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Influence of macronutrients and activated charcoal on intake of sagebrush 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 activated charcoal influenced intake of sagebrush, a shrub that contains terpenes. Sheep (lambs 3 mo age, 33 kg) and goats (kids 5 mo age, 28 kg) were fed supplements high in energy, high in protein, or offered a choice between the two supplements before and after receiving sagebrush for 4 h/d. The effect of activated charcoal, a compound that reduces bioavailability of toxins, was assessed by offering charcoal mixed with a carrier or the carrier alone while animals consumed sagebrush for 4 h/d. The effects of supplemental macronutirents were tested before and after charcoal supplementation. Activated charcoal did not increase intake of sagebrush by sheep or goats (P > 0.05), but supplemental macronutrients influenced intake of sagebrush by both species (P < 0.05). Intake of sagebrush was markedly higher for animals fed the high-protein supplement than for animals fed the high-energy supplement (P < 0.05). Lambs (before and after charcoal supplementation) and kids (after charcoal supplementation) offered a choice between the two supplements used sagebrush at levels comparable to animals supplemented with protein. They consumed more CP than animals fed the high-energy supplement and more ME than lambs fed the high-protein supplement (P < 0.05). Thus, providing a choice between supplements enhanced intake of sagebrush and enabled animals to achieve a more balanced intake of macronutrients. Preference for supplement and sagebrush also depended on an animal’s immediate past history with a supplement. Sheep and goats previously fed the high-energy or the high-protein supplement preferred, respectively, the high-protein or the high-energy supplement when given a choice (P < 0.05). The changes in preference for supplement caused a decreased intake of sagebrush by groups that preferred the energy concentrate, and an increased intake of sagebrush by groups that preferred the protein concentrate. Thus, supplements of different energy and protein densities consistently altered intake of sagebrush by sheep and goats.

Key Words: Artemisia tridentata • Charcoal • Food Preferences • Goats • Sheep • Supplements

	Introduction

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Sagebrush (Artemisia tridentata Nutt.) occurs on millions of hectares of rangeland in the western United States. Though sagebrush is abundant and nutritious, terpenes limit intake of sagebrush (Ngugi et al., 1995). Tolerance of terpenes may depend on nutrient intake because macronutrients are required to bio-transform and excrete toxins (Illius and Jessop, 1995). Supplemental energy enhances consumption of sagebrush by sheep fed alfalfa hay, a good source of protein (Banner et al., 2000).

Offering animals choices between supplements that differ in concentrations of energy and protein is one way to assess the relative importance of energy and protein on use of sagebrush. Ruminants learn about the consequences of food ingestion (Provenza, 1995, 1996), and they discriminate between the postingestive effects of protein and energy (Villalba and Provenza, 1999). If terpenes in sagebrush influence nutrient balance, animals may modify their preferences for foods that differ in macronutrients.

The negative effects of sagebrush also may be attenuated by decreasing the bioavailability of terpenes. Activated charcoal adsorbs toxins (Levy, 1982) and enhances intake of poisonous plants (Poage et al., 2000). Grain supplements containing charcoal enhance intake of sagebrush by sheep fed a basal ration of alfalfa pellets (Banner et al., 2000), but the effects of charcoal on use of sagebrush, in the absence of macronutrient supplementation, are not known.

Our objectives were to determine how use of mountain big sagebrush by sheep and goats was influenced by supplementation: (1) with foods of different energy and protein densities or (2) with activated charcoal.

	Materials and Methods

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The study was conducted at the Green Canyon Ecology Center, located at Utah State University in Logan. We used 24 commercial crossbred lambs weighing 33 kg (SEM = 0.9) and 24 dairy kid goats weighing 28 kg (SEM = 0.9). Each group had a mix of both sexes. Throughout the experiments, animals had free access to trace mineral blocks and fresh water.

Sagebrush
Mountain big sagebrush (Artemisia tridentata Nutt. ssp. Vaseyana (Rydb.) Beetle) was hand-harvested from 10 May to 20 June 1999 from the foothills near Logan in northern Utah. Sagebrush leaves and twigs were clipped, placed in woven, polyethylene feed sacks, and frozen within 4 h of collection. Several days after freezing, the frozen sagebrush was ground with a chipper to 1 to 2 cm in length, mixed for uniformity, placed in plastic bags in 5 kg amounts, and returned to a freezer. Every day during the trial, bags of sagebrush were removed from the freezer, thawed, and fed. Representative samples of sagebrush were collected, pooled, freeze-dried, and analyzed for crude protein content (AOAC, 1975) (Table 1). Sagebrush contained 55% moisture.

Exposure to Supplements.
The main objective of this procedure was to give animals further experience with the supplements. For 5 d, lambs and kids had access to the high-energy supplement from 1200 to 1245. Refusals were collected and weighed. For the next 5 d, the high-protein supplement replaced the high-energy supplement. All animals had alfalfa pellets ad libitum from 0800 to 1200.

The high-energy supplement was offered first because ruminants normally show strong preferences for energy-dense foods (Villalba and Provenza, 1999). Familiarity with the supplemental ingredients (Table 1) and the feeding regime facilitated lambs eating the high-protein supplement.

Initial Preference Tests.
We determined the animals’ supplement preferences before Trial 1 by first offering lambs and kids alfalfa pellets from 0800 to 1200 and then offering 450 g of the high-energy supplement and 450 g of the high-protein supplement from 1200 to 1245. Intake of each supplement was determined from preference tests conducted daily for 2 d.

Exposure to Sagebrush.
To give further exposure and obtain baseline information on sagebrush intake, all animals then were offered sagebrush from 0800 to 1200 for 5 d. From 1200 to 1245 all animals received alfalfa pellets. After the last day of familiarization with sagebrush, animals were randomly assigned to three groups (eight animals/group) balanced according to sagebrush intake.

Trial 1: Influence of Macronutrients on Intake of Sagebrush
This trial determined the effects of supplements high in energy or protein or the preferred combination of both supplements on intake of sagebrush. At 0800 each morning, animals received 300 g of the high-energy supplement (Group 1), 300 g of the high-protein supplement (Group 2), or a choice between 300 g of each supplement (Group 3) for 5 min. Intake of the supplements was recorded. After supplementation, all animals had ad libitum access to sagebrush for 4 h. Refusals were collected and sagebrush intake was calculated. Immediately after collecting sagebrush, all animals were again offered the supplements, as described above, but the time of exposure was 30 min and the amount of supplement offered was 500 g. Throughout the study, supplements were offered ad libitum. Additional supplements, up to 700 g, were offered to animals with higher levels of intake. We restricted the time, rather than the amount of supplements offered to any group of animals. Intake of the supplements was measured and no other food was offered until the next day. The procedure was repeated for 9 d.

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

Trial 2: Influence of Activated Charcoal on Intake of Sagebrush
The objective of this trial was to assess the effects of activated charcoal on use of sagebrush. Animals from Trial 1 (Groups 1, 2, and 3) were rerandomized and four animals/group were assigned to two new feeding regimes (12 animals/group). Animals in Group 1 (treatment) received a 70:30 (w/w) mixture of grape pomace, a low-quality food, and activated charcoal (Norit Americas, Inc.), whereas animals in Group 2 (control) received only grape pomace.

From 0900 to 1300, all animals had ad libitum access to sagebrush offered with either grape pomace:charcoal (treatment) or grape pomace (control). 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 7 d.

Trial 3: Influence of Macronutrients on Intake of Sagebrush After Supplementation with Activated Charcoal
The objective of this trial was to determine the effects of macronutrient supplementation on intake of sagebrush after macronutrient intake was restricted during supplementation with activated charcoal. The protocol was the same as Trial 1. Animals from Trial 2 were randomly assigned to three new groups (eight animals/group). Groups 1, 2, and 3 were offered supplements high in energy, high in protein, or a choice between the two foods, respectively. The procedure was repeated for 7 d. Exposure to the supplements after sagebrush ingestion was reduced to 15 min to prevent acidosis (high-energy supplement) or ammonia toxicity (high-protein supplement) by animals previously restricted in macronutrient intake during Trial 2.

Statistical Analyses
The statistical design for the study was a split-plot. When F-values were significant (P < 0.05), means were compared using the LSD test. Throughout the study, intake of sagebrush 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), crude protein (CP), and CP:ME ratio that each animal consumed/kg^0.75 with the supplements.

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

Sagebrush, ME, CP, and Single Supplement Intake.
The dependent variables for these analyses were sagebrush intake, calculated intakes of ME and CP, and supplement intake for the groups without a supplement choice (Groups 1 and 2); 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), animals were nested within groups. Group (1, 2, or 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 Sagebrush Intake.
To determine whether sagebrush consumption changed from Trial 2 (activated charcoal supplementation) to Trial 3 (macronutrient supplementation), sagebrush intake during the last 3 d of each trial was analyzed as described above, and Trial and d were the repeated measures in the analysis. Animals in Trial 2 were coded with the group number (1, 2 or 3) assigned during Trial 3.

	Results

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Exposure to Supplements
Lambs consumed 21 g/kg^0.75 (SEM = 0.7) of the high-energy supplement and 50 g/kg^0.75 (SEM = 1.0) of alfalfa pellets during the initial 5 d of exposure. For the next 5 d, they consumed 15 g/kg^0.75 (SEM = 0.6) of the high-protein supplement and 74 g/kg^0.75 (SEM = 1.2) of alfalfa pellets. Kids consumed 21 g/kg^0.75 (SEM = 0.7) of the high-energy supplement and 45 g/kg^0.75 (SEM = 1.2) of alfalfa pellets during the first 5 d of exposure. For the next 5 d, they consumed 16 g/kg^0.75 (SEM = 0.7) of the high-protein supplement and 62 g/kg^0.75 (SEM = 1.1) of alfalfa pellets.

Initial Preference Tests
Prior to the trials, when alfalfa pellets were the basal diet, sheep and goats preferred the supplement high in energy to the supplement high in protein: 22 vs 7 (SEM = 1.4) and 22 vs 9 g/kg^0.75 (SEM = 1.9), respectively (P < 0.001).

Exposure to Sagebrush
Initial intake of sagebrush did not differ among groups on d 1 to 5 when alfalfa pellets were the basal diet, prior to macronutrient supplementation (group and group x day interaction, P > 0.05; Figure 1). Sheep and goats increased sagebrush intake across days (P < 0.001; Figure 1).

View larger version (24K): [in this window] [in a new window]   Figure 1. Intake of sagebrush (Artemisia tridentata ssp. vaseyana) for 4 h/d by sheep and goats. Double arrows indicate different supplementation regimes. Days 1 to 5 (Familiarization Period): Animals had alfalfa pellets for 45 min after receiving sagebrush. Days 6 to 14: Animals were given an energy concentrate, a protein concentrate, or a choice between the two foods before (for 5 min) and after (for 30 min) receiving sagebrush. Days 15 to 17: All groups had a choice between the energy concentrate and the protein concentrate before (for 5 min) and after (for 30 min) receiving sagebrush. Values are means for eight animals; standard errors are represented by vertical bars.

Lambs without a choice ate more of the high-energy supplement (Group 1) than the high-protein supplement (Group 2) before, but not after, eating sagebrush. In contrast, the reverse pattern occurred before and after goats ate sagebrush (P < 0.001; Figure 2). When offered a choice (Group 3), preference tended to be higher for the supplement high in energy than for the supplement high in protein (lambs, P = 0.15 [PM]; goats, P = 0.13 [AM] and P = 0.19 [PM]), but a significant difference was manifest only by lambs before sagebrush consumption (P < 0.05; Figure 2).

View larger version (37K): [in this window] [in a new window]   Figure 2. Intake of supplements by sheep and goats before (AM) and after (PM) eating sagebrush. During Trials 1 (d 1 to 9) and 3, 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 10 to 12), all animals had a choice between the energy concentrate and the protein concentrate. Values are means for eight animals; standard errors are represented by vertical bars.

From d 1 to 9, lambs offered a choice between supplements selected a lower ratio of CP/ME before (AM) than after (PM) sagebrush ingestion: 64 vs 71 g/Mcal (SEM = 1.1; P < 0.001). Goats did not differ in the CP/ME ratio selected before and after sagebrush ingestion: 73 vs 74 g/Mcal (SEM = 1.2; P > 0.05).

When all groups were offered a choice of the two supplements from d 10 to 12, animals previously offered the high-energy (Group 1) or the high-protein (Group 2) supplement subsequently preferred the high-protein or the high-energy supplement, respectively (Figure 2). Animals previously fed the high-energy supplement (Group 1) increased intake of sagebrush, whereas animals previously fed the high-protein supplement (Group 2) decreased intake of sagebrush; animals previously fed a choice of the two supplements (Group 3) maintained the same levels of sagebrush intake (group x day interaction; Figure 1, P < 0.001). Lambs and goats previously exposed to the high-energy supplement selected a higher ratio of CP/ME than lambs and goats previously exposed to the high-protein supplement (lambs: 85 vs 57 g/Mcal, SEM = 3.3; goats: 88 vs 74 g/Mcal, SEM = 5.3; P < 0.05). Averaged across groups, lambs selected a lower ratio of CP/ME before (AM) than after (PM) sagebrush ingestion (68 vs 71 g/Mcal; SEM = 1.0; P < 0.05), while goats did not differ in the CP/ME ratio before or after sagebrush ingestion (80 vs 80 g/Mcal; SEM = 1.2; P > 0.05).

Trial 2: Influence of Activated Charcoal on Intake of Sagebrush
Intake of sagebrush was not influenced by activated charcoal (group and group x day interaction, P > 0.05). Averaged across days, lambs and goats in the control and treatment groups consumed 43 and 41 g/kg^0.75 (SEM = 4.2) and 55 vs 47 g/kg^0.75 (SEM = 4.4), respectively. Lambs consumed 8 and 6 g/kg^0.75 (SEM = 1.27) and goats consumed 15 and 13 g/kg^0.75 (SEM = 1.50) of grape pomace and grape pomace:charcoal, respectively (P > 0.05).

Trial 3: Influence of Macronutrients on Intake of Sagebrush After Supplementation with Activated Charcoal
Supplementation influenced intake of sagebrush (group: lambs, P > 0.05; goats, P < 0.05, group x day: lambs, P < 0.05; goats P < 0.001; Figure 3). From d 4 to 7 (sheep) and d 3 to 7 (goats), animals fed the high-energy supplement (Group 1) ate less sagebrush than animals fed the high-protein supplement (Group 2) or than animals offered a choice of supplements (Group 3; P < 0.05; Figure 3). Averaged across days, animals offered a supplement high in energy, high in protein, or given a choice consumed 43, 49, or 47 g/kg^0.75 (lambs, SEM = 4.5), and 37, 58, or 61 g/kg^0.75 (goats, SEM = 5.0) of sagebrush. Sagebrush intake increased across days for animals offered the high-protein supplement or a choice of supplements (P < 0.05; Figure 3), while it decreased (goats; P < 0.05) or did not change (lambs, P > 0.05) for animals offered the high-energy supplement.

View larger version (20K): [in this window] [in a new window]   Figure 3. Intake of sagebrush (Artemisia tridentata ssp. vaseyana) for 4 h/d by sheep and goats during Trial 3. Animals were supplemented with an energy concentrate, a protein concentrate, or a choice between the two foods before (for 5 min) and after (for 15 min) receiving sagebrush. Values are means for eight animals; standard errors are represented by vertical bars.

Compared with Trial 2 when they were supplemented with charcoal, lambs and goats ate more sagebrush when fed the high-protein supplement (lambs: 59 vs 30 g/kg^0.75; goats: 67 vs 55 g/kg^0.75, P < 0.001) or when offered a choice (lambs: 55 vs 34; goats: 66 vs 53 g/kg^0.75; P < 0.001), in Trial 3. Compared to the grape pomace-charcoal supplement in Trial 2, the high-energy supplement increased (lambs: 39 vs 47 g/kg^0.75; P < 0.05) or decreased (goats: 53 vs 31 g/kg^0.75; P < 0.001) sagebrush consumption in Trial 3 (group x Trial interaction, P < 0.001).

	Discussion

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Effect of Supplementation on Intake of Sagebrush
In our study, animals were supplemented with high-energy, high-protein, or a choice of high-energy and high-protein supplements. We offered variable levels of energy and protein rather than restrict animals to foods that were isonitrogenous (and varied in energy) or isocaloric (and varied in protein) for two reasons. First, animals offered a choice could manifest preferences for two nutritionally different supplements. This enabled individuals to meet their multiple nutrient requirements by mixing two suboptimal but complementary foods with sagebrush (Simpson and Raubenheimer, 1999). That result is more difficult to achieve, particularly with a time constraint, when animals are restricted to one nutritional plane (isocaloric) or another (isonitrogenous). Second, we wanted to compare intake of sagebrush by animals given a choice with animals fed only a supplement high in energy and low in protein or a supplement lower in energy but higher in protein. All animals had ad libitum access to the supplements during each trial, so their intakes of sagebrush were influenced by the type of supplements they selected.

Intake of sagebrush was influenced by the balance of protein to energy in the diets of sheep and goats. In Trials 1 and 3, intake of sagebrush was higher for animals fed the high-protein supplement than the high-energy supplement (Figures 1 and 3). High intake of energy concentrates reduces ruminal pH and shifts ruminal bacteria toward greater amylolitic and lower cellulolytic populations, thereby decreasing intake and digestion of the fibrous portion of low-quality diets (Head, 1953; Mertens and Loften, 1980). In high amounts, intake of supplements also can substitute for use of forages (Caton and Dhuyvetter, 1997). While intake of energy was higher for animals fed the high-energy supplement than the high-protein supplement, a reduced appetite due to greater intake of energy was not likely the cause for lower intake of sagebrush. Lambs (Trial 1) and goats (Trial 3) offered a choice of supplements ingested more energy—and protein—than animals fed the high-energy supplement. The additional protein evidently enabled animals offered a choice to eat more sagebrush than animals fed the high-energy supplement. Thus, intake of sagebrush was influenced more by the balance of protein to energy than by the amount of energy ingested.

Intake of sagebrush evidently increased need for protein. The range of CP:ME ratios selected by lambs and kids ingesting sagebrush (60 to 70 g CP/Mcal ME) was higher than reported in other studies (Egan, 1977) or recommended in tables of nutrient requirements (sheep: 53; NRC, 1985; goats: 39 g CP/Mcal ME; NRC, 1981). Lambs also selected ratios of P:E that were higher after (PM) than before (AM) sagebrush ingestion, again suggesting that sagebrush increased protein demands.

Protein intake increased by 3 and 6 g/kg^0.75 in Trial 3 (macronutrient supplementation) compared to Trial 2 (charcoal supplementation). The increase in protein intake was associated with an increase in sagebrush intake of 29 and 13 g/kg^0.75 for sheep and goats, respectively. Protein supplementation was associated with higher levels of sagebrush intake than those reported when sagebrush was mixed with grass hay (Ngugi et al., 1995) or supplemented with barley and alfalfa pellets (Banner et al., 2000).

Supplemental protein enhances intake of low-quality foods by sheep when P:E ratios are below 23 g DP/Mcal DE (Egan, 1977). Digestible P:E ratios are in this range for sagebrush (NRC, 1985). Protein promotes more rapid clearance of food from the reticulo-rumen, due to the increased availability of nitrogen to rumen microorganisms (Redman et al., 1980), and alleviates the inhibition of fiber digestion caused by starch in energy concentrates (El-Shazly et al., 1961).

Lambs and kids showed a low preference for the supplement high in protein during the initial preference tests, when alfalfa was the basal diet, but their preference for the high-protein food increased when the basal diet was sagebrush. Their low preference for the high-protein supplement during the initial preference tests likely was due to the high-protein basal diet (alfalfa) and their exposure to the high-protein supplement for 5 d prior to the test (Villalba and Provenza, 1999).

Influence of Terpenes on Intake of Sagebrush
Sagebrush contains an array of toxins—monoterpenes, methacrolein, and sesquiterpene lactones (Kelsey et al., 1982; Bray et al., 1991; Meyer and Karasov, 1991)—that reduce the amount of sagebrush ruminants can consume. Intake of sagebrush-hay rations is inversely related to the concentration of sagebrush in the mix (Ngugi et al., 1995). Monoterpenes have antimicrobial and digestibility-reducing effects (Oh et al., 1968; Connolly et al., 1980). More recently, their mode of action also has been linked to systemic disruption of cell membranes and liver damage after absorption from the gut (Foley and McArthur, 1994).

Ingestion of toxins increases needs for macronutrients. During detoxification, many toxins are conjugated with compounds like glutathione, glycine, and sulfate to increase their water solubility (Cheeke and Schull, 1985; Cheeke, 1998). This process yields organic acids that perturb acid base balance (Foley et al., 1995 ; 1999). Conjugation and maintenance of acid base balance deplete glucose and catabolize amino acids (Illius and Jessop, 1995). Thus, when animals receive supplemental nutrients, they should be able to eat more of foods that contain toxins (Illius and Jessop, 1996).

Our results suggest the kind and amounts of nutrients are critical for increasing tolerance of toxin-containing plants. Supplemental protein enhanced use of sagebrush, likely by enhancing detoxification processes, whereas supplemental energy decreased intake of sagebrush, likely by potentiating the inhibitory effects of monoterpenes on cellulolytic activity of rumen microbes (Nagy and Tengerdy, 1968). In previous studies, lambs supplemented with an energy concentrate ate more sagebrush than unsupplemented animals (Banner et al., 2000), and lambs fed a protein-energy concentrate or an energy concentrate consumed similar amounts of sagebrush (Burritt et al., 2000). In both studies, animals received a basal diet of alfalfa pellets. In light of the present study, it is likely that energy supplementation per se did not enhance use of sagebrush. Rather, the protein supplied with the basal diet of alfalfa pellets, in combination with the energy, likely influenced intake of sagebrush by affecting P:E ratios.

Effects of Supplement Choice on Intake of Sagebrush
Lambs (Trials 1 and 3) and goats (Trial 3) offered a choice of supplements used sagebrush at levels comparable to animals supplemented with protein (Figures 1 and 2). They consumed similar or more ME and much more CP than animals fed the high-energy supplement, and more ME and less CP than animals fed the high-protein supplement. Thus, animals given a choice selected a combination of foods that yielded a more balanced ingestion of ME and CP than animals that received single supplements. The supplement combination also enhanced use of sagebrush, except during Trial 1, when due to high intakes of energy, goats offered a choice ingested sagebrush in amounts comparable to animals supplemented with energy (Figure 1). When goats ate less of the high-energy supplement during Trial 3 (Figure 2), sagebrush use was comparable to animals supplemented with protein (Figure 3).

Preference for supplement also depended on immediate past history with the supplement. Animals previously fed only the high-energy or the high-protein supplement preferred, respectively, the high-protein or the high-energy supplement, when given a choice (Figure 2), and that influenced intake of sagebrush (Figure 1, d 14 to 17). Intake of sagebrush decreased for groups that preferred the energy concentrate, and increased for groups that preferred the protein concentrate.

More generally, the influence of macronutrients on intake of sagebrush depends (1) on the kind and amount of toxin ingested (Villalba et al., 2002), (2) on requirements due to growth, gestation, and lactation, and (3) on environmental conditions (NRC, 1985, Forbes, 1995). For instance, mature ewes at maintenance selected a high-energy food (50% corn–50% beet pulp) over a high-protein food (85% alfalfa, 15% soybean meal) when offered a choice while foraging on sagebrush (unpublished data). The lower protein requirements of ewes compared to lambs, the needs of ewes for energy while foraging on sagebrush pastures during cool fall days, and the protein supplied by the high-energy supplement plus the sagebrush, can explain the contrasting supplement selection by ewes in a field trial during fall relative to lambs and kids in a pen trial during summer.

Effects of Charcoal on Intake of Sagebrush
Adsorbents like activated charcoal can attenuate the effects of toxins in sagebrush. Positively charged surfaces in activated charcoal bind with the negatively charged molecular surface of most toxins, decreasing their bioavailability (Poage et al., 2000). While activated charcoal did not enhance use of sagebrush in our study, lambs on a basal diet of alfalfa pellets supplemented with a charcoal-barley mix (0.7, 1.7 g charcoal/kg^0.75) ate more sagebrush than lambs supplemented only with barley (Banner et al., 2000). In the present study, lambs and goats consumed comparable or higher levels of activated charcoal (1.7 and 4 g charcoal/kg^0.75, respectively), but restricted nutrient intake apparently limited their use of sagebrush. Sagebrush consumption increased when activated charcoal (Trial 2) was replaced by supplemental macronutrients (Trial 3). Collectively, these results suggest that macronutrients were more important than charcoal for increasing intake of sagebrush and that activated charcoal may enhance sagebrush use only when the appropriate combination of supplemental macronutrients is available (Banner et al., 2000).

Comparative Responses of Sheep and Goats
The limited data on the comparative foraging behavior of sheep and goats focuses primarily on descriptions of diet selection (Gordon and Illius, 1992). Differences in anatomy and physiology can account for dissimilarities in food selection. Goats have an incisor arcade that is narrower and more pointed than that of sheep (Gordon and Illius, 1988). Goats also can achieve higher bite rates than sheep (Gordon et al., 1996), perhaps due to higher chewing efficiency or willingness to swallow larger particles (Domingue et al., 1991). Goats also appear to digest plants with toxins more extensively than do sheep (Gordon and Illius, 1992). Differences in use of toxin-containing plants may be due to differential ruminal metabolism of toxins (Kronberg and Walker, 1993) or differential biotransformation of absorbed toxins.

Goats in our study generally ingested higher levels of sagebrush and more supplement than did sheep. Higher nutrient demands from eating more sagebrush may account for goats’ higher preference for the protein supplement. When sheep and goats consume low-quality diets, their use of energy is similar, but goats apparently are more efficient than sheep at using nitrogen (Watson and Norton, 1982, Doyle et al., 1984). Sheep apparently adjust foraging time to a greater extent than do goats with respect to CP (Kronberg and Malechek, 1997). Nevertheless, goats in our study selected higher P:E ratios than did sheep. In contrast to sheep, goats offered the protein concentrate (Trial 1) and then offered a choice between the protein concentrate and an energy concentrate continued to consume high amounts of the protein concentrate (Figure 2).

	Implications

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Managers use various techniques—herbicides, fire, and mechanical manipulation—to reduce the over-abundance of plants like sagebrush. Herbicides and fire may adversely affect environments and mechanical manipulations often are unjustifiable economically. Livestock can be used as an environmentally safe and economically sound alternative to reduce the abundance of sagebrush on rangelands. However, sagebrush contains toxins that limit its use by livestock. Supplemental macronutrients can help overcome this problem and increase the forage value of this abundant shrub. Sagebrush intake was enhanced by high-protein availability and depressed by high-energy/low-protein availability. Lambs and goats allowed to choose from a high-protein and a high-energy supplement consumed amounts of sagebrush comparable to animals supplemented with protein. A choice of supplements enhanced use of sagebrush and enabled more balanced intake of macronutrients by sheep and goats.

	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 7345.

² We acknowledge S. Hammond and E. Yardley for technical support.

Received for publication November 21, 2001. Accepted for publication April 12, 2002.

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