Journal of Chemical Ecology, Vol. 17, No. 2, 1991

EXPERIENCE E A R L Y IN LIFE AFFECTS V O L U N T A R Y INTAKE OF B L A C K B R U S H BY GOATS

R.A. DISTEL and F.D. PROVENZA* Range Science Department Utah State University Logan, Utah 84322-5230 (Received July 23, 1990; accepted October 10, 1990)

Abstract--Low nutritional quality and high levels of condensed tannins adversely affect voluntary intake of blackbrush (Coleogyne ramosissima Torr.) by goats. We studied: (1) how consumption of blackbmsh or alfalfa pellets by young goats affected their consumption of those foods later in life, and (2) whether previous ingestion of blackbrush or alfalfa pellets affected the excretion of condensed tannins and total phenols from blackbrush in urine and feces, production of proline-rich proteins in saliva, excretion of nitrogen in feces, and mass of the liver, kidneys, parotid glands, and reticnlommen in goats. From 6 to 26 weeks of age, experienced goats were exposed to blackbrush, while inexperienced goats ate alfalfa pellets. Following exposure, both groups were offered older-growth blackbmsh twigs (OG) or a choice between OG and current season's blackbmsh twigs (CSG). A similar feeding trial was repeated nine months after exposure, and, in addition, both groups were offered a choice between OG ad libitum and alfalfa pellets at six levels of availability. Immediately after exposure, experienced goats ingested 95 % more (P < 0.01) OG per unit of body weight than inexperienced goats, but both groups rejected CSG. Nine months after exposure, experienced goats ingested 27% more (P < 0.01) OG than inexperienced goats. Experienced goats ingested 30 % more OG than inexperienced goats at every level of alfalfa pellet availability. The fate of condensed tannins and total phenols was similar for both groups, but experienced goats excreted 63 % more (P < 0.05) uronic acids per unit of body weight. Neither experienced nor inexperienced goats produced proline-rich proteins in saliva. Experienced goats excreted 32% more (P < 0.01) nitrogen in feces per unit of nitrogen ingested than did inexperienced goats. The mass of the reticulorumen was 30% greater (P < 0.05) for experienced than for inexperienced goats one month after exposure, but did not differ 10 months after exposure. The mass of the liver, kidneys, and parotid glands did not differ between treatments. The results *To whom correspondence should be addressed. 431 0098-0331/91/0200-0431506.50/0 9 t991 Plenum Publishing Corporation

432

DISTEL AND PROVENZA show that experience early in life can have profound and persistent effects on consumption of diets high in chemical defenses and low in nutritional quality. The results also suggest that several physiological and morphological factors are involved.

Key Words--Experience, condensed tannins, plant defense, intake, goats, blackbrush, Coleogyne ramosissima.

INTRODUCTION

Empirical evidence indicates that prior experience ingesting foods increases preferences for those foods by animals (Arnold, 1964; Leuthold, 1971; Arnold and Maller, 1977; Martin, 1978; Frost, 1981; Narjisse, 1981; Bartmann and Carpenter, 1982; Otsyina, 1983). Moreover, experiences early in life apparently have a more lasting effect than those later in life (reviewed by Arnold and Dudzinski, 1978; Matthews and Kilgour, 1980; Provenza and Balph, 1988; Squibb et al., 1990). Neurological, physiological, and morphological processes are amenable to change early in life and can be altered permanently so animals can better forage in the environment where they are reared (Provenza and Balph, 1987, 1988, 1990). Some authors suggest that previous experience, particularly in early life, may counter chemical defenses of plants, thus attenuating their negative effects on voluntary intake (Provenza and Balph, 1987, 1988; Lindroth, 1988). Low nutritional quality and high levels of condensed tannins adversely affect voluntary intake of blackbrush by goats (Provenza et al., 1983, 1990; Provenza and Malechek, 1984). However, consumption of blackbmsh by young goats could influence the physiological processes that deter consumption later in life. Mammals have evolved physiological (i.e., reduced uptake or transport of toxins) and biochemical (i.e., enzymatic transformation of toxins) adaptations to cope with plant chemical defenses such as tannins (Lindroth, 1988). For example, deer (Robbins et al., 1987) and rats (Mehansho et al., 1983) produce proline-rich proteins in saliva that bind tannins. In rats, the ingestion of tannins induces a marked increase in the size of the parotid salivary glands (Mehansho et al., 1983). Voles (Lindroth and Batzli, 1983) and snowshoe hares (Bryant et al., 1985) possess enzyme systems that detoxify phenolic compounds through conjugation with glucuronic acid. Such enzymes are located primarily in the liver and kidneys (Freeland and Janzen, 1974), but also occur in visceral tissues such as the rumen wall (Smith, 1986). Experience early in life could also influence morphological processes. For example, the alimentary canal of red deer (Milne et al., 1978) and mule deer (Baker and Hobbs, 1987) may enlarge when they ingest poor-quality forages. Because distension of the reticulorumen wall may limit intake of roughage that

EARLY EXPERIENCE AFFECTS LATER INTAKE BY GOATS

433

is low in nutrients (Grovum, 1988), young goats may adapt to a low-nutrient, high-fiber diet such as blackbrush by increasing gut size. The objectives of this study were to determine the effects of experience early in life with blackbrush or alfalfa pellets on: (1) voluntary intake of those foods by goats; (2) the fate of blackbrush condensed tannins and total phenols, production of proline-rich proteins in saliva, and excretion of nitrogen in feces of goats; and (3) the mass of the liver, kidneys, parotid glands, and reticulorumen of goats.

METHODS AND MATERIALS

Plant Material. Blackbrush is a small ( < 1 m) shrub that grows in nearly monospecific stands on millions of hectares in the southwestern United States (Bowns and West, 1976). Blackbrush is low in crude protein (4-7%) and low in digestible dry matter (38-48%) (Provenza et al., 1983). In addition, blackbrush twigs are high in phenols, which are composed primarily of condensed tannins (T. Clausen, personal communication). The allocation of tannins varies according to plant part (Provenza and Malechek, 1983). Current season's growth (CSG) from previously browsed plants contains 2.4 times more tannins than does older growth (OG) from unbrowsed plants. Even though CSG is higher than OG in crude protein (7% vs. 5%) and digestible dry matter (48% vs. 38%), goats prefer OG to CSG (Provenza and Malechek, 1984, 1986). Condensed tannins cause goats to avoid CSG (Provenza et al., 1990; Clausen et al., 1990). Exposure. Thirty Spanish goats 6 weeks old were randomly assigned to either a treatment (experienced) or a control (inexperienced) group. Inexperienced goats ate alfalfa pellets in Logan in northeastern Utah, while experienced goats browsed blackbrush near Gunlock in southwestern Utah. Goats were exposed to blackbrush or alfalfa pellets from 6 to 26 weeks of age. This period covered the transition from monogastric to ruminant, which may be a "sensitive period" in the development of food preferences in ruminants (Squibb et al., 1990). The young goats remained with their mothers during exposure to enhance the establishment and persistence of their dietary habits (Thorhallsdottir et al., 1990a,b; Nolte et al., 1990; Mirza and Provenza, 1991). Experienced goats browsed blackbrush from November 10, 1989, to February 10, 1990, on a 5-ha pasture that had been heavily browsed by another group of goats during the winter of 1988-1989. Vegetation on the site was a monospecific understory of blackbrush shrubs with an overstory of scattered juniper (Juniperus osteosperma) trees. The removal of terminal buds by herbivores during winter, when blackbrush is dormant, stimulates production of CSG during spring (Provenza et al., 1983). Thus, the blackbrush plants were composed of both OG and CSG. We observed young goats browsing on both

434

DISTEL AND PROVENZA

OG and CSG, and virtually all OG and CSG terminal branches had been browsed by the end of exposure. Experienced and inexperienced goats were weaned 10 days before feeding trials began to minimize the effects of weaning stress on the results. Feeding Trial 1. The objective of the first trial was to determine the intake of blackbrush by experienced and inexperienced goats immediately after exposure. The goats were penned individually in southwestern Utah and offered alfalfa pellets ad libitum for four days. They then were offered chopped blackbrush OG ad libitum for the next seven days (period 1), a choice between OG and chopped blackbrush CSG ad libitum for the next seven days (period 2), and a choice between half of their ad libitum intake of OG in period 2 and CSG ad libitum for the final five days (period 3). Goats had access to the food from 0900 to 1700 hr daily. All the animals were weighed at the beginning and at the end of each period. Intake of blackbrush was expressed as grams of ovendry tissue weight ingested per kilogram of body weight. Both blackbrush OG and CSG were harvested by hand daily and chopped, in a mechanical chipper, into segments 1-3 cm in length. When a choice was involved (periods 2 and 3), OG and CSG were offered in separate food boxes. The position of the food boxes containing OG or CSG was changed each day to avoid bias. All animals had ad libitum access to water and salt. At 1700 hr goats were given enough Calfmanna, a concentrate-based pellet high in protein (25 %) and energy, to cover 30% of their estimated energy requirements (NRC, 1981). The amount of Calfmanna offered was adjusted according to the body weight of each individual. Feeding Trial 2. The objective of the second trial was to determine the intake of blackbrush by goats nine months after initial exposure. After the first feeding trial, both groups of goats grazed the same grass-legume (Dactylis glomerata-Medicago spp.) pasture for eight months. The procedures described in trial 1 were repeated, except that the length of period 3 was reduced to two days; the production of CSG was low as a result of drought, which precluded feeding CSG for five days as in trial 1. Feeding Trial 3. The objective of the third trial was to determine the intake of blackbrush OG by goats when offered OG ad libitum and alfalfa pellets at six levels of availability. After trial 2, we determined the ad libitum intake of alfalfa pellets for each individual. Experienced and inexperienced goats then were offered a choice between blackbrnsh OG ad libitum and alfalfa pellets ad libitum on day 1. Subsequently, the amount of alfalfa pellets offered was reduced by 20 % each day, whereas OG was offered ad libitum. On the last day of the trial, only OG was offered ad libitum. The intake of OG was measured throughout the trial. Fate of Condensed Tannins and Total Phenols. During trial 1, five goats from each group were penned individually in metabolism stalls to collect feces

EARL"( EXPERIENCE AFFECTS LATER INTAKE BY GOATS

435

and urine. Urine samples were collected in a bucket containing 1 ml of concentrated mercuric chloride to prevent microbial growth. All samples were frozen at - 2 0 ~ Before the chemical analyses, blackbmsh and fecal samples were freeze-dried and ground to pass a l-ram screen. Blackbrush, fecal, and urine samples were analyzed for days 3 and 5 (representative of period 1), 11 and 13 (representative of period 2), and 17 and 19 (representative of period 3). Phenols were extracted from blackbrush and feces according to a procedure developed by A.E. Hagerman (personal communication). Blackbrush and feces were extracted with 10% acetic acid in ether to remove pigments, and then with 1% sodium dodecyl sulfate in 70% acetone to remove phenols. A 150-rag sam-ple was mixed with 5 ml of acetic acid in ether, extracted for 10 min, centrifuged (2000 rpm), and the supernatant was discarded. This procedure was repeated four times. The sample then was mixed with 5 ml of sodium dodecyl sulfate in acetone, centrifuged for 10 min, and the supernatant was collected. This procedure was repeated four times. Finally, the samples were analyzed for condensed tannins (Porter et al., 1986) and total phenols (Price and Butler, 1977). Condensed tannins and total phenols were expressed as blackbmsh condensed tannin equivalents per unit organic matter, using standard curves prepared daily for the conditions used in the analysis. Condensed tannins used as standards were extracted from blackbrush CSG and purified according to the method of Hagerman and Butler (1980). Urine samples were analyzed for total phenols (Price and Butler, 1977) and uronic acids (Bitter and Muir, 1962). The presence of uronic acids is an index of phenol detoxification in animals (Lindroth and Batzli, 1983). Total phenols in urine were expressed as blackbmsh condensed tannin equivalents, whereas uronic acids in urine were expressed as glucuronolactone equivalents. Excretion of Nitrogen in Feces. Blackbrush and fecal samples were analyzed for nitrogen using the Digesdahl procedure (Hach Company, 1987). Production of Proline-Rich Proteins in Saliva. Saliva samples from five goats in each group were analyzed for days when fecal and urine samples were collected. Samples were collected in the morning before feeding. A sponge, placed in the mouth of each animal, was removed after the animal had chewed on it for 2-3 min, which enabled us to collect total mixed saliva (some salivary glands secrete only when the animal is chewing). The sponge was squeezed to collect saliva in a small tube containing 100/xl of phenylmethyl-sulfonyl fluoride, an antiproteolytic agent. One milliliter of saliva was collected per individual. The samples were frozen immediately in Dry Ice and later placed in a freezer at - 2 0 ~ Saliva samples were analyzed for proline-rich proteins according to the method of Austin et al. (1989). Mass of Reticulorumen, Liver, Kidneys, and Parotid Glands. One month after exposure, four goats from each group that was in trial 1 were sacrificed and the fresh mass of the liver, kidneys, parotid glands, and reticulorumen was

436

DISTEL AND PROVENZA

determined. The contents of the reticulorumen were removed and the reticulorumen was washed and dried with a towel before weighing. The mass of the reticulorumen was measured in four goats from each group again 10 months after exposure. After goats had ingested blackbrush OG, we determined how many alfalfa pellets they would ingest voluntarily to ascertain whether physical limitation in rumen volume might limit intake. Both groups of goats were offered blackbrush OG from 0900 to 1300 hr and then alfalfa pellets from 1300 to 1700 hr on the day following trial 1. Statistical Analyses. The statistical design for the analyses of variance was a randomized block with experience as the main effect and individual goats as blocks. Periods and foods within periods were analyzed separately in trials 1 and 2. When the same measurement was repeated for more than one day, a repeated measures analysis was used (Winer, 1971).

RESULTS

Feeding Trial 1. Experienced goats ingested 12 g/kg of body weight or 95 % more (P < 0.01) OG than inexperienced goats in periods 1 and 2 (Figure 1). On average, experienced goats ingested 332 g of OG, whereas inexperienced goats ingested 232 g of OG. In period 2, both groups rejected CSG when allowed to choose between OG and CSG. However, when OG was restricted and CSG was offered ad libitum in period 3, both groups ingested all the OG and experienced goats ingested 6 g/kg of body weight or 46 % more (P < 0.01) CSG than did inexperienced goats. The average intake of CSG was 245 g and 225 g for experienced and inexperienced goats, respectively. Feeding Trial 2. Experienced goats ingested 7 g/kg of body weight or 27 % more (P < 0.01) OG than inexperienced goats nine months after exposure (Figure 2). On average, experienced goats ingested 716 g of OG, whereas inexperienced goats ingested 618 g of OG. When allowed to choose between OG and CSG in period 2, both groups rejected CSG. In period 3 when OG was restricted and CSG was offered ad libitum, goats did not differ (P > 0.05) in the amount of CSG they ingested. Feeding Trial 3. When allowed to choose between OG and alfalfa pellets, experienced goats ingested more (P < 0.05) OG than inexperienced goats at every level of alfalfa pellet availability (Table 1). On average, experienced goats ingested 6 g/kg of body weight or 30 % more than inexperienced goats. Body Weight. In trial 1, there was no change in the weight of experienced goats (13 kg), but inexperienced goats lost an average of 4 kg, from 21 to 17 kg. Differences in the nutrient and energy contents of blackbrush OG and alfalfa pellets caused the differences in weight at the beginning of trial 1. There were

437

EARLY EXPERIENCE AFFECTS LATER INTAKE BY GOATS

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EXPERIENCED-OG 0 - - 0 INEXPERIENCED-OG 9 - - 9 EXPERIENCED-CSG Z%--/~ INEXPERIENCED-CSG4 1 . - - f

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DAY

FIG. 1. Intake of blackbrush by goats immediately after exposure. Each value represents the mean of 14 (experienced) or 16 (inexperienced) goats. Goats were offered OG in period 1, a choice between OG and CSG in period 2, and a choice between half their ad libitum intake of OG in period 2 and CSG ad libitum in period 3. Differences between treatments are significant (P < 0.01), except for intake of CSG in period 2. There was no treatment by day interaction (P > 0.05). The standard errors of the means for periods 1, 2, and 3 are 1.0, 1.0 (OG) and 0.5 (CSG), and 0.7, respectively.

no changes in the weights of experienced (25 kg) or inexperienced (27 kg) goats during trial 2. Fate of Condensed Tannins and Total Phenols. Experienced goats ingested more (P < 0.05) condensed tannins and total phenols than inexperienced goats in trial 1 (Table 2). Experienced and inexperienced goats did not differ (P > 0.05) either in the amount (milligrams excreted per gram ingested) of condensed tannins and total phenols in feces or in the amount of total phenols and uronic acids in urine (Table 3). However, experienced goats excreted more (P < 0.05) uronic acids than inexperienced goats during the trial when expressed on a per unit body weight basis (Table 3). Excretion of Nitrogen in Feces. Experienced goats excreted 32 % more (P < 0.01) nitrogen in feces, per unit of nitrogen ingested, than inexperienced goats and ingested a higher (P < 0.01) proportion of nitrogen as blackbrush protein (45% vs. 30%) throughout trial 1 (Table 4). Production of Proline-Rich Proteins in Saliva. No proline-rich proteins were detected by electrophoresis in the saliva of either experienced or inexperienced goats.

438

DISTEL AND PROVENZA 50

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TABLE 1. INTAKE OF BLACKBRUSH O G BY GOATS a

Intake of blackbrush OG by goats (g/kg body weight)

Alfalfa pellet availability (Ad Libitum %)

Experienced

Inexperienced

SE

100 80 60 40 20 0

13a 16a 19a 29a 34a 36a

9b 9b 13b 22b 28b 32b

1.5 1.9 2.6 2.8 2.2 1.5

aBlackbrush OG was offered ad libitum throughout the trial, while alfalfa pellets were offered at six levels of availability, Each value represents the mean of 10 (experienced) or 12 (inexperienced) goats. bab, means within a row are different (LSD 0.05).

439

EARLY EXPERIENCE AFFECTS LATER INTAKE BY GOATS TABLE 2. INGESTION OF CONDENSED TANNINS AND TOTAL PHENOLS BY GOATS IN TRIAL 1a Ingestion (mg/kg body wt) Condensed tannins

Total phenols

Day

Experienced

Inexperienced

Experienced

Inexperienced

Period 1 3 5 Period 2 11 13 Period 3 17 19

1400a

700b

1890a

940b

1240a

740b

1720a

1030b

1970a 2060a

1450b 1430b

2920a 3120a

2160b 2160b

2800a 2320a

1760b 1730b

4170a 3550a

2900b 2650b

a Each value represents the mean of five goats. The standard error of a mean for condensed tannins is 130 and for total phenols is 183. bab, means within a row, within a chemical class, are different (LSD 0.05).

TABLE 3. EXCRETION OF CONDENSED TANNINS, TOTAL PHENOLS, AND URONIC ACIDS IN TRIAL 1a Goats Excretion

Experienced

Inexperienced

SE

Tannins in feces (mg/g ingested) Phenols in feces (mg/g ingested) Phenols in urine (mg/g ingested) Uronic acids (mg/g phenol ingested) Uronic acids (mg/kg body weight)

60a 94a lla 29a 85a

62a 86a 12a 26a 52h

13 15 3 11 21

aEach value represents the mean of five goats for days 3, 5, 11, 13, 17, and 19. ~ means within a row are different (LSD 0.05).

Mass of Reticulorumen, Liver, Kidneys, and Parotid Glands. O n e m o n t h a f t e r e x p o s u r e , t h e m a s s o f t h e r e t i c u l o r u m e n w a s 3 0 % h i g h e r ( P < 0 . 0 5 ) in e x p e r i e n c e d t h a n in i n e x p e r i e n c e d g o a t s ( T a b l e 5). T r e a t m e n t s d i d n o t differ e i t h e r i n t h e t h i c k n e s s o f t h e r e t i c u l o r u m e n w a l l o r in t h e d e v e l o p m e n t o f r u m e n papillae. Thus, the larger reticulorumen mass meant experienced goats had a larger reticulorumen capacity. There were no differences (P < 0.05) between t r e a t m e n t s in t h e m a s s o f l i v e r , k i d n e y s , o r p a r o t i d g l a n d s . T e n m o n t h s a f t e r

440

DISTEL AND PROVENZA TABLE 4. EXCRETION OF NITROGEN IN FECES OF GOATS IN TRIAL 1a

Nitrogen excretion (mg/g ingested) Day Period 1 3 5 Period 2 11 13 Period 3 17 19

Experienced

Inexperienced

363a 374a

209b 313b

392a 374a

293b 325a

454a 462a

348b 347b

a Each value represents the mean of five goats. The standard error of a mean is 27. bab, means within a row are different (LSD 0.05).

TABLE 5. MASS OF RETICULO RUMEN, LIVER, KIDNEYS, AND PAROTID GLANDS 1 MONTH AFTER EXPOSURE AND MASS OF RETICULO RUMEN 10 MONTHS AFTER EXPOSUREa

Mass (g/kg body wt) Time after exposure One month Experienced Inexperienced Ten months Experienced Inexperienced

Reticulo rumen

Liver

Kidneys

Parotid glands

25a 18b

22a 19a

5a 4a

la la

25a 23a

a Each value represents the mean of four goats. The standard errors of the means for reticulo mmen mass at one and 10 months after exposure are 2 and 1, respectively. The standard errors of the means for liver, kidneys, and parotid glands are 2.4, 0.4, and 0.1, respectively. bab, means within a column are different (LSD 0.05).

exposure the mass of the reticulorumen did not differ (P > 0.05) between treatments. Experienced and inexperienced goats ate nearly the same amount of OG from 0900 to 1300 hr (Table 6) as they ate from 0900 to 1700 hr on day 14 of trial 1 (Figure 1). The alfalfa pellets ingested from 1300 to 1700 hr were 83%

441

EARLY EXPERIENCE AFFECTS LATER INTAKE BY GOATS TABLE 6. INTAKE OF BLACKBRUSH O G ( 0 9 0 0 - 1 3 0 0 hr) AND ALFALFA

PELLETS(1300-1700 hr)a Intake (g/kg body wt) Food

Experienced

Inexperienced

SE

Blackbrush OG Alfalfa pellets Total

29a 24a 53a

19b 21 a 40b

1 2 3

a Each value represents the mean of 14 (experienced) or 16 (inexperienced) goats. /'ab, Means within a row are different (LSD 0.05).

and 111% of the OG ingested from 0900 to 1300 hr by experienced and inexperienced goats, respectively. DISCUSSION

First we discuss the mechanisms that may underlie the responses of experienced and inexperienced goats, then we address artifacts of the experimental design that may have influenced those responses, and finally we consider implications of the findings for the biological manipulation of shrubs.

Mechanisms that May Underlie Responses of Goats Experience early in life with blackbrush increased voluntary intake of blackbrush OG and CSG by goats in trial t and increased intake of and preference for blackbrush OG in trials 2 and 3. These results may reflect differences between experienced and inexperienced goats in: (1) preference for blackbrush OG and CSG, (2) ability to detoxify phenols in OG and CSG, and (3) distension of the reticulorumen. Preferenee for Blackbrush OG and CSG. Preference was the relative amount of blackbrush OG ingested when goats could choose between blackbrush OG and alfalfa pellets or between blackbrush OG and CSG. Exposure to blackbrush increased preference for blackbrush OG when offered with alfalfa pellets (Table 1). Animals develop preferences for foods ingested early in life in the presence of social models such as the mother (reviewed by Immelmann 1975; Matthews and Kilgour, 1980; Provenza and Balph, 1987, 1988, 1990; Provenza et al., 1991). The enhanced ingestion of blackbrush OG shows that goats can develop a preference for a food high in tannins and low in nutritional quality. Grazing experience early in life also increases the preference of sheep for various unpalatable plants, and these differences persisted for at least two

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years even when sheep were exposed to a wide range of forages (Arnold, 1964; Arnold and Mailer, 1977). When allowed to choose between blackbrush OG and CSG, goats always preferred OG. The fact that the concentration of condensed tannins in CSG is over twice that in OG may explain the preference. Alternatively, differences in the structure of condensed tannins in OG and CSG may have affected the goat's response and are known to cause snowshoe hares to prefer bitterbrush CSG to blackbrush CSG (Clausen et al., 1990). The ratio of epicatechin to catechin in condensed tannins from blackbrush CSG is five times higher than that of condensed tannins from bitterbrush CSG. Condensed tannin structure can vary with the age of plant parts. The epicatechin-catechin ratio of condensed tannins of Pinus taeda differs between the phloem and the heartwood (Karchesy and Hemingway, 1980), and most condensed tannins from the ripe fruit of Cydonia oblonga are bound covalently with sugars whereas condensed tannins from unripe fruit have no carbohydrate substituents (Porter et al., 1985). Porter (1984) argues that branching increases with the molecular weight of condensed tannins. Thus, condensed tannins from OG, which form larger polymers, may branch more than newly formed condensed tannins from CSG and may be less aversive to goats than those in CSG (Provenza et al., 1990). Inexperienced goats initially ingested more (P < 0.01) CSG than experienced goats (Figure 1, day 8). Intake of CSG by inexperienced goats subsequently declined and their consumption of OG increased. Goats associate the taste of CSG with postingestive distress caused by condensed tannins and quickly learn an aversion to blackbrush CSG, presumably when condensed tannins stimulate the emetic system of the midbrain and brain stem (Provenza et al., 1990, 1991). Toxins can stimulate the emetic system by afferent nerves from throughout the body (e.g., gastrointestinal tract, cardiovascular system, liver, kidneys) (Borison, 1986; David et al., 1986; Grahame-Smith, 1986; Kosten and Contreras, 1989). This explains why inexperienced goats ingested more (P < 0.01) CSG on day 8 than on subsequent days. Experienced goats also ingested more (P < 0.01) CSG initially than in subsequent days, perhaps because chopping CSG changed cues such as sight, odor, and taste that goats use to recognize CSG (Provenza et al., 1990, 1991). This is a likely explanation for why experienced goats sampled more CSG on day 8 than on day 9 (Figure 1). Detoxification of Phenols. Only a small proportion of blackbrush condensed tannins and total phenols were excreted in feces (6 % and 9 %, respectively) and urine (1% of phenols ingested) (Table 3). Condensed tannins form strong hydrogen bonds with proteins, polysaccharides, and other cell constituents (Swain, 1979), and the technique used in this study may not have recovered all of them (A.E. Hagerman, personal communication).

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In both treatments, the most nitrogen was excreted in feces during period 3 of trial 1, the time when goats ingested the most tannins. Experienced goats excreted more nitrogen in the feces than inexperienced goats (Table 4), perhaps because they ingested more tannins and because a higher proportion of the nitrogen they ingested was from the poorly digestible twigs of blackbrush. Nastis and Malechek (1981) also found that goats' excretion of nitrogen in feces increased with the tannin content of an alfalfa-gambel oak diet. An alternative explanation for the fact that only a small proportion of blackbrush condensed tannins was excreted in the feces and urine is that part of the condensed tannins were depolymerized, absorbed from the gastrointestinal tract, and then detoxified. Depolymerization of condensed tannins, particularly procyanidins (Hemingway and McGraw, 1983; Hemingway et al., 1983), occurs readily under acidic conditions similar to those in the digestive tract of mammalian herbivores (Butler et al., 1986; Clausen et al., 1990). Condensed tannin depolymerization products may be toxic (McLeod, 1974; Kumar and Singh, 1974; Lindroth and Batzli, 1984; Mehansho et al., 1985; Butler et al., 1986). For example, voles die when fed quebracho tannins (Lindroth and Batzli, 1984), and hamsters fail to sexually mature or die if condensed tannin levels in their diets are sufficiently high (Mehansho et al., 1985, 1987). Condensed tannins also increase mixed-function oxidase activity in the liver and levels of phenols in urine (Sell and Rogler, 1983), which suggests that they are absorbed and metabolized to some extent. Experienced goats excreted 63 % more uronic acid than inexperienced goats (85 vs. 52 mg/kg body weight; Table 3), perhaps because experienced goats ingested more condensed tannins and total phenols (Table 2), or because they were better able to detoxify phenols. A greater ability to detoxify condensed tannins would lower concentrations of phenols in blood and might affect intake. Phenols in blood may stimulate emetic receptors in the midbrain and brain stem, thereby causing a conditioned food aversion and reducing intake (Provenza et al., 1990, 1991). Thus, experienced goats may have ingested more blackbrush OG and CSG, and consequently more blackbrush condensed tannins and total phenols, because they were better able to detoxify them. If detoxification required time for induction, this lag could explain the difference in intake of CSG between experienced and inexperienced goats during period 3 of trial 1, and the lack of any difference during period 3 of trial 2. Freeland and Janzen (1974) hypothesize that mammalian herbivores ingest toxins in amounts that they can detoxify. By the end of trial 1, average tannin intake by experienced and inexperienced goats was 2560 and 1745 mg/kg body weight, respectively (average of days 17 and 19 in Table 2), which may be the maximum amount of tannins goats could detoxify. Sheep (Burritt and Provenza, 1989) and goats (Provenza et al., 1990, 1991) regulate intake of foods that contain lithium chloride (LiC1), a potentially toxic compound. When LiC1 is

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mixed with an otherwise nutritious food, sheep and goats will voluntarily ingest between 25 and 50 mg/kg body weight (DuToit et al., 1991). Our results do not support the hypothesis that mixed-feeders such as goats (Hofmann, 1988) produce proline-rich proteins in saliva as a mechanism for neutralizing tannins (Robbins et al., 1987). Different animal species may have evolved different mechanisms to cope with tannins. Species such as goats may have evolved the ability to detoxify tannins or their metabolites to some extent, while species such as deer apparently evolved proline-rich proteins as a partial defense against tannins (Robbins et al., 1987). Distension of Reticulorumen. Our data support the hypothesis that the capacity of the reticulorumen can increase when ruminants ingest foods high in fiber and low in nutrients (Milne et al., 1978; Baker and Hobbs, 1987), but do not support the hypothesis that experience early in life causes a permanent increase in the capacity of the reticulorumen. One month after exposure experienced goats had greater reticulorumen capacity than did inexperienced goats, but after eight months of grazing on the same grass-legume pasture the size of the reticulorumen did not differ between treatments. Grovum (1988) argues that intake of roughage can be limited by the distension of the reticulorumen. While experienced and inexperienced goats ingested a considerable amount of alfalfa pellets immediately after they ingested blackbrush OG (Table 6), distension of the reticulorumen may still have limited their intake of blackbrush OG. Difference in the density of blackbrush OG and alfalfa pellets may explain the results in Table 6. The blackbrush OG ingested by experienced and inexperienced goats (29 and 19 g/kg body weight) occupied volumes of 112 and 73 cm 3, while the alfalfa pellets ingested by experienced and inexperienced goats (26 and 22 g/kg body weight) occupied volumes of 34 and 29 cm 3. Furthermore, finely ground and pelleted alfalfa breaks down much more quickly when masticated than the coarse, fibrous chipped blackbmsh OG. Thus, rumen volume and distension of the reticulorumen may partially explain the large differences in intake between experienced and inexperienced goats in trial 1, and the reduction in such differences in trial 2.

Experimental Artifacts That May Have Influenced Responses of Goats The differences in intake of blackbmsh OG and CSG between treatments were greater immediately after exposure (Figure 1) than nine months after exposure (Figure 2). The nutritional plane of goats during the experiment and the exposure of inexperienced goats to blackbrush during trial 1 may help explain this result. Effects of Nutritional Plane on Responses of Goats. Experienced and inexperienced goats were on drastically different planes of nutrition during exposure and differed in fat reserves and body weight during trial 1. The difference in

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the amount of back fat and kidney fat between groups was apparent when the animals were sacrificed to measure visceral mass. The difference in fat reserves between treatments was probably even greater at the beginning of trial 1 when inexperienced goats weighed 21 kg and experienced goats weighed 13 kg. During trial 1, inexperienced goats lost 430 g/day during the first seven days (3 kg) and 70 g/day during the next 14 days (1 kg). By the end of trial 1, inexperienced goats weighed 17 kg while experienced goats weighed 13 kg. Inexperienced goats probably lost weight when they mobilized fat reserves to compensate for their low intake of blackbrush. There were no differences between treatments in fat reserves and body weight (26 kg vs. 24 kg for inexperienced vs. experienced goats) after both groups of goats grazed a grass-legume pasture for eight months. The disparity in nutritional plane between experienced and inexperienced goats prior to trial 1 and the similarity in nutritional plane for the eight months prior to trial 2 may partially explain the differences in intake immediately after exposure and nine months later. Rats deprived of protein early in life ingest more food, and utilize protein more efficiently, than normal rats (Beck et al., 1983). Protein levels in blackbrush are not adequate to meet requirements of young goats (Provenza et al., 1983; NRC, 1981), and long-term protein deprivation may have caused experienced goats to ingest more blackbrush than experienced goats during trial 1. Unfortunately, we know of no data concerning the role of protein deprivation early in life on intake in ruminants. In addition, levels of body fat affect insulin levels in the blood, which may also affect intake and could explain why inexperienced goats consumed less food immediately following exposure than experienced goats. Evidence supports this hypothesis in rats, but there is no evidence to support this hypothesis in ruminants (Weston, 1982; deJong, 1985; Grovum, 1988). Experience of the Control Group. Exposing inexperienced goats to blackbrush during the first trial also may help explain why the differences in intake of blackbrush OG between treatments were larger immediately after exposure than nine months later. In the feeding trial nine months after exposure, young goats in the control group had been exposed to blackbrush for 20 days. This duration of exposure relatively early in life, which represented 20 % of the exposure of the treatment group, may have increased inexperienced goats' intake of blackbrush OG.

Implications Goats are an alternative to chemical or mechanical control of undesirable plant species (Wood, 1987). Goats also can be used to modify the growth form of shrubs to increase their productivity and nutritional value (Provenza et al., 1983). Our results show that young goats exposed to blackbrush consequently

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ate more of the shrub, findings which could enhance biological manipulation efforts. For example, if goats (average weight of 30 kg) forage on blackbrushdominated rangeland for 90 days during the dormant season, as was typical of the studies of Provenza et al. (1983), 15 experienced goats would ingest 284 kg more blackbrush than 15 inexperienced goats based on the data in Figure 2 (0.007 kg more ingested/kg body weight/day x 30 kg body weight/goat x 15 goats x 90 days). These differences would be much larger using the intakes shown in Figure 1: experienced goats would ingest 486 kg more blackbrush than inexperienced goats (0.012 kg more ingested/kg body weight/day x 30 kg body weight/goat x 15 goats x 90 days). The results of Flores et al. (1989ac) indicate that the differences in intake of blackbrush between experienced and inexperienced goats would be even greater under free-ranging conditions because herbivores learn foraging skills. In our studies, goats were offered chopped blackbrush twigs in food boxes so that plant form and foraging skills would not confound the results.

CONCLUSIONS

Results from our study show that experience early in life with different diets can have profound and persistent effects on intake. The results also suggest that several physiological and morphological factors are involved. Clearly, more research is needed to understand the relationship between preference and the ability to detoxify condensed tannins. It is also important to study the interrelationships among intake, rumen volume, rate of passage of particles through the gastrointestinal tract, and factors such as rumination efficiency, which are influenced by experience and could affect intake (Welch and Hooper, 1988). Moreover, the role of nutrient deficiency on intake of blackbrush by young animals must be understood. Finally, it is important to determine how age at exposure and duration of exposure to blackbmsh influence physiological and morphological processes that affect intake of and preference for blackbrush by goats. Acknowledgments--Financialsupport of the Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET) de ta Republica Argentina through a fellowship to R.A. Distel is acknowledged. Additional support was provided by a grant from National Science Foundation (BSR8614856) and a grant from the United States Agency for International Development. We thank Dr. Ann E. Hagerman for assistance with the chemical analyses and the analysis of goat's saliva for proline-rich proteins, and Johan and Mary Jane DuToit for assistance with some of the field trials. This is journal paper No. 4035, Utah Agric. Exp. Sta., Utah State Univ., Logan, Utah 843224845.

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Experience early in life affects voluntary intake of blackbrush by goats.

Low nutritional quality and high levels of condensed tannins adversely affect voluntary intake of blackbrush (Coleogyne ramosissima Torr.) by goats. W...
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