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CROP QUALITY & UTILIZATION

Protein Degradability and Forage Quality in Maturing Alfalfa, Red Clover, and Birdsfoot Trefoil

^a Southwest Res. and Extension Center, University of Arkansas, 362 Hwy 174N, Hope, AR 71801 USA
^b W.K. Kellogg Foundation, Battle Creek, MI USA
^c Dep. of Animal Sci., Michigan State University, East Lansing, MI 48824 USA
^d Cooperative Extension, Univ. of Maine, Orono, ME 04469 USA

kcassida{at}uaex.edu

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 
Choice of forage species and harvest management may influence protein degradability in cattle diets. We measured forage yield, undegradable intake protein (UIP), and forage quality at three maturities in two cuttings (spring, regrowth) of alfalfa (Medicago sativa L.), red clover (Trifolium pratense L.), and birdsfoot trefoil (Lotus corniculatus L.) in the seeding and two subsequent years. At equivalent harvest stage, trefoil and clover were usually lower in neutral detergent fiber (NDF) and acid detergent fiber (ADF) than alfalfa. Crude protein (CP) was lower in red clover than trefoil or alfalfa for three of six cuttings, but red clover was equal to or higher in UIP than alfalfa or trefoil in all cuttings. Trefoil never had higher UIP concentrations than alfalfa. Trefoil tannin concentrations increased with maturity and were positively correlated with UIP concentration. For all species, UIP was positively correlated with NDF and ADF and negatively correlated with in situ dry matter disappearance (ISDMD). In alfalfa and trefoil, UIP was negatively correlated with CP. In all legumes, dry matter yield (DMY), NDF, and ADF increased with maturity, while CP and ISDMD decreased. Changes in NDF, ADF, CP, and ISDMD with maturity were usually most rapid in trefoil. The proportion of UIP increased with maturity for all species in all cuttings, but it never comprised more than 240 g kg^-1 CP. Small increases in UIP as legumes matured were gained at the expense of other forage quality measures.

Abbreviations: ADF, acid detergent fiber • CP, crude protein • DM, dry matter • DMY, dry matter yield • ISDMD, in situ dry matter disappearance • NDF, neutral detergent fiber • UIP, undegradable intake protein on dry matter basis • UIPIP, undegradable intake protein as fraction of intake (crude) protein

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 
BALANCING RATIONS for ruminants requires knowledge of the proportion of feed protein that escapes ruminal degradation (National Research Council, 1985). While degradability characteristics of concentrate feeds have been widely reported, there is less information for forages, which form the bulk of most cattle diets. Most data indicate that the standard of comparison for legume forages, alfalfa, contains much less UIP than do concentrate feed protein sources (National Research Council, 1985; Broderick et al., 1992; Hoffman et al., 1993; Griffin et al., 1994).

Legumes that contain condensed tannins may have lower protein degradabilities than alfalfa. Condensed tannins bind to protein and decrease ruminal protein degradability of forage, but the bonds are reversible at the low pH found in the abomasum, thus releasing the protein for enzymatic digestion. However, tannins also precipitate bacterial enzymes in the rumen, interfering with plant cell wall degradation and sometimes lowering dry matter (DM) digestibility in high-tannin forages (Reed, 1995). Marten et al. (1987) suggested that condensed tannins in birdsfoot trefoil may enhance animal performance by increasing UIP, and Miller and Ehlke (1994) demonstrated that trefoil varieties high in tannin are also high in UIP. Red clover contains negligible amounts of tannin (Jackson et al., 1996; Messman et al., 1996), but quinones produced when soluble phenolic compounds are oxidized by polyphenol oxidase (Jones et al., 1995) may play a similar role in binding protein (Albrecht and Broderick, 1990; Albrecht and Muck, 1991; Broderick and Albrecht, 1997).

Forage maturity at harvest can influence protein degradability. Griffin et al. (1994) demonstrated that in situ protein degradability of alfalfa decreased with maturity in spring growth, but maturity only decreased protein degradability of regrowth in one of the two study years. In all cuttings, rumen protein degradability decreased as NDF and ADF increased and CP decreased. Hoffman et al. (1993) also found that rumen protein degradability decreased with advancing maturity of a single year's spring growth of alfalfa, red clover, and birdsfoot trefoil. However, Broderick et al. (1992) found no effect of maturity on protein degradability in alfalfa when an in vitro measurement method was used.

We evaluated the effects of cutting and maturity on legume protein degradability and its relationship to other forage quality factors in order to determine whether forage harvest management or species selection might be effective methods for manipulating UIP concentrations of livestock diets. We compared forage yield, quality, and UIP of alfalfa, red clover, and birdsfoot trefoil stands through three growing seasons. Our objectives were (i) to measure UIP of alfalfa, red clover, and birdsfoot trefoil in spring and regrowth cuttings and (ii) to evaluate the effect of maturity at harvest on forage quality and UIP of the three species.

	Materials and methods

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 
Field plots were on a Conover loam (fine loamy, mixed, mesic Ochraqualfs) soil at the Michigan State University Agronomy Farm in East Lansing. Soil testing in the spring of 1991 indicated that no fertilizer was required (pH 7.4, 134 kg K ha^-1, 403 kg P ha^-1). Glyphosate (N-phosphonomethyl glycine, 2.2 kg a.i. ha^-1) was applied before moldboard plowing (20-cm depth), and EPTC (S-ethyl dipropylthiocarbamate, 3.3 kg a.i. ha^-1) was applied immediately before planting. On 15 May 1991, `Big Ten' alfalfa and `Arlington' red clover were drill-seeded at 18 kg ha^-1 after inoculation with Rhizobium meliloti, and `Norcen' birdsfoot trefoil was drill-seeded at 11 kg ha^-1 after inoculation with R. trifoli. Plot size was 0.9 by 7.6 m. In 1991, broadleaf weeds were controlled postplanting with Butyrac 200 [4-(2,4-dichlorophenoxy) butyric acid at 1.1 kg a.i. ha^-1], while weeds were removed by hand in subsequent years. Fertilizer P and K was broadcast after the spring cutting in 1993 according to Michigan Soil and Plant Nutrient Laboratory soil test recommendations for alfalfa. Insecticides to control potato leafhopper (Empoasca fabae Harris) and irrigation water were applied as needed in all years.

The experimental design was a randomized complete block with four replications and a split-split-plot arrangement. Cutting (spring, regrowth) was the main plot, legume species was the subplot, and maturity (three growth stages) was the sub-subplot. Legumes were harvested three times in each cutting at 7- to 15-d intervals. The second harvest in each cutting occurred on the date when alfalfa was at recommended harvest stage (one-tenth bloom). Spring harvest dates were 10, 19, and 31 July 1991 (seeding year); 15 and 26 May and 6 June 1992; and 17 May and 1 and 16 June 1993. Regrowth harvest dates were 13, 22, and 29 Aug. 1991 (seeding year); 1, 9, and 18 July 1992; and 6, 15, and 22 July 1993. Forage was cut 2.5 cm above ground level with a sickle-bar mower. All plots were clipped after the last harvest date of the spring cutting to initiate regrowth. Weather data were obtained from the Michigan State University Regional Weather Service (Fig. 1) .

View larger version (31K): [in this window] [in a new window]   Fig. 1 (A) Normal and actual monthly precipitation and (B) average air temperature at East Lansing, MI from 1991 to 1993

In situ incubations were conducted using two rumen-cannulated Holstein steers (Bos taurus) as described by Griffin et al. (1994). Mixed alfalfa–smooth bromegrass (Bromus inermis L.) hay averaging 600 g kg^-1 NDF, 360 g kg^-1 ADF, and 130 g kg^-1 CP was offered free-choice to steers. Steers (550, 465, 318 kg in 1991, 1992, 1993, respectively) ate 7 to 14 kg hay DM d^-1, supplemented with 0.5 kg d^-1 of soybean [Glycine max L. (Merr.)] meal, and had water available at all times. In situ residues were initially dried at 55°C and then equilibrated to ambient conditions in an air-conditioned lab before weighing. Values for ISDMD and UIP were corrected to a DM basis using 928 g kg^-1 as an average hygroscopic DM concentration for samples from this laboratory. In situ dry matter disappearance was calculated as weight loss of samples during rumen incubation. Concentration of UIP in residues was expressed both on a DM basis (UIP) and as a fraction of the original intake (crude) protein (UIPIP).

Statistical analyses were conducted using SAS (SAS Institute, 1988). Analyses of variance were conducted separately for each cutting within years because of significant interactions among year, cutting, date, and species main effects. Maturity effects were compared using orthogonal contrasts (Date 1 vs. 2 and 3, Date 2 vs. 3). Analysis of variance for rates of change within cuttings was used to clarify significant species x date interactions for forage quality factors. Based on visual estimation of bloom percentage, red clover plots were consistently less mature than alfalfa or trefoil plots on the same date, an observation also made by Buxton et al. (1985). Because this should bias yields in favor of the more mature species and bias quality in favor of the less mature species, we made species comparisons using data from single harvest dates that were closest to one-tenth bloom (second harvest date for alfalfa and birdsfoot trefoil, third harvest date for red clover). Species means were separated using Fisher's LSD (P < 0.05) with a protection level of P < 0.10 for the main effect. Correlations among forage quality and protein degradability measurements were calculated for each species using the entire data set. Birdsfoot trefoil tannin data were analyzed within years as a split plot, with cutting as main plot and sampling date as the subplot.

	Results

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 
When compared at the one-tenth bloom stage, total DMYs were higher for red clover than for alfalfa or birdsfoot trefoil in 1992 and 1993 (Table 1) . In the seeding year, the DMY was similar between cuttings, while in the established years, yields were lower in regrowth cuttings than in spring cuttings for all species. In the seeding year, trefoil and clover had higher growth rates than alfalfa (Table 1) in the first cutting. Trefoil growth rates exceeded alfalfa (both cuttings) and clover (regrowth) in 1993.

View this table: [in this window] [in a new window]   Table 2 Composition of alfalfa (ALF), red clover (RC), and birdsfoot trefoil (BFT) harvested at approximately one-tenth bloom in spring and summer regrowth cuttings from 1991 to 1993.

The relationship of forage composition and ISDMD to maturity of forage was evaluated across harvest dates within cuttings (Fig. 2) . Concentrations of NDF and ADF increased in all three legumes in all cuttings as forage matured, and CP and ISDMD decreased with maturity. Undegradable intake protein (g kg^-1) showed some increase with maturity for all species in all cuttings except 1993 regrowth. Because of the consistent decline in CP concentration with maturity, UIPIP increased with maturity for all species in all cuttings. There were seventeen instances of statistically significant species x harvest date interactions for NDF, ADF, CP, or ISDMD within cuttings. Twelve of these instances were attributed to differences in the rate of change of forage composition among species (Table 3) . The most consistent source of rate-influenced interactions (ten instances) was a faster rate of compositional change (spread across all variables) for birdsfoot trefoil compared with alfalfa or red clover. The remaining two rate-influenced interactions were a result of faster decline of NDF and ADF in alfalfa than in clover and trefoil in seeding-year regrowth.

View larger version (55K): [in this window] [in a new window]   Fig. 2 (A) In situ dry matter degradability (ISDMD), (B) acid detergent fiber (ADF), and (C) neutral detergent fiber (NDF), (D) crude protein (CP), (E) undegradable intake protein on crude protein basis (UIPIP), and (F) undegradable intake protein on dry matter basis (UIP) of alfalfa, birdsfoot trefoil, and red clover harvested at three harvest dates in spring and regrowth cuttings from 1991 to 1993. Cuttings indicated with SxD had a significant species x harvest date interaction for that variable

View this table: [in this window] [in a new window]   Table 3 Rate of change in alfalfa (ALF), red clover (RC), and birdsfoot trefoil (BFT) composition and in situ dry matter disappearance in spring and summer regrowth cuttings.

Tannin concentration in birdsfoot trefoil (Table 4) increased with maturity in two spring and two regrowth cuttings. In the seeding year, tannin was higher in the first cutting than in regrowth, but in both years with established stands, tannin concentration was higher in regrowth than in spring growth. Tannin concentration was positively correlated with NDF, ADF, UIP, and UIPIP concentration (r = 0.59, 0.53, 0.69 and 0.75, respectively; P < 0.01) and negatively correlated with CP and ISDMD (r = -0.55 and -0.70; P < 0.01). Linear regression of UIP on tannin concentration gave prediction equations of: UIP = 1.11 + 0.18(tannin) (r² = 0.47; P < 0.01) and UIPIP = 4.09 + 1.28(tannin) (r² = 0.57; P < 0.01).

	Discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

Similar values for NDF and ADF have been reported for alfalfa and trefoil (Buxton and Hornstein, 1986; Hoffman et al., 1993). Buxton and Hornstein (1986) reported values for NDF in red clover that were slightly lower than these. Buxton et al. (1985), McGraw and Marten (1986), and Hoffman et al. (1993) reported no differences in CP among these legumes. Hoffman et al. (1993) reported that ISDMD was higher in red clover than in alfalfa or birdsfoot trefoil at all maturities, but McGraw and Marten (1986) reported no differences for DM degradability of alfalfa and trefoil when measured in vitro. Similar rates of change in forage cell wall concentration with maturity have been described for these legumes (Buxton and Hornstein, 1986; Hoffman et al., 1993; Griffin et al., 1994). Crude protein declines of similar extent have been recorded (Buxton et al., 1985; Hoffman et al., 1993), but Buxton et al. (1985) reported a constant rate of decline at 2 g CP kg^-1 d^-1 for the three species.

While our UIPIP concentrations are similar to those of Griffin et al. (1994) for alfalfa, they are low in comparison to other reported ranges for alfalfa (150–300 g kg^-1 CP, Hoffman et al., 1993), birdsfoot trefoil (150–540 g kg^-1 CP, Hoffman et al., 1993; Miller and Ehlke, 1994), and red clover (150–300 g kg^-1 CP, 1.gif" BORDER="0">man and Nordkvist, 1983; Hoffman et al., 1993). The use of freeze-drying for sample preservation and in vitro methods to determine UIPIP may explain the higher values found in some of those studies (man and Nordkvist, 1983; 21–54 g kg^-1, Miller and Ehlke, 1994). However, Hoffman et al. (1993) used oven-dried samples and the in situ method and also found higher maximum UIPIP (279, 249, and 300 g kg^-1 CP for alfalfa, birdsfoot trefoil, and red clover, respectively) than was found in our study. They found no differences in UIPIP among these species in a spring cutting. However, our results suggest species differences in protein degradability may be more likely in regrowth cuttings. Environmental or plant morphology differences in mid summer vs. late spring may explain this effect.

The variability in UIP concentration across the 3 yr of this study deserves comment. Experimental design and the presence of interactions prevented statistical comparison of years, but legumes showed numerically lower UIP concentrations in the unusually cool summer of 1992 (Fig. 1) than in the more typical years of 1991 or 1993. Griffin et al. (1994) conducted their experiment concurrently with this one at the same site and also observed that UIP concentrations in alfalfa appeared to be lower in 1992 than in 1991. They suggested that protein degradation in alfalfa is possibly influenced by environmental conditions, and our data indicate that birdsfoot trefoil and red clover may show a similar response.

Increased in situ UIP with advancing maturity has been reported for alfalfa (Nordkvist and 1.gif" BORDER="0">man, 1986; Hoffman et al., 1993; Griffin et al., 1994), birdsfoot trefoil, and red clover (Hoffman et al., 1993). In contrast, Broderick et al. (1992) found no effect of forage maturity on in vitro UIP in alfalfa. Unfavorable correlations among UIP and other measures of forage quality in alfalfa have been previously reported by Griffin et al. (1994), who concluded that potentially negative effects on animal performance of increasing cell wall and decreasing CP and ISDMD are probably greater than potential small benefits in UIP to be gained by harvesting forages at greater maturities.

Our tannin concentrations were within reported ranges for birdsfoot trefoil (Chiquette et al., 1989; Albrecht and Muck, 1991; Roberts et al., 1993; Miller and Ehlke, 1994). However, measured tannin concentrations were probably lower than would have been observed in fresh forage because tannin is oxidized during drying (Reed, 1995). Terrill et al. (1990) found similar tannin levels in sericea lespedeza [Lespedeza cuneata (Dumont) G. Don.] oven-dried samples and hay, but levels in hay were half those detected in freeze-dried samples. Therefore, our numbers are more representative of tannin concentrations in birdsfoot trefoil hay than in fresh forage and should be interpreted with caution in regards to grazed trefoil. Use of dried samples may also help explain the lower than expected UIP found in trefoil if tannins altered by drying were less able to complex with dietary protein in the rumen. Such a direct link has not been reported, but Terrill et al. (1989) observed that trefoil hay had higher cell wall digestibility in sheep (Ovis aries) than fresh-frozen trefoil, which would be expected if tannin binding activity was decreased by drying. Measured tannin concentration accounted for only 47% of the variation in trefoil UIP as indicated by regression — the remaining variation must be attributable to other chemical factors related to plant maturity and environment.

We assumed that the response of tannin to maturity in trefoil was similar to that in other tannin-containing legumes. Tannin levels of sericea lespedeza increase with maturity and are higher in summer than initial spring cuttings for both oven-dried (Donelly, 1959) and freeze-dried (Cope and Burns, 1974) samples. The regrowth response was attributed to higher summer temperatures (Donelly, 1959; Fales, 1984). Roberts et al. (1993) observed decreased tannin concentrations of birdsfoot trefoil in harvests made from July to September, but temperature was not reported. In each year of our experiment, tannin concentrations were higher in the cutting made during the warmer month (July vs. August for the seeding year, and June vs. July for established years).

Indirect evidence suggests that birdsfoot trefoil may improve animal performance by providing more UIP than other legumes. Marten et al. (1987) attributed better weight gain by heifers grazing birdsfoot trefoil compared with those on alfalfa to a beneficial effect of trefoil tannin on UIP, but forage UIP and tannin concentrations were not measured. Marten and Jordan (1979) also reported higher weight gains for lambs grazing pastures containing trefoil than those grazing alfalfa–grass mixtures, but attributed the effect to higher overall forage quality. Marten et al. (1990) found higher in vitro DM degradability for red clover than for alfalfa and birdsfoot trefoil, but similar average daily gains in lambs, and attributed this to stocking rates low enough to allow animals to select diets of high nutritive value on all pastures. In our experiment, birdsfoot trefoil UIP concentrations did not exceed those of alfalfa, and trefoil sometimes contained more ISDMD than alfalfa. Kraim et al. (1990) also reported that trefoil DM digestibility can equal that of alfalfa. Chiquette et al. (1989) reported that a high-tannin birdsfoot trefoil had lower CP digestibility than a low-tannin strain, but DM and ADF digestibility were not affected. Miller and Ehlke (1994) suggested that selecting birdsfoot trefoil cultivars for 27 to 85 g catechin equivalent kg^-1 will cause a small potential loss in DM degradability in relation to a large potential gain in UIP. The effects of sample preparation and analysis technique make it difficult to extrapolate laboratory results to actual grazing animal performance. Drying probably decreases birdsfoot trefoil UIP concentrations, as discussed above, while UIP in alfalfa hay is higher than in freeze-dried samples (Broderick et al., 1992). The potential complexity of interacting factors highlights the need for experiments to look directly at the effect of changing legume UIP concentrations on animal performance rather than relying solely on laboratory measures of UIP.

Red clover was always as high or higher in UIP and UIPIP than alfalfa or trefoil. Jones et al. (1995) suggested that phenolic compounds in red clover may bind protein and inhibit its degradation by microbes, but they were not able to develop a reliable assay. In vitro protein degradation in red clover is similar to the inhibited degradation observed in tannin-containing legumes (Albrecht and Broderick, 1990; Broderick and Albrecht, 1997). In contrast, Hoffman et al. (1993) reported that protein degradation rates measured in situ were faster for red clover than for trefoil or alfalfa and that the final extent of degradation did not differ among species. Clearly more research and an assay for binding phenolic compounds will be necessary to further investigate this area.

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 
In this study the relative increase in UIP from first to third harvest maturity was up to 54%, but UIP never comprised more than 41 g kg^-1 forage or 240 g kg^-1 CP for any legume. Small increases in UIP were gained at the expense of other measures of forage quality, suggesting that harvesting at later maturities to increase UIP will be counterproductive for birdsfoot trefoil and alfalfa. However, the lack of correlation between CP and UIP concentrations in red clover may indicate a possibility that this species could be selected for high CP and high UIP simultaneously. Further research is warranted to determine if small differences in legume protein degradability can have a favorable impact on animal performance and to identify the exact mechanisms by which plant species or stage of maturity affect protein degradability.Association of Official Analytical Chemists 1990

	ACKNOWLEDGMENTS

 
The authors thank J. Paling for assistance with data collection.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 
Research conducted in cooperation with the Michigan State Univ. Agric. Exp. Stn.

Received for publication November 30, 1998.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion Conclusions REFERENCES

 

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