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ANIMAL NUTRITION |
,1
* Department of Animal & Range Sciences, Montana State University, Bozeman 59717;
and
Department of Animal Sciences, The Ohio State University, Columbus 43210; and
Department of Animal and Veterinary Science, University of Idaho, Moscow 83844
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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0.02) with increasing alfalfa substitution. In Exp. 2, treatments were arranged as a 2 x 2 factorial testing alfalfa substitution [none or 25% (as-fed basis)] to orchardgrass hay (Dactylis glomerata L.) and intake level [restricted to 1% of BW daily (DM basis) or ad libitum]. Nutrient intakes were lowest (P 0.05) by steers fed restricted diets, intermediate by steers fed orchardgrass ad libitum, and greatest by steers fed orchardgrass plus alfalfa ad libitum. Intake level and forage source had no effect (P 
0.23) on total tract apparent digestibility of all nutrients except CP. Steers fed orchardgrass plus alfalfa had 33% greater (P = 0.01) total tract apparent digestibility for CP than those fed orchardgrass alone. Lag time of DM and NDF disappearance was not affected (P 0.20) by alfalfa supplementation or intake level. Rate of DM and NDF disappearance of orchardgrass was faster (P 0.01) in steers fed orchardgrass plus alfalfa, at both restricted and ad libitum levels of feeding, than in animals fed orchardgrass alone. Mean retention times of large and small particles of orchardgrass tended to be shorter (P 0.06) when steers consumed ad libitum vs. restricted diets. Small orchardgrass particles tended to have a faster (P = 0.09) rate of passage under ad libitum feeding conditions and with alfalfa addition. Ad libitum intake was associated with a shorter mean retention time of orchardgrass and faster rate of passage of small orchardgrass particles, whereas alfalfa addition increased the rate of passage of small orchardgrass particles and the rate of DM and NDF disappearance.
Key Words: alfalfa • cattle • digestion rate • particle size • passage rate • restricted feeding
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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), and clearance of digesta from the rumen is the primary process that limits forage intake by ruminants (Ulyatt et al., 1986). To improve forage intake, the rate of removal from the rumen must be increased by increasing the rate of digestion, the rate of passage, or both (Moseley and Jones, 1979). Forage particles must be reduced in size before they can exit the rumen (Troelsen and Campbell, 1968; Poppi et al., 1981; Allen, 1996). Small particles (<1.18 mm) have been shown to comprise 72% of the DM in the rumen (Poppi et al., 1981), and the rate of passage of small particles from the rumen has been considered to be the main factor controlling mean retention time of digesta and hence voluntary intake in cattle and sheep (Poppi et al., 1981; Shaver et al., 1988; Okine and Mathison, 1991).
Increased feed intake is associated with a decreased extent of DM and cell wall digestion (Robertson and Van Soest, 1975) due to a reduction in the amount of time digesta spends in the rumen (Staples et al., 1984). Okine and Mathison (1991) demonstrated DMI affected the particle size escaping the rumen. Increased intake and animal performance has been reported with the addition of a legume to grass diets (Bowman and Asplund, 1988), an improvement attributed to the positive associative effects of legumes, which have a faster digestion (Andrighetto et al., 1993) and passage rate (Moseley and Jones, 1979) compared with grasses. It is unclear whether the positive associative effects of legumes can overcome limitations to intake that may be imposed by very low-quality forages. A better understanding of these factors is necessary to increase our ability to manipulate the factors limiting intake of forage by cattle.
The objectives of this study were to evaluate 1) the effects of alfalfa substitution to big bluestem hay diets on fiber digestion, and 2) the effects of alfalfa substitution to an orchardgrass hay diet fed at ad libitum or restricted intake on fiber digestion and passage characteristics of small and large forage particles.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Experiment 1
Two ruminally cannulated cows (654 kg of BW) were used to evaluate the effects of alfalfa (Medicago sativa L.) and big bluestem (Andropogon gerardi Vitman) combinations on rate and extent of in situ DM and NDF disappearance and carboxymethylcellulase (CMCase) activity. Forage combinations evaluated consisted of 0% alfalfa-100% big bluestem, 25% alfalfa-75% big blue-stem, 50% alfalfa-50% big bluestem, and 100% alfalfa-0% big bluestem. Cows were fed 10 kg of orchardgrass hay (Dactylis glomerata L.; 6.9% CP and 81.6% NDF, DM basis) once daily. Trace mineralized salt and water were provided for ad libitum access.
Forage samples were ground through a 1-mm screen in a Wiley mill (Arthur H. Thomas, Philadelphia, PA) and analyzed for DM (AOAC, 1999; Method 934.01), N (AOAC, 1999; Method 976.05), and ADF, NDF, and ADL (Van Soest et al., 1991). Concentrations of NDF and ADF were not adjusted for residual ash content, and sodium sulfite and 
-amylase were omitted from the neutral detergent solution. Hemicellulose was calculated as the difference between NDF and ADF, and cellulose was calculated by subtracting ADL and insoluble ash from ADF (Van Soest et al., 1991). A description of forage composition is presented in Table 1.
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) as a marker of cellulolytic, particle-associated microbial activity. Forage residues remaining in the dried bags were analyzed for DM and NDF. The DM and NDF disappearance were fit to a model using PROC NLIN (SAS Inst. Inc., Cary, NC). Initial estimates of the rate constant (Kd) and lag time of DM and NDF disappearance were necessary for model fitting and were calculated as described by Bowman and Firkins (1993). The basic model used was from Mertens and Loften (1980). Dry matter and NDF were partitioned into 3 fractions, defined as immediately soluble (fraction A), disappearing at a measurable rate (fraction B), and undegradable (fraction C). The rate constant for fraction B, lag time, and fractions B and C were determined from the nonlinear model, whereas fraction A was calculated as: 100 – fraction B – fraction C. Fractions A, B, and C, the rate constant for fraction B, and lag time were analyzed using PROC GLM of SAS, with treatment in the model. Means were separated using the least significant difference test when the treatment effect was P < 0.05.
Carboxymethylcellulase activity data collected over time were statistically analyzed using the repeated option of PROC MIXED of SAS (Littell et al., 1998) for a covariance structure of autoregressive within animals and random between animals. Mean comparisons were made among the least squares means using the PDIFF option. Differences were considered significant at P < 0.05.
For all data, linear and quadratic effects of concentration of alfalfa were tested using contrast statements. Because the treatment levels were unequally spaced (i.e., 0, 25, 50, and 100% alfalfa), PROC IML of SAS was used to generate coefficients for unequally spaced contrasts.
Experiment 2
Four ruminally cannulated steers (416 ± 1 kg initial BW) were assigned randomly to 1 of 4 treatments in a 4 x 4 Latin square design with a 2 x 2 factorial arrangement of treatments, to evaluate digestion and passage of small and large forage particles when alfalfa was supplemented to a grass hay diet and fed at ad libitum or restricted intake. The 4 dietary treatments consisted of 1) orchardgrass (Dactylis glomerata L.) hay offered for ad libitum intake, 2) restricted feeding of orchardgrass hay, 3) orchardgrass hay plus alfalfa hay in a ratio of 3:1 offered for ad libitum intake, and 4) restricted feeding of orchardgrass hay plus alfalfa hay in ratio of 3:1. The concentration of alfalfa was selected to represent a concentration commonly fed as a supplement to low-quality grass hay by beef cattle producers. The orchardgrass hay was harvested at the late dough stage, and alfalfa hay was harvested at 25% bloom. Orchardgrass and alfalfa hays were chopped with a forage chopper as harvested to 20-cm lengths. Forage analyses were as outlined for Exp. 1.
Feeding level for restricted-fed steers was 4.5 kg/d as-fed (1% of BW daily on a DM basis, based on initial BW), whereas steers fed for ad libitum intake were offered diets at 110% of ad libitum intake of the previous day. The combination of orchardgrass plus alfalfa hays was mixed in a ratio of 3:1 (as-fed) before each feeding. An estimate of the amount of orchardgrass plus alfalfa hay needed was made, and the appropriate amount of each hay was weighed and mixed by hand in a large tub. Feed was offered in 2 equal portions at 0600 and 1800, and orts were weighed and recorded at these times. Steers were housed on a concrete floor in individual 3.7 x 3.7-m pens. Water was available for ad libitum consumption. Fifty grams of trace mineralized salt were offered daily to each animal. Animals were weighed in the morning at the beginning and end of each period.
Nylon bags measuring 10 x 23 cm, with an average pore size of 50 µm, were used for determination of the rate and extent of DM and NDF disappearance and CMCase activity. At 0600 on d 11 of each period, in situ bags were placed into the rumens of the steers and exposed to ruminal fermentation for 3, 6, 12, 18, 24, 30, 36, 42, 48, 60, 72, 84, and 96 h. Four bags, each containing approximately 4 g of orchardgrass and ground through a 2-mm screen in a Wiley mill, were used for each time point. After removal from the rumen, 1 bag was frozen immediately for later determination of CMCase activity (Bowman and Firkins, 1993) with the addition that CMCase activity was corrected by subtracting background glucose concentration in the enzyme extract. The other 3 bags were handled as in Exp. 1, and the DM and NDF fractions A, B, and C, the rate constant for fraction B, and lag time of DM and NDF disappearance were calculated as described for Exp. 1.
Representative samples of alfalfa and orchardgrass hay fed to the steers, obtained by coring 40, small square bales, were dry-sieved through a screen opening size of 2.36 mm to separate large particles from small particles. The particles that passed through the sieve were defined as small, and the particles remaining on top of the sieve were defined as large. Large and small particles of orchardgrass were labeled with Yb and Tb, respectively, and large and small particles of alfalfa were labeled with Dy and Er, respectively, using the procedure of Ellis and Beever (1984). The particles were soaked with the rare earth chlorides for 24 h in solutions containing 80 mg of each metal per gram of forage material. Soaking was at room temperature in plastic buckets with sufficient liquid to cover the forage material. After soaking, labeled forage material was strained through 8 layers of cheesecloth and washed 6 times with distilled water to remove unbound label. Labeled forage was then dried in a forced-air oven at 55°C for 48 h.
Each period of the 4 x 4 Latin square consisted of 10 d of diet adaptation followed by 4 d of sample collection. On d 11 of each period, approximately 25 g each of the respective labeled forage particles was dosed into the rumen at 0600. Fecal grab samples were taken at 6, 12, 18, 24, 30, 36, 42, 48, 60, 72, 84, 96, and 108 h after dosing, dried in a forced-air oven at 55°C for 48 h. Feed and ort samples were also collected at each feeding, dried, and composited by animal within each period.
Feed, orts, and fecal samples were ground through a 1-mm screen in a Wiley mill and analyzed for DM, N, NDF, ADF, and ADL. Cellulose and hemicellulose were calculated as in Exp. 1. Acid detergent lignin was used as an internal marker to estimate fecal output (Van Soest, 1994), which was calculated by dividing daily ADL intake by the ADL fecal concentration. Feed intake and fecal output estimates were used to estimate total tract apparent digestibility of nutrients. Labeled forages (i.e., alfalfa large particles, alfalfa small particles, orchardgrass large particles, and orchardgrass small particles) and fecal samples were analyzed for Dy, Er, Yb, and Tb by neutron activation (Gray and Vogt, 1974).
Fecal Dy, Er, Yb, and Tb were fitted to a 1-compartment, time-dependent (gamma 2 time dependency) model (Ellis et al., 1979), as follows: Y = K0 x (T–tau) x 
x e[–K1 x (T–tau)], where Y = the marker concentration in fecal samples; K0 = initial marker concentration in the rumen; K1 = age-dependent passage parameter; T = time after marker pulse-dose; and tau = the time delay (i.e., the time between the marker dose and the first appearance in the feces).
Data were fitted to the model using the nonlinear procedure (Marquardt method) of SAS. Total tract flow rate of large and small particles of orchardgrass and alfalfa, their retention time (h), pool size (kg), and output (kg/d) were estimated according to the methods of Ellis et al. (1984) and Krysl et al. (1988) as: flow rate = K1 x 0.59635; retention time = (2/K1) + tau; pool size = dose/(KO x K1 x 0.59635); and output = (dose/KO) x 24h. Several assumptions were made to aid in interpretation of particle passage data. Large particle flow rate was assumed to include the rate of reduction of large particles to the small particle pool and then their subsequent passage out of the rumen, whereas small particle flow rate was assumed to measure passage alone. Hence, large particle output (kg/d) measured the quantity of large particles that were reduced in size and passed from the rumen per day. Large particles that were reduced in size and passed from the rumen were assumed to be similar in chemical composition and specific gravity to small particles. Also, the markers were assumed to behave similarly. Rate of particle reduction was assumed to be similar between forages and between treatments. Finally, behavior and passage of the ground, small particles that were dosed were assumed to be equal to that of particles formed by mastication of the large marked particles.
Data collected over time (CMCase activity) were statistically analyzed using the repeated option of PROC MIXED (Littell et al., 1998) for a covariance structure of autoregressive within animals and random between animals. Mean comparisons were made among the least squares means using the PDIFF option. Differences were considered significant at P < 0.05.
Dry matter and NDF fractions A, B, and C, the rate constant for fraction B, lag time, and all other parameters were analyzed using PROC GLM of SAS for a 4 x 4 Latin square design with a 2 x 2 factorial arrangement of treatments. The 2 main effects were intake level and forage source, with individual animal as the experimental unit. The sums of squares of the model were separated into animal, period, and treatment effects. Contrast statements were used to evaluate the effects of intake level, forage source, and their interaction. When a significant (P < 0.05) F-statistic was detected for the interaction, means were separated using the least significant difference method.
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RESULTS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The fraction of DM that was immediately soluble (fraction A) and the DM disappearance rate (Kd) increased linearly (P 0.03), whereas the fraction of DM that disappeared at a measurable rate (fraction B) decreased linearly (P = 0.01) as increasing concentration of alfalfa was added to big bluestem (Table 2). No difference (P 0.18) was observed in the DM fraction that was undegraded (fraction C; average 20.5%), or in the lag time (mean 2.6 h).
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0.02) with the addition of increasing concentration of alfalfa (Table 2
). No differences (P 0.32) were observed in the immediately soluble NDF fraction (average 3.9%) or the lag time for NDF disappearance (mean 4.7 h).
Carboxymethylcellulase activity peaked at 9 h for 100% alfalfa-0% big bluestem, declined, and remained at a constant low level during 24 to 72 h (Figure 1). Carboxymethylcellulase activity was greater (P 0.04) for 100% alfalfa-0% big bluestem than for the other treatments at 6 and 9 h of incubation; however, 100% alfalfa-0% big bluestem had the lowest (P 0.04) CMCase activity from 36 to 48 h. Carboxymethylcellulase activity was lowest at 3 h for 0% alfalfa-100% big bluestem and gradually reached a peak at 48 h. Both 25% alfalfa-75% big bluestem and 50% alfalfa-50% big bluestem exhibited 2 CMCase peaks. The first peak occurred at 9 h and was intermediate (P 0.03) between the CMCase level for 100% alfalfa-0% big bluestem and 0% alfalfa-100% big bluestem. The second CMCase peak occurred at 42 h for both 25% alfalfa-75% big bluestem and 50% alfalfa-50% big bluestem. Carboxymethylcellulase activity was linearly related (P 0.01) to DM and NDF disappearance of 100% alfalfa-0% big bluestem (DM, r2 = 0.33; NDF, r2 = 0.30) and 0% alfalfa-100% big bluestem (DM, r2 = 0.48; NDF, r2 = 0.48), but not to 25% alfalfa-75% big bluestem or 50% alfalfa-50% big bluestem (P 0.10).
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Interactions (P 0.05) were observed between intake level and forage source for intakes of DM, NDF, ADF, cellulose, hemicellulose, CP, and ADL (Table 3). Nutrient intakes were lowest (P 0.05) when steers were fed either of the restricted diets, intermediate when steers consumed orchardgrass ad libitum, and highest when steers consumed orchardgrass and alfalfa ad libitum. Dry matter intake for the restricted diets was set at 1% of BW daily. Steers offered orchardgrass ad libitum consumed DM at 1.6% of BW daily, whereas those offered orchardgrass plus alfalfa ad libitum consumed DM at 2.0% of BW daily, an increase of 25% over the orchardgrass alone diet.
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0.23) by intake level or forage source (Table 4). Crude protein total tract apparent digestibility did not differ (P = 0.41) between intake levels, but was 33% greater (P = 0.01) in steers consuming orchardgrass plus alfalfa than in those consuming orchardgrass alone.
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0.12) of intake level or forage source on the orchardgrass DM or NDF fraction A, or the DM or NDF fraction that disappeared at a measurable rate (Table 5). Lag time for in situ DM and NDF disappearance of orchardgrass was not affected (P 0.20) by intake level or forage source, and averaged 2.5 and 2.2 h for DM and NDF, respectively. Intake level did not affect (P 0.18) the orchardgrass DM or NDF undegraded fractions (average 37.9% for DM and 44.2% for NDF) or disappearance rates (average 0.040 h–1 for DM and 0.041 h–1 for NDF). Orchardgrass undegraded DM and NDF fractions were 10 and 6% greater (P 0.02), respectively, when incubated in steers consuming orchardgrass plus alfalfa compared with orchardgrass alone. Rate of DM disappearance was 35% greater (P = 0.001), and rate of NDF disappearance was 16% greater (P = 0.01) for orchardgrass when incubated in steers consuming orchardgrass plus alfalfa compared with orchardgrass alone.
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). Intake level had no effect (P 0.18) on CMCase activity at any incubation time. Carboxymethylcellulase activity was 35 and 15% greater (P 0.002) at 18 and 24 h, respectively, when orchardgrass was incubated in steers consuming orchardgrass plus alfalfa, than in steers consuming orchardgrass alone.
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). Small orchardgrass particles had a 9% shorter (P = 0.03) MRT in steers consuming diets offered at ad libitum rather than restricted intake. Intake level did not affect (P 0.13) MRT of large or small alfalfa particles. Forage source did not affect (P 0.11) MRT of large or small orchardgrass or alfalfa particles. Rate of passage of large orchardgrass particles was not influenced (P = 0.16) by intake level; however, small orchardgrass particles tended to have a faster (P = 0.09) rate of passage when diets were offered ad libitum. Rate of passage of large and small alfalfa particles was not affected (P 0.15) by level of intake. Rate of passage of large orchardgrass particles was not influenced (P = 0.71) by forage source; however, small orchardgrass particles tended to have a faster (P = 0.09) rate of passage when alfalfa was fed with orchardgrass. Rate of passage of large and small alfalfa particles was not affected (P 0.22) by forage source.
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DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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reported that 25% red clover substitution for orchardgrass had no effect on in situ rate of NDF disappearance or lag time; however, it increased extent of NDF disappearance. Bowman and Firkins (1993) also reported that grasses had a greater in situ extent of NDF disappearance at 72 h, slower rate of NDF disappearance, and similar lag time compared with legumes. Hunt et al. (1985) reported that substituting increasing proportions of alfalfa for tall fescue had a positive linear effect on rate of DM disappearance and no effect on rate of NDF disappearance. In addition, a quadratic effect on in vitro extent of DM and NDF disappearance was observed, with a greater extent for 25% alfalfa plus tall fescue compared with fescue only (Hunt et al., 1985). We evaluated alfalfa in combination with a warm season grass, whereas Hunt et al. (1985) evaluated alfalfa in combination with a cool season grass.
Extent of in situ DM and NDF disappearance of orchardgrass was also not different (P 0.22) for samples incubated in animals consuming orchardgrass plus alfalfa compared with orchardgrass only diets. This differs from results in Exp. 1, where extent of in situ NDF disappearance at 72 h was lower for the grass plus alfalfa compared with grass only treatment. In Exp. 1 combinations of alfalfa and big bluestem were incubated in situ whereas in Exp. 2 only orchardgrass was incubated in situ. Orchardgrass and alfalfa had similar ADL levels in Exp. 2, whereas the big bluestem used in Exp. 1 contained less ADL than alfalfa. This could explain the differences seen in the impact alfalfa substitution for grass had on in situ digestion. Alfalfa had a lower potential extent of fiber digestion compared with orchardgrass (Mertens and Loften, 1980), and total tract NDF digestibility was lower with alfalfa supplementation (Prigge et al., 1990; Vanzant and Cochran, 1994). Other researchers have reported that alfalfa addition to Caucasian bluestem had no effect on in vitro NDF extent of digestion (Bowman and Asplund, 1988). Our results also differ from reports that extent of in situ NDF disappearance increased with red clover supplementation to orchardgrass (Bowman et al., 1991) and alfalfa supplementation to barley straw (Haddad, 2000). Bowman et al. (1991) attributed this effect to a ruminal environment improved by legumes supplying deficient nutrients or a readily fermented cell wall substrate for cellulolytic bacteria. We did observe improved DM and NDF disappearance with alfalfa substitution early in incubation.
Similar to our data, Alwash and Thomas (1971) reported that rate of disappearance of cotton threads was faster at a low level of feeding, and Okine and Mathison (1991) reported that total tract NDF digestion rate decreased with increasing level of intake. Staples et al. (1984) attributed decreased fiber digestion rate to a lower ruminal pH observed with greater intakes. Increased rate of disappearance observed with orchardgrass plus alfalfa diets could be explained by greater disappearance observed at early time points. Our data agree with Haddad (2000) who reported faster in situ NDF digestion rates when alfalfa was fed with barley straw, and Smith et al. (1972) who reported that legumes were less digestible but had a faster rate of digestion compared with grasses. Hunt et al. (1988) reported that in situ rate of NDF disappearance of wheat straw decreased linearly and quadratically due to alfalfa substitution with ad libitum, but not restricted feeding; however, similar to our data, the rate of NDF disappearance was numerically greater for 25% alfalfa addition compared with wheat straw only at both levels of feeding. Ndlovu and Buchanan-Smith (1985) reported a faster rate of DM and cell wall disappearance of corn cobs, barley straw, and bromegrass hay with alfalfa supplementation. In contrast to our data, Mertens and Loften (1980) reported that alfalfa and orchardgrass had similar rates of fiber digestion. Alfalfa addition appears to have altered the rumen environment thereby affecting rate of digestion of orchardgrass, possibly through increased cell solubles and N available when alfalfa was fed (Ndlovu and Buchanan-Smith, 1985; Bowman and Asplund, 1988). Ndlovu and Buchanan-Smith (1985) and Hunt et al. (1988) reported that ruminal pH, ammonia, and some VFA concentrations were altered by alfalfa supplementation.
Our data agree with Mertens and Loften (1980) who reported that alfalfa and orchardgrass had similar lag times. Haddad (2000) also reported no difference in lag times when various amounts of alfalfa were included in the diet with barley straw. Ndlovu and Buchanan-Smith (1985) reported that alfalfa had no effect on lag time when supplemented to 3 low-quality forages.
Results of Exp. 1 and the report of Bowman and Firkins (1993) agree in that CMCase activity peaked earlier for legumes than grasses. In Exp. 2, in vivo DM and NDF digestibility and in situ CMCase activity did not differ between intake levels or forage sources. However, we observed differences in rate of in situ DM and NDF disappearance of orchardgrass between forage sources, which were not explained by CMCase activity at these time points. Silva et al. (1987) reported that particle bound CMCase activity reflects final degradability and accentuates small differences associated with rate of degradation. We did not observe 2 peaks in CMCase activity in Exp. 2 as in Exp. 1, possibly because big bluestem and alfalfa were incubated in situ in Exp. 1, whereas only orchardgrass was placed in the in situ bags in Exp. 2.
Bowman and Firkins (1993) suggested that alfalfa addition to big bluestem increased the rate of NDF disappearance by increasing CMCase concentration early in fermentation. This could explain why NDF disappearance continued to increase in the latter part of incubation for big bluestem and combination treatments, but reached a plateau for alfalfa.
Silva et al. (1987) reported that DM disappearance was highly correlated with CMCase activity at 24 and 48 h (r = 0.98 and 0.94, respectively). Bowman and Firkins (1993) also reported a high regression coefficient between NDF disappearance and CMCase cumulative area under the curve (r2 = 0.93). In our study, regression coefficients between CMCase activity and DM or NDF disappearance were lower than the reports of Silva et al. (1987) and Bowman and Firkins (1993); however, these authors evaluated data at specific time points, whereas we combined data from all time periods for correlation analysis.
Substitution of legumes for grass hay has increased DMI by cattle (Hunt et al., 1988; Prigge et al., 1990; Das and Singh, 1999) and sheep (Moseley and Jones, 1979; Bowman and Asplund, 1988; Bird et al., 1994). Other researchers have reported that the response to legume substitution is affected by level of feeding. Hunt et al. (1985) reported that DMI increased linearly with increasing proportions of alfalfa in a tall fescue diet when lambs were fed ad libitum but not restricted diets; however, their data show that addition of 25% alfalfa to tall fescue diets increased intake 8 and 29% for restricted and ad libitum diets, respectively. Cherney et al. (1990) also reported an interaction between level of intake and forage source for OM and NDF intake.
Legumes may increase intake due to a lower NDF content compared with grasses (Bowman and Asplund, 1988) or due to a higher N content. Neutral detergent fiber content contributes to bulk fill and is thought to be related to forage intake (Reid et al., 1988), and in our study, alfalfa had a lower NDF content than orchardgrass. Bowman and Asplund (1988) concluded that increased DMI in sheep fed grass supplemented with alfalfa was due to an increased amount of N and cell solubles available for microbial growth and fiber digestion. Supplemental protein increased forage intake by stimulating microbial growth, thereby increasing percentage of DM digested, whether measured in vivo (Coleman and Wyatt, 1982) or in vitro (McCollum and Galyean, 1985), and rate of passage (Coleman and Wyatt, 1982; McCollum and Galyean, 1985; Petersen, 1987). Van Soest (1994) reported that depressions in DMI occurred when the CP content of the forage was less than 7%. In the current study, the CP content of the orchardgrass fed was 6.5%, which is below the threshold level of protein content described in the preceding study. This may help explain why the DMI by steers fed orchardgrass was less than that of those fed mixtures of orchardgrass and alfalfa. Similarly, Moore et al. (1999) suggested that supplementation decreased intake when the forage TDN:CP was <7, and increased intake when this ratio was >7. In our study the TDN:CP of orchardgrass was 7.7; therefore, the addition of alfalfa would be expected to increase intake. When a restricted amount of hay was fed, we were substituting rather than supplementing alfalfa; nevertheless, this relationship was true when animals were fed ad libitum diets. This is likely a function of a greater CP content of the orchardgrass plus alfalfa diet.
Another possible explanation for the increased DMI seen with alfalfa addition to orchardgrass hay fed ad libitum is the potential for animals to have selected for alfalfa, possibly a more palatable and digestible forage. Cattle have been shown to prefer forages that have greater total nonstructural carbohydrate contents (Fisher et al., 2002), and the chemical composition of the alfalfa in Exp. 1 and 2 would fit this explanation.
Our results agree with reports that total tract OM and fiber digestibility of grass-based diets were not affected by intake level (Varga and Prigge, 1982; Firkins et al., 1986) or legume supplementation (Lagasse et al., 1990; Bird et al., 1994). In contrast to our data, Alwash and Thomas (1971) reported that total tract OM digestibility decreased with increased level of feeding. Firkins et al. (1986) reported similar ruminal and total tract OM digestion between high and low intake levels; however, ruminal NDF digestion was greater for the low intake level whereas total tract NDF digestion was similar between intake levels. Digestion occurring in the lower tract may compensate for decreased ruminal digestion (Moseley and Jones, 1979; Owens and Goetsch, 1986; Okine and Mathison, 1991) and could help explain the lack of differences observed in total tract nutrient digestibility as a result of increased intake level and legume substitution in the current study.
Hunt et al. (1985) reported that concentration of alfalfa had a quadratic effect on total tract DM and NDF digestibility when lambs were fed ad libitum with a tendency for increased digestibility with combination diets (alfalfa and tall fescue); however, when lambs were fed restricted amounts, increasing concentration of alfalfa caused DM digestibility to increase linearly, whereas NDF digestibility decreased linearly. These authors suggested that positive associative effects for grass-legume diets may not be present for all combinations and suggested that further research was needed to determine the exact cause of these associative effects. In other experiments, alfalfa addition to grass diets decreased total tract NDF digestibility but had no effect on total tract DM digestibility (Prigge et al., 1990; Vanzant and Cochran, 1994). Prigge et al. (1990) reported similar NDF digestibilities for a 25% alfalfa:75% switchgrass combination and a 100% switchgrass diet (63.8 and 62.1%, respectively), which agrees with our results. In contrast, DM and fiber digestibility increased when wheat straw was supplemented with Egyptian clover (Das and Singh, 1999) and when barley straw was supplemented with alfalfa (Haddad, 2000). Bowman et al. (1991) reported that legumes increased total tract OM digestibility when fed with late, but not with early, maturity orchardgrass. Similarly, Lagasse et al. (1990) substituted alfalfa in Bermudagrass (warm season) or orchardgrass (cool season) diets and reported that alfalfa addition did not change OM and NDF digestibility of grass; however, they concluded that feed intake and digestibility response to alfalfa supplementation varied with grass characteristics. The effect of legume supplementation on digestibility cannot be generalized, and variation in results between studies is most likely due to the inherent chemical constituents of the grass. In addition, lack of differences in digestibility observed in our study could be due to the technique used to estimate total tract digestibility.
Generally, increased intake will increase the flow of liquid and particulate matter (Van Soest, 1994), which could result in decreased digestion because particle retention time is decreased. Similarly, Okine and Mathison (1991) reported decreased cumulative ruminal NDF digestion with increasing intake. Our results may differ because we estimated extent in situ, which means samples were in the rumen the same amount of time and not subject to the kinetics of rumen turnover. This agrees with Van Soest (1994) who suggested that digesta spend less time in the rumen at higher intake. Varga and Prigge (1982) reported no difference in retention time due to intake level, but Alwash and Thomas (1971), Okine and Mathison (1991), and Luginbuhl et al. (1994) did observe shorter MRT of particulate matter with increased forage intake.
Other researchers have also reported that legume substitution reduced total tract and ruminal retention time in cattle and sheep (Moseley and Jones, 1979; Prigge et al., 1990); however, Varga and Prigge (1982) reported no difference in retention time due to forage source. Ndlovu and Buchanan-Smith (1985) reported shorter MRT when corn cobs, but not barley straw or bromegrass hay, were supplemented with 30% alfalfa. Our MRT values for large and small particles of orchardgrass were similar to the values reported by Bowman et al. (1991) with and without 25% red clover substitution with orchardgrass. However, in contrast to our data, MRT of small orchardgrass particles was not influenced by 25% red clover substitution, whereas the MRT of large orchardgrass particles was longer in animals fed diets containing red clover and orchardgrass than those fed orchardgrass only (Bowman et al., 1991). We used a 2.36-mm screen to separate large and small particles, whereas Bowman et al. (1991) used a 1.68-mm screen; therefore, some of their large particles likely would have been defined as small particles in our study, which could explain differences between their results and ours. Our data would support those of Poppi et al. (1981) and Shaver et al. (1988) who concluded that small, and not large, particles were important in influencing DM retention time in the rumen and thus affecting voluntary forage intake (Campling, 1970; Thornton and Minson, 1973).
Okine and Mathison (1991) reported increased ruminal and total tract NDF passage rates when intake increased from 1 to 1.7x maintenance; however, several researchers did not find any differences in passage rate of digesta due to intake level (Varga and Prigge, 1982; Luginbuhl et al., 1994). Only passage rate of small orchardgrass particles was influenced by alfalfa substitution. Small orchardgrass particles were probably the predominant fraction in the rumen because 75 to 100% of the diets were orchardgrass and 60 to 72% of the rumen contents were probably small particles (Poppi et al., 1981; Shaver et al., 1988).
Other workers (Varga and Prigge, 1982; Lagasse et al., 1990) did not report any difference in the passage rate of digesta due to alfalfa supplementation; however, passage rate increased linearly (Vanzant and Cochran, 1994) and quadratically (Prigge et al., 1990) with increasing amount of alfalfa supplementation. Hunt et al. (1988) reported that alfalfa supplementation increased passage rate of alfalfa and wheat particles (linearly and quadratically, respectively) under conditions of ad libitum intake, but only passage of alfalfa particles were affected when feeding was restricted. Ndlovu and Buchanan-Smith (1985) reported faster passage rates with 30% alfalfa supplementation to corn cobs, but not barley straw or bromegrass hay, and attributed the effect to increased N or the greater digestibility of alfalfa cell wall. Once again, differences between studies could be due to the size of the screen used to separate large and small particles or the forage characteristics of the legume and grass fed. Our data support those of Woodford and Murphy (1988) and Okine and Mathison (1991) who concluded that small particle passage is important in regulating digesta passage from the rumen and forage intake.
In conclusion, nutrient intake increased with alfalfa included in the diet of steers fed orchardgrass hay for ad libitum intake, and ad libitum intake was associated with shorter MRT of large and small orchardgrass particles, a faster rate of passage of small orchardgrass particles, and a decreased rate of fiber digestion. Alfalfa substitution increased the rate of passage of small orchardgrass particles and the rate of DM and NDF digestion, and tended to reduce the mean retention time of small particles. These data support research suggesting that clearance of small particles from the rumen is the major factor affecting voluntary intake in cattle and sheep. Alfalfa addition to grass hay had different effects with warm and cool season grasses, and the mechanism of improved digestion with legume substitution has not been clearly established. Future research should investigate specific gravity, fluid kinetics, and digestion of large and small particles as well as passage rates of each.
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2 Corresponding author: jbowman{at}montana.edu
Received for publication October 17, 2006. Accepted for publication September 26, 2007.
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), 25% alfalfa-75% big bluestem (
), 50% alfalfa-50% big bluestem (
), and 100% alfalfa-0% big bluestem (x) forage combinations in Exp. 1. a–cWithin a time, the lowercase letters above the data points indicate statistical significance; means without a common superscript letter differ (P 