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Influence of macronutrients and polyethylene glycol on intake of a quebracho tannin diet by sheep and goats^1,2

Department of Rangeland Resources, Utah State University, Logan 84322-5230

³ Correspondence:
phone: (435) 797-2539; fax: (435) 797-3796; E-mail:
villalba{at}cc.usu.edu.

	Abstract

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We determined if supplemental macronutrients or polyethylene glycol (PEG) influenced intake of a tannin diet. Sheep (lambs 5 mo age, 36 kg) and goats (kids 7 mo age, 32 kg) were fed supplements high in either energy or protein or offered a choice between the two supplements before and after receiving a meal containing 15% quebracho tannin. The effect of PEG, a compound that attenuates the negative effects of tannins, was assessed by offering PEG while animals consumed the tannin diet for 4 h/d. Intake of the tannin diet was influenced by both macronutrients and PEG. Animals that chose their own supplements or that received the high-protein supplement, consumed more of the tannin diet than animals fed the high-energy supplement: 34 and 36 vs 31 g/kg^0.75 (lambs) and 41 and 39 vs 34 g/kg^0.75 (kids), respectively (P < 0.05). Animals supplemented with PEG ate much more of the tannin diet than unsupplemented animals: 70 vs 39 g/kg^0.75 (lambs) and 63 vs 34 g/kg^0.75 (kids), respectively (P < 0.001). Sheep and goats consumed more tannin food when given PEG than when supplemented with macronutrients (51 and 38 g/kg^0.75, P < 0.001). Sheep and goats offered a choice between supplements consumed more CP than animals fed the high-energy supplement and more ME than animals fed the high-protein supplement (P < 0.05). In so doing, they selected a combination of foods that yielded a more balanced intake of macronutrients, while achieving high levels of intake of the tannin food. Sheep and goats can be used as an environmentally safe and economically sound means to reduce the abundance of tannin-rich vegetation. Macronutrients and PEG enhance use of tannin-containing plants, which may increase production of alternate forages and create a more diverse mix of species in a plant community.

Key Words: Food Preference • Goats • Polyethylene Glycol • Sheep • Supplements • Tannins

	Introduction

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Soluble phenolics are the most widely distributed plant secondary compounds (Rhoades and Cates, 1976). Tannins are a diverse group of soluble phenolics that form complexes with proteins and carbohydrates and in the process reduce forage digestibility (Robbins et al., 1991). Besides affecting digestibility, tannins may be degraded in the gut and absorbed, exerting their toxic actions systemically (Provenza et al., 1990). Absorbed tannins must be excreted in urine and feces, which incurs metabolic cost (McArthur and Sanson, 1993). Thus, tannins may simultaneously reduce digestion and have postabsorptive (e.g., toxic) effects that influence an animal’s nutritional state.

Offering animals choices between foods that differ in macronutrient content is one way to assess how tannins influence needs for macronutrients. Ruminants learn about the consequences of food ingestion (Provenza, 1995, 1996), and they discriminate between the postingestive effects of energy and protein (Villalba and Provenza, 1999). Thus, the influence of tannins on nutrient balance may cause animals to modify their preferences for macronutrients.

Substances that limit tannin bioavailability also affect the nutritional status and preferences of animals consuming tannin diets. Polyethylene glycol (PEG) is a polymer that binds tannins irreversibly, reducing the negative effects of tannins on food intake (Silanikove et al., 1994), digestibility (Silanikove et al., 1996), and preferences (Titus et al., 2001).

Sheep and goats are mixed feeders, capable of using substantial amounts of high-tannin browse in their diets, frequently under conditions of water scarcity (Landau et al., 2000). Thus, sheep and goats have the potential to be utilized to control tannin-rich vegetation under range conditions. The objective of this study was to determine how use of tannin-containing foods by sheep and goats was influenced by supplementation (1) with foods that differed in concentrations of energy and protein or (2) with PEG.

	Materials and Methods

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The study was conducted at the Green Canyon Ecology Center, located at Utah State University in Logan. Twenty-four commercial crossbred lambs (5 mo of age, 10 females and 14 males) weighing 36 kg (SEM = 0.8) and 24 dairy kid goats (7 mo of age, 9 females and 15 males) weighing 32 kg (SEM = 0.9) were used. Throughout the experiments, animals were individually penned and had free access to trace mineral blocks and fresh water.

Familiarization Period
The tannin-containing diet was formulated using quebracho tannin, a source of condensed tannin extracted from the South American quebracho tree (Aspidosperma quebracho). Quebracho tannin is a complex of tannins, flavonoids, and other phenolic compounds (Mole and Waterman, 1987). The quebracho we used contains approximately 85% tannin (Titus and Provenza, unpublished results).

To give animals experience with the 15% quebracho-tannin diet (Table 1) and obtain baseline information on intake, all animals were offered the tannin diet from 0800 to 1200 for five consecutive days so the food was no longer novel (Burritt and Provenza, 1996). From 1200 to 1245, animals received alfalfa pellets. The animals were familiar with supplements high in energy and high in protein (Table 1) from a previous study (Villalba et al., 2002). Representative samples of the diet and supplements were collected, pooled, and analyzed for CP (AOAC, 1975).

Supplements were offered before and after the tannin diet to assess supplement preference immediately before and after ingesting tannins. We did not want to satiate the animals before they ate the tannin diet, so we restricted the amount of exposure to the supplements. After tannin ingestion, we extended the time of exposure to the supplements to allow the animals to achieve higher levels of nutrient intake.

After the 9-d period, all animals were offered a choice between the high-energy and high-protein supplements before and after consuming the tannin diet for 4 h/d. The procedure was repeated for 3 d.

Trial 2: Influence of PEG on Intake of the Tannin Diet
The objective of this trial was to assess the effects of PEG on use of a tannin diet. Animals from Trial 1 were rerandomized and 4 animals/group were assigned to two new groups (12 animals/group). Animals in Group 1 (treatment) received a combination of PEG (MW 3,350; Spectrum Chemical, Los Angeles, CA) and barley grain with decreasing proportions of barley from d 1 to d 4: 60:40; 70:30; 70:30; 80:20, respectively, to facilitate ingestion of PEG. From d 5 to d 8, animals in Group 1 were offered PEG without grain. Animals in Group 2 (control) received the amount of barley offered to Group 1 from d 1 to d 4. From d 5 to d 8, Group 2 did not receive any supplement.

From 0900 to 1300 all animals had ad libitum access to the tannin diet offered with (treatment) or without (control) PEG. Refusals were collected and weighed, and intake was determined. At 1300 all animals were offered 250 g of alfalfa pellets. The procedure was repeated for 8 d.

Trial 3: Influence of Macronutrients on Intake of the Tannin Diet after Supplementation with PEG
The objective of this trial was to determine how offering a choice of supplements high in energy and high in protein affected use of a tannin diet by sheep and goats previously supplemented with PEG. After Trial 2, animals supplemented with PEG (treatment) or not (control) were offered choices of the high-energy and high-protein supplements before and after ingesting the tannin diet, as described for Trial 1. The procedure was repeated for 4 d.

Statistical Analyses
The statistical design for the study was a split plot, and separate analyses were conducted for each species. When F values were significant (P < 0.05), means were compared using the LSD test. Throughout the study, intake of the tannin diet and supplements was converted to grams of food ingested/kg of metabolic body weight (kg^0.75), and we estimated the amount of metabolizable energy (ME_intake: [Intake of Food High in Energy x ME_{Food High in Energy}] + [Intake of Food High in Protein x ME_{Food High in Protein}] ), crude protein (CP_intake: [Intake of Food High in Energy x CP_{Food High in Energy}] + [Intake of Food High in Protein x CP_{Food High in Protein}]) and CP/ME ratio that each animal consumed/kg^0.75 with the supplements.

Preference Tests.
Intake of supplement during preference tests was the dependent variable. For Group 3 (Trial 1), animals and supplements (high-energy or high-protein) were the whole-plot factors and day was the subplot. When all animals had a choice of supplements (last 3 d of Trial 1, Trial 3), animals were nested within groups. Group (1, 2, or 3 [Trial 1]; PEG or Control [Trial 3]) was the between-subject factor, while supplement (high-energy or high-protein) and day were the within-subject factors in the split plot.

Tannin Diet, ME, CP, and Single Supplement Intake.
The dependent variables for these analyses were tannin diet intake, calculated intakes of ME and CP, and supplement intake for the groups without a supplement choice; animals were nested within groups. Group was the between-subject factor, and day was the repeated measure in the analysis.

Protein/Energy Ratios.
Animals and time of supplementation (AM: before sagebrush ingestion or PM: after sagebrush ingestion) were the whole-plot factors and day was the subplot. When all animals had a choice between supplements (last 3 d of Trial 1, Trial 3), animals were nested within groups. Group (1, 2, or 3 [Trial 1]; PEG or Control [Trial 3]) was the between-subject factor, while time of supplementation (AM or PM) and day were the within-subject factors in the analysis.

Effects of Supplementation on Tannin Diet Intake.
To determine whether intake of the tannin diet changed from Trial 2 (PEG supplementation) to Trial 3 (macronutrient supplementation), intake of the tannin diet during the last 2 d of each trial was analyzed as described above, and Trial and day were the repeated measures in the analysis.

	Results

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Familiarization Period
Intake of the tannin diet did not differ among groups during the familiarization period (group effect and group x day interaction, P > 0.05; Figure 1). Averaged across the 5 d, sheep and goats in Groups 1, 2, and 3 consumed, respectively, 18, 18, and 18 g/kg^0.75 (SEM = 2.6) and 30, 27, and 29 g/kg^0.75 of the tannin diet (SEM = 2.2) (P > 0.05). Sheep increased intake of the diet across days (P < 0.001), but goats did not (P > 0.05; Figure 1).

View larger version (18K): [in this window] [in a new window]   Figure 1. Intake of a 15% quebracho tannin diet by sheep and goats during Trial 1. Double arrows indicate different supplementation regimes. Days 1–5 (Familiarization Period): Animals had alfalfa pellets for 45 min after receiving the tannin diet. Days 6–14: Animals were given an energy concentrate (Group 1), a protein concentrate (Group 2), or a choice between the two foods (Group 3) before (for 5 min) and after (for 15 min) receiving the tannin diet. Days 15–17: All groups had a choice between the energy concentrate and the protein concentrate before (for 5 min) and after (for 15 min) eating the tannin diet. Values are means for 8 animals; standard errors are represented by vertical bars.

Lambs without a choice ate more of the high-energy supplement (Group 1) than of the high-protein supplement (Group 2; P < 0.05; Figure 2). Animals fed the high-energy supplement (Group 1) consumed more ME and less CP than animals fed the high-protein supplement (Group 2) (P < 0.05; Table 2). When offered a choice (Group 3), lambs and goats preferred the high-energy to the high-protein supplement (P < 0.05; Figure 2). Intake of ME by animals offered a choice of the two supplements (Group 3) was higher than for animals fed the high-protein supplement (Group 2) and for goats fed the high-energy supplement (PM) but lower than for lambs fed the high-energy supplement (AM). Intake of CP by Group 3 (choice) was higher than for Group 1 (high-energy) and lower than for Group 2 (high-protein) (P < 0.05; Table 2).

View larger version (26K): [in this window] [in a new window]   Figure 2. Intake of supplements by sheep and goats before (AM) and after (PM) eating a 15% quebracho tannin diet. During Trial 1 (d 6–14), animals were given an energy concentrate (Energy), a protein concentrate (Protein), or a choice between the two foods (Choice). During the last 3 d of Trial 1 (d 15–17) and 3, all animals had a choice between the energy concentrate and the protein concentrate. In Trial 3 animals were formerly supplemented (PEG) or not (Control) with polyethylene glycol. Values are means for 8 (Trial 1) or 12 (Trial 3) animals; standard errors are represented by vertical bars.

When all animals were offered a choice of the two supplements after d 9, there were no differences in intake of the tannin diet (P >0.05; Figure 1). Animals previously offered the high-energy (Group 1) or the high-protein (Group 2) supplement ate both supplements when offered a choice (group effect, P > 0.05; group x supplement interaction, P > 0.05; Figure 2). Because of the similar pattern of consumption of both supplements, there were no differences in ME or CP intake among groups (P > 0.05; Table 2). Averaged across groups, lambs and goats selected a lower ratio of CP/ME before (AM) than after (PM) consuming the tannin diet: 70 vs 73 g/Mcal (SEM = 1.3; P < 0.1) and 71 vs 73 g/Mcal (SEM = 0.6; P < 0.05).

Trial 2: Influence of PEG on Intake of the Tannin Diet
Supplemental PEG increased intake of the tannin diet by sheep and goats (Figure 3). Averaged across days, animals in the control and treatment (PEG) groups consumed 39 and 70 g/kg^0.75 (lambs; SEM = 3.2; P < 0.001) and 34 and 63 g/kg^0.75 (goats; SEM = 2.7; P < 0.001), respectively. In contrast to the control group, animals in the treatment group increased intake of the tannin diet across days (day effect, P < 0.001; group x day interaction, P < 0.001).

View larger version (14K): [in this window] [in a new window]   Figure 3. Intake of 15% quebracho tannin diet by sheep and goats during Trials 2 and 3. The double arrows indicate different supplementation regimes. Days 1–8 (Trial 2): Animals were (PEG) or were not (control) offered polyethylene glycol (PEG) while consuming a 15% quebracho tannin diet for 4 h/d. Days 9–12 (Trial 3): All animals had a choice between the energy concentrate and the protein concentrate before (for 5 min) and after (for 15 min) eating the tannin diet. Values are means for 12 animals; standard errors are represented by vertical bars.

View larger version (10K): [in this window] [in a new window]   Figure 4. Intake of polyethylene glycol (PEG) by sheep and goats during Trial 2. PEG was offered along with a 15% quebracho tannin diet. Polyethylene glycol was mixed with decreasing amounts of barley grain daily until d 4 (60:40; 70:30; 70:30; 80:20). Plain PEG was offered from d 5 to d 8. Values are means for 12 animals; standard errors are represented by vertical bars.

Animals formerly supplemented or not with PEG during Trial 2 ate both supplements when offered choices between high-energy and high-protein foods in Trial 3 (group and group x supplement interactions, P > 0.05; Figure 2). Because of the similar pattern of consumption of both supplements, there were no differences in intake of ME or CP (P > 0.05; Table 2). Animals selected a lower ratio of CP/ME before (AM) than after (PM) consuming the tannin diet: 69 vs 73 g/Mcal (lambs, SEM = 0.8; P < 0.001) and 63 vs 69 g/Mcal (goats, SEM = 0.5; P < 0.001).

	Discussion

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Effect of Single Supplements on Tannin Intake
Sheep and goats fed the supplement high in protein ate more high-tannin food than animals fed the supplement high in energy (Figure 1), which suggests that protein was more limiting than energy for utilizing tannins. Proteins complex selectively with tannins (Hagerman and Butler, 1980). Thus, protein supplementation may have prevented the formation of tannin-enzyme complexes and minimized the negative effects of tannins. Nitrogen supplementation improves the digestibility of forages that contain condensed tannins (Petersen and Hill, 1991). Thus, increased nitrogen retention on low-nitrogen diets or increased availability of nitrogen on high-nitrogen diets is likely to offset the metabolic costs of tannins (McArthur and Sanson, 1993).

Supplemental energy depressed intake of the high-tannin food. Excess supplementation can result in substitution when grazed nutrients are exchanged for supplemental nutrients (Caton and Dhuyvetter, 1997). In addition, the negative synergistic effects of energy concentrates (e.g., low ruminal pH, decreased cellulolytic populations of bacteria) (Head, 1953; Mertens and Loften, 1980) and tannins (Foley et al., 1999) on rates of fiber digestion and particle size reduction also could have affected intake of the tannin diet by animals fed the high-energy supplement. However, the amount of tannin diet ingested was too low to be interpreted solely as an effect of rumen fill and low-fiber digestion. It is more likely that nitrogen limitation and the postabsorptive (e.g., toxic) effects of tannins were more important than gut fill on use of the tannin diet by animals fed the high-energy supplement. The cyclic pattern of tannin diet consumption—gradual increases followed by declines (Figure 1)—is characteristic of nutritious foods that contain toxins (Pfister et al., 1997).

Effects of Supplement Choice on Tannin Intake
Sheep and goats offered a choice between high-energy and high-protein supplements consumed more of the tannin diet than animals fed only the high-energy supplement. They consumed similar or higher amounts of ME (only lambs consumed lower amounts of ME during AM) and more CP than animals offered the high-energy supplement and more ME and less CP than animals fed the high-protein supplement (Table 2). Thus, when offered a choice, animals selected a combination of foods that yielded a more balanced intake of macronutrients, while achieving high levels of intake of the tannin food.

When all animals had a choice of supplements, differences in intake of the tannin food disappeared (Figure 1). This was due mainly to a decrease in intake of the tannin diet by animals previously supplemented with protein. The sharp decline in ingestion of the tannin food on d 14 by goats offered a choice also attenuated differences. This decline in intake may have been due to the negative effects of foods high in energy on intake of the tannin diet (Figure 1) or to the cyclic patterns of ingestion of foods that contain toxins (Pfister et al., 1997).

Ingestion of quebracho tannin has both ruminal (e.g., digestibility inhibition) and postabsorptive (e.g., toxic) effects that impose metabolic costs to herbivores (McArthur and Sanson, 1993; Dawson et al., 1999). Tannins alter the supply of VFA and nitrogen to the host (Makkar et al., 1995; Landau et al., 2000), and degraded and absorbed tannins must be metabolized and excreted (McArthur and Sanson, 1993), which requires the ingestion of adequate types and amounts of macronutrients (Illius and Jessop, 1995; 1996).

One way to assess how tannins influence macronutrient needs is to offer choices between foods that differ in macronutrients. We hypothesized that if lambs consumed tannins and then were offered foods differing in their ratios of protein/energy, they would self-select a supplement that best met their needs. Tannin-rich diets likely are experienced as low-protein diets. Lambs and goats given a choice selected ratios of CP/ME that ranged between 65 to 75 g CP/Mcal ME, which are similar to those found when sagebrush (Artemisia tridentata Nutt.) is the basal diet (Villalba et al., 2002) and higher than those recommended in tables of nutrient requirements (NRC, 1981, 1985). Ratios of CP/ME were higher after than before ingestion of the tannin diet, which is also consistent with the notion that tannins increased need for protein.

When offered choices between the high-energy and high-protein supplements, animals previously fed single supplements showed equal preference for the two foods (Figure 2). This contrasts with the preferences of sheep and goats for a supplement high in either energy or protein when the basal diet is sagebrush or alfalfa pellets (Villalba et al., 2002). Tannin ingestion evidently promoted an internal state different from sagebrush or alfalfa, reflected in a more balanced use of both supplements (Figure 2).

Sheep and goats also ate both the high-energy and the high-protein supplements in Trial 3 after being supplemented with PEG in Trial 2 (Figure 2). Even though PEG attenuates the negative effects of tannins and PEG-treated groups consumed much more of the tannin food than controls (Figure 3), this was not reflected in a different pattern of macronutrient selection. Controls that did not receive PEG in Trial 2 increased consumption of the tannin food when supplemented with macronutrients in Trial 3. However, PEG enhanced intake of the tannin food much more than macronutrients. Sheep and goats given PEG during Trial 2 consumed, respectively, 51 and 38 g/kg^0.75 more tannin food than when supplemented with macronutrients during Trial 3.

Effects of PEG on Tannin Intake
Polyethylene glycol is a polymer that binds to tannins irreversibly over a wide range of pH, thus alleviating the negative effects of tannins (Landau et al., 2000). Supplemental PEG increases intake of tannin-containing plants by sheep, goats (Pritchard et al., 1988; Titus et al., 2000, 2001), and cattle (Hanningan and McNeill, 1998). In our study, PEG substantially increased intake of food high in tannin (Figure 3). Thus, the low intakes of quebracho tannin during Trials 1 and 3, when PEG was absent, cannot be accounted solely by digestion inhibition and gut fill. We suggest that intake of the tannin diet was constrained primarily by lesions of gut mucosa and toxicity (Reed, 1995; Dawson et al., 1999), which explains not only the lower levels of intake of the tannin diet without PEG, but also the abrupt declines in intake when PEG supplementation was suspended (Figure 4).

Minimal PEG:condensed tannin ratios of 1:4 to 1:8 for goats (Silanikove et al., 1997), and 1:2 for sheep (Silanikove et al., 1994) may totally neutralize the negative effects of condensed tannins on the intake of tannin-rich foods. In our study, lambs and goats supplemented with PEG consumed 70 and 63 g tannin diet/kg^0.75, which represents 11 and 10 g tannin/kg^0.75, respectively. Sheep and goats consumed about 13 and 7 g PEG/ kg^0.75 by the end of the trial, which suggests that animals were consuming adequate amounts of PEG to neutralize the negative effects of quebracho tannin.

Polyethylene glycol is not absorbed by animals and serves as an inert marker in digestion trials (Bauman et al., 1971). Nevertheless, lambs and goats continued to ingest PEG in amounts that were adequate to neutralize the levels of tannins ingested with their basal diet. These results are consistent with previous findings that sheep self-regulate their intake of PEG according to the amount of quebracho tannin in their diets (Provenza et al., 2000) and recognize the benefits of ingesting PEG when fed diets high in tannins (Villalba and Provenza, 2001).

Comparative Responses of Sheep and Goats
Goats typically utilize tannin-rich foods better than do sheep (Landau et al., 2000). Food intake and dry matter digestibilities of tannin-containing forages are often higher for goats than for sheep (Silanikove et al., 1996), and goats often use protein more efficiently than do sheep (Watson and Norton, 1982; Doyle et al., 1984; Kronberg and Malechek, 1997). Differences in ruminal fermentation and adaptation of rumen microbes to tannins also may enable goats to more efficiently use tannin-rich foods (Landau et al., 2000).

Some animals have adaptations, like production of salivary proteins with a high affinity for binding tannins, that minimize the potential for tannin toxicity (Robbins et al., 1987). Sheep and goats do not produce such salivary proteins (Austin et al., 1989; Distel and Provenza, 1991), but goats secrete more saliva containing a higher level of nitrogen than do sheep (Domingue et al., 1991). A 50% reduction in soluble nitrogen in extrusa samples from the esophagus of goats consuming blackbrush, a tannin-containing shrub (Provenza and Malecheck, 1984), also suggests that even when high-affinity proteins may not be present in the saliva of goats, other salivary proteins may contribute to forming complexes with tannins and alleviating their negative effects.

Despite the aforementioned differences in tannin utilization by sheep and goats, the pattern and amount of tannin food consumed by both species in the present study was very similar (Figures 1 and 3). The pattern of supplement preference also was similar, though goats consumed more supplement than did sheep (Figure 2). It is likely that the supplementation programs we used minimized differences in tannin utilization.

	Implications

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Sheep and goats can be used as an environmentally safe and economically sound alternative to reduce the abundance of tannin-rich foliage, which would likely enhance production of grasses and create a more diverse mix of plants. This study suggests that polyethylene glycol is more important than macronutrients at influencing use of quebracho tannin by livestock. Nevertheless, quebracho tannin use was higher with protein than with energy supplementation. Thus, energy supplementation in areas dominated by plants high in tannins, where potential sources of protein, such as grasses and forbs are unavailable, may limit use of high-tannin plants by livestock. Lambs and goats allowed to select their own supplement ingested levels of tannins comparable to animals supplemented with protein, but they achieved a more balanced ingestion of macronutrients than animals receiving single supplements. Choices between energy and protein concentrates and polyethylene glycol may enhance use of tannin-containing plants by livestock.

	Footnotes

 
¹ This research was supported by grants from the Utah Agricultural Experiment Station. This paper is published with the approval of the Director, Utah Agricultural Experiment Station, Utah State University, Logan, UT, as journal paper number 7365.

² We acknowledge S. Hammond for technical support.

Received for publication December 17, 2001. Accepted for publication June 11, 2002.

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