage of stems for Kenblue was not different from the other cultivars. Touchdown had 21% more leaves than Kenblue, 40% more leaves than Limousine, and 46% more leaves than Ascot. The leaf composition of Kenblue, Limousine, and Ascot were not different from each other. The high leaf composition of Touchdown resulted in it having the greatest leaf/stem ratio of all cultivars.
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Table 3. Differences in structural composition (percentage head, leaf, and stem) and seed yield at seed ripe (S5) of four Kentucky bluegrass cultivars (Ascot, Kenblue, Limousine, and Touchdown) in 2003 and 2004 field experiments.
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Forage Composition
Structural Components
Kentucky bluegrass structural components (head, leaf, and stem) varied in forage nutrient composition, and were often related to aboveground plant biomass or plant height (Table 4). Despite the differences in nutrient composition among the structural components, and the differences in structural composition among cultivars, forage nutrient composition was not affected by cultivar. Forage nutrient value is greatest when NDF, ADF, and lignin concentration are low, and when CP and IVTD concentration are high. Neutral detergent fiber is a predictor of animal feed intake, ADF and lignin are predictors of forage digestibility, CP is required by a ruminant animal for microbial protein and by all animals for amino acid production, and IVTD is an indicator of feed energy and animal performance. Neutral detergent fiber concentration of stems was 27% greater than leaves and 32% greater than heads, and the NDF concentration of leaves and heads were not different from each other. Acid detergent fiber concentration of stems was 45% greater than leaves and 60% greater than heads, and the ADF concentration of leaves was 10% greater than heads. Stems were greater in ADF and NDF concentration than heads or leaves, which can be attributed to more structural carbohydrate fibers (cellulose, hemicellulose, and lignin) in the stems necessary for cell wall development of the xylem, phloem, epidermis, and sclerenchyma tissues (Barnes et al., 2003). The ADF fraction contains cellulose and lignin and the NDF fraction contains cellulose, lignin, and hemicellulose. Hemicellulose is 60 to 70% digestible, cellulose is 40 to 50% digestible, and lignin is 0 to 10% digestible, as compared with sugars, starches, and soluble proteins that are 95% or more digestible (Kellems and Church, 2003). The lignin concentration of heads was 60% greater than stems and 134% greater than leaves, and the lignin concentration of stems was 46% greater than leaves. Lignin is an important component of secondary cell walls responsible for structural strength (Hopkins, 1995). Seed heads and stems require secondary cell walls to help maintain plant rigidity. Crude protein concentration of heads was 22% greater than leaves and 237% greater than stems, and the CP concentration of leaves was 177% greater than stems (Table 4). Nitrogen (CP = 6.25 x N) concentration is greatest in the leaves until they reach full expansion, after which, the plant exports N to other developing leaves and seed heads (Hopkins, 1995). Perennial plants such as Kentucky bluegrass also translocate N from senescing leaves to roots for storage over the winter (Hopkins, 1995). In vitro true digestibility of heads was 5% greater than leaves and 34% greater than stems, and the IVTD concentration of leaves was 28% greater than stems. In vitro true digestibility is a laboratory prediction of in vivo digestibility and is negatively correlated with NDF concentration (Kellems and Church, 2003). For reasons explained earlier, seed heads and leaves contain less structural carbohydrates (NDF) than stems, and are thus more digestible than stems. The IVTD concentration of stems was less in 2004 than 2003, which was associated with a nonsignificant increase in ADF, NDF, and lignin concentration in 2004, which was likely due to stand aging.
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Table 4. The impact of Kentucky bluegrass structural components (head, leaf, and stem) and covariates (aboveground biomass and height) on forage nutrient composition (NDF, neutral detergent fiber; ADF, acid detergent fiber; lignin; CP, crude protein; and IVTD, in vitro dry matter digestibility) at anthesis (R4) in 2003 and 2004 field experiments. See Table 5 for whole plant forage nutrient composition at anthesis (R4).
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Table 5. Impact of growth stage (R0, boot; R4, anther emergence/anthesis; S5, endosperm dry/seed ripe) pooled across cultivars (Ascot, Kenblue, Limousine, and Touchdown) on Kentucky bluegrass forage nutrient composition (NDF, neutral detergent fiber; ADF, acid detergent fiber; lignin; CP, crude protein; and IVTD, in vitro true digestibility) in 2003 and 2004 field experiments.
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Whole Plant Composition
The nutrient composition of Kentucky bluegrass forage was not affected by cultivar (data not shown) despite differences in biomass, height, and structural composition among cultivars (Tables 2 and 3). Crude protein and IVTD decreased, and ADF, NDF, and lignin increased with increasing plant maturity (Table 5). Nutrient composition is often most affected by growth stage, and decreases as plants mature because of increasing structural carbohydrate concentration (Barnes et al., 2003; Kellems and Church, 2003). Averaged across years, NDF concentration increased 38% between boot and anthesis and 6% between anthesis and seed ripening, and was greater in 2004 than 2003 across all growth stages. Averaged across years, ADF concentration increased 58% between boot and anthesis and 7% between anthesis and seed ripening, and was greater in 2004 than in 2003 at boot and anthesis. Lignin concentration increased 71% between boot and anthesis and 73% between anthesis and seed ripening, and was comparable across years. Averaged across years, CP concentration decreased 48% between boot and anthesis and 18% between anthesis and seed ripening, and was lower in 2004 than in 2003 at anthesis and seed ripening. Kentucky bluegrass CP was reported to decrease 0.28% d–1 (% change in composition) between stem elongation (E0) and flowering (Mason and Lachance, 1983). In this study, the CP composition decreased 0.57% d–1 in 2003 and 0.21% d–1 in 2004 between boot and flowering, indicating that the rate of CP decrease can vary. In vitro true digestibility decreased 17% between boot and anthesis and 12% between anthesis and seed ripening, and was lower in 2004 than 2003 at all growth stages. In previous studies, it was reported that Kentucky bluegrass IVTD decreased 0.45% d–1 (% change in composition) between stem elongation and flowering (Mason and Lachance, 1983). In this study, the IVTD composition decreased 0.47% d–1 in 2003 and 0.49% d–1 in 2004 between boot and flowering. These results suggest that the rate of IVTD decrease in Kentucky bluegrass was relatively consistent across years and cultivars.
The cause of generally lower forage nutrient level (high NDF and ADF and low CP and IVTD) in 2004 compared to 2003 was not clearly understood. Forage yield was lower in 2004 than 2003 (Table 2), but the structural composition of Kentucky bluegrass cultivars was comparable across years, suggesting that the lower forage nutrient level in 2004 might have been correlated to lower forage yield. Other studies have attributed forage nutrient composition differences between years to abiotic conditions (Barnes et al., 2003; Buxton, 1996; Cherney and Volenec, 1992). Greater growing season temperature, lower nutrient availability, and moisture stress commonly increases cell wall thickness, reduces leaf/stem ratio, and decreases the nonstructural carbohydrate concentration of cool-season grasses (Barnes et al., 2003). In this study, precipitation was 25 cm greater during 2004 (70 cm) than 2003 (45 cm), the average daily temperature between boot and seed ripening was 10.3°C in 2003 and 10.6°C in 2004, and there were no visible symptoms of nutrient deficiency.
Anecdotal information from Kentucky bluegrass producers suggests that the forage nutrient level of postharvest residue decreases after the second seed harvest year (Holman and Hunt, unpublished data, 2006). The forage nutrient level decline in 2004 appeared to be associated with an increase in the cellulose and hemicellulose structural carbohydrate concentration (ADF fraction) of the stems (Tables 4 and 5). A measurement of the nonstructural carbohydrates in alfalfa crowns and roots showed that nonstructural carbohydrates decreased in both crowns and roots as the stand aged from a young to midaged stand, and decreased in roots only as the stand aged from a midaged to old stand (Suzuki, 1991). The affect of stand age on nutrient composition has not been broadly documented nor are the mechanisms of the interaction fully understood. It is possible that the structural carbohydrate concentration in the aboveground plant biomass increased as the Kentucky bluegrass stand aged in this study.
Total Digestible Dry Matter and Crude Protein Yield
Total digestible dry matter and CP yield (biomass x CP or IVTD) were determined to help identify the optimum time to harvest Kentucky bluegrass for maximum total digestible dry matter and CP yield. Total digestible dry matter and CP yield were affected by cultivar, growth stage, and year for the reasons explained earlier for effects on forage yield and nutrient composition (Table 6). Total digestible dry matter yield was lowest at boot compared to anthesis or seed ripening, while anthesis and seed ripening were not different from each other. Total digestible dry matter yield ranged from 168 to 219 g m–2 among cultivars at boot, and was not different among cultivars. At anthesis, the total digestible dry matter yield for Touchdown was 74% greater than Ascot and 30% greater than Limousine, and was not different than Kenblue. Kenblue total digestible dry matter yield at anthesis was 49% greater than Ascot and was not different than Limousine, and Limousine was not different than Ascot. At seed ripe, Touchdown total digestible dry matter yield was not different from the other cultivars except Ascot, which was 31% less, and Ascot was not different from the other cultivars. These results indicate that harvesting at either anthesis or seed ripe would maximize total digestible dry matter yield (Table 6), since harvesting at seed ripe increased forage yield but reduced IVTD concentration (Table 4).
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Table 6. The effect of Kentucky bluegrass cultivar (Ascot, Kenblue, Limousine, and Touchdown) and growth stage (R0, boot; R4, anther emergence/anthesis; S5, endosperm dry/seed ripe) on total digestible dry matter and crude protein yield in 2003 and 2004 field experiments.
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Ascot total CP yield did not change with plant maturity because of its low aboveground biomass production. The total CP yield of the other cultivars was more variable. Total CP yield was not different between anthesis and seed ripe for all cultivars. Total CP yield of Kenblue and Touchdown increased between boot and anthesis, and was unchanged for Limousine and Ascot (Table 6). Total CP yield generally increased between boot and anthesis due to an increase in forage yield (Table 2). Touchdown total CP was 68% greater at anthesis than boot. Kenblue was unique in that total CP yield was not different between seed ripening and boot, but was 42% greater at anthesis than boot. With the exception of Ascot, whose total CP yield did not change across growth stages, harvesting at anthesis or seed ripe would maximize total CP yield.
A similar study found that Kentucky bluegrass forage yield and total digestible dry matter yield increased with plant maturity until flowering, and that total CP yield decreased from stem elongation onward (Mason and Lachance, 1983). Although this study indicated that harvesting at seed ripe compared with anthesis did not affect total CP and digestible dry matter yield, forage nutrient level was reduced and forage yield was only occasionally increased when harvest was delayed until seed ripe. Thus, it would be advisable to harvest Kentucky bluegrass at or near anthesis to maximize both forage yield and nutrient level.
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CONCLUSIONS
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Differences in nutrient composition among the structural components was not surprising since this has been well documented (Barnes et al., 2003). Forage nutrient composition was not different among the cultivars despite differences in structural composition (percentage head, leaf, and stem) among the cultivars. Of the structural components, stems were lowest in CP and IVTD concentration, heads were highest in CP and second highest in IVTD concentration, and leaves were highest in IVTD concentration. Cultivars with high stem composition (Ascot and Limousine) were also high in head composition, and since heads were high in forage nutrient level, the forage nutrient level of cultivars with high stem composition was improved. In part, this helps explain why cultivars with high stem composition, which were hypothesized to be lower in forage nutrient level, had comparable forage nutrient composition as cultivars with low stem composition (Touchdown).
Those cultivars that produced the most aboveground biomass had the greatest plant height, leaf/stem ratio, total IVTD yield, and total CP yield. Therefore, selectively breeding for less structural carbohydrate concentration or greater nonstructural carbohydrate concentration in tall cultivars might result in a cultivar with high forage yield and improved nutrient level. However, this would likely also increase the potential for stand lodging, a problem in tall cultivars harvested for seed. A strategy for reducing stand lodging during the years of seed production would be to spray Kentucky bluegrass with a growth-regulating herbicide such as trinexapac-ethyl [4-(cyclopropyl-a-hydroxymethylene)-3,5-dioxo-cyclohexanecarboxylic acid ethylester] or prohexadione calcium (calcium 3-oxido-5-oxo-4-propionylcyclohex-3-enecarboxylate) that results in shortened plant height and reduced stand lodging. The principle factors that influenced forage nutrient composition in this study were plant maturity (growth stage) and year (stand age). More research is needed to understand the mechanisms that caused an increase in the structural carbohydrate concentration of the stems, and thereby reduced forage nutrient level as the stand aged.
The results from this study indicate that Kentucky bluegrass seed producers can harvest Kentucky bluegrass as a forage crop with a forage nutrient level comparable with other grass hay. Seed producers should consider harvesting Kentucky bluegrass as a forage crop alternative after seed yield has declined when forage prices are high and there is a local demand for the forage crop.
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ACKNOWLEDGMENTS
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This research was funded in part by the Idaho State Department of Agriculture, Western Sustainable Agriculture Research and Education (Grant SW03-021), the Washington Turfgrass Seed Commission, and the Idaho Agricultural Experiment Station. Authors thank Bill Johnston, Charles Golob, Stephanie Kane, Brandon Davis, Katherine Beavers, Tianna Fife, and the anonymous reviewers for their assistance.
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NOTES
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Contribution 07-73-J from the Kansas Agric. Exp. Stn. Mention of a trade name is for identification only and does not imply endorsement or preference to other products not mentioned.
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REFERENCES
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ABSTRACT
INTRODUCTION
