Preferences for foods varying in macronutrients and tannins by lambs supplemented with polyethylene glycol

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1 Preferences for foods varying in macronutrients and tannins by lambs supplemented with polyethylene glycol C. H. Titus*, F. D. Provenza*, A. Perevolotsky, and N. Silanikove, *Department of Rangeland Resources, Utah State University, Logan and Volcani Center, P.O. Box 6, Bet Dagan, Israel ABSTRACT: Supplemental polyethylene glycol (PEG) increases intake when animals eat foods high in tannins, but little is known about how PEG affects preference for foods that vary in concentrations of macronutrients and tannin. We investigated how varying macronutrients and tannins (commercially available extracts from quebracho trees) affected food intake, and we assessed the degree to which PEG (MW 3350) affected intake of tannin-rich foods by sheep. From 0715 to 1800 daily, lambs were offered diets that varied in concentrations of macronutrients: high energy/low protein (75% barley/25% alfalfa), medium energy/medium protein (35% barley/65% alfalfa), and low energy/high protein (100% alfalfa). Preference for these diets was determined in the absence of tannin, and then, in Trials 1 to 3, tannin was added in increasing concentrations (from 5 to 20%) to the diets with high and medium levels of energy. In Trial 4, tannin (10%) also was added to the low-energy diet. Lambs were supplemented with either 50 g of PEG mixed with 50 g of ground barley or 50 g of ground barley alone from 0700 to 1715 daily; lambs always consumed all of these supplements. In the absence of added tannins, all lambs preferred high energy/low protein > medium energy/medium protein > low energy/high protein. As tannin levels increased, preference for the high- and medium-energy foods decreased, and all lambs preferred foods that were lower in tannins and higher in protein. Lambs supplemented with PEG ate more macronutrients and tannins than unsupplemented lambs, and the effect became increasingly apparent as tannin levels increased from Trials 1 to 4. We conclude that the effectiveness of supplemental PEG may be low if alternative forages are equal or superior in nutritional quality and contain fewer metabolites with adverse effects. In such cases, animals would likely prefer alternatives to high-tannin foods. Key Words: Tannin, Energy, Protein, Sheep, Polyethylene Glycol 2000 American Society of Animal Science. All rights reserved. J. Anim. Sci : Introduction Macronutrients (energy and protein) and tannins interact to influence food preference. Lambs prefer foods high in macronutrients, and they acquire preferences even for poorly nutritious foods like straw when straw is eaten before intraruminal infusions of energy (Villalba and Provenza, 1997a,c) or protein (Villalba and Provenza, 1997b). Tannins suppress food intake by reducing macronutrient digestibility or by causing illness (Nastis and Malechek, 1981; Furstenburge and Hoven, 1994). Tannins bind to proteins, cell walls, and cell solubles (Kumar and Vaithiyanathan, 1990) and negatively affect rumen microbial and intestinal enzyme 1 This research was supported by grants from the Binational Agricultural Research and Development Fund (BARD) and the Utah Agricultural Experiment Station, Utah State University, Logan Approved as Journal paper number Received June 24, Accepted November 19, activities (Silanikove et al., 1994, 1996a; Aharoni et al., 1998). The reduction in substrate and microbial and enzymatic activity reduces levels of ammonia and volatile fatty acids in the rumen (Makkar et al., 1995), which is likely to adversely affect preference (Villalba and Provenza, 1996, 1997a,b,c). Tannins also cause rapid (< 1 h) and dramatic decreases in food intake (Provenza et al., 1990, 1994; Silanikove et al., 1997b), best accounted for by oral astringency, lesions of gut mucosa, and toxicity (Kumar and Singh, 1984; Reed, 1995). Polyethylene glycol (PEG) binds to tannins and increases macronutrient availability (Makkar et al., 1995), even in species as diverse as rats (Horigome et al., 1988), sheep (Gilboa, 1995), and cattle (Hannigan and McNeill, 1998). As a result, sheep and goats supplemented with PEG eat as much as 50% more high-tannin foods (Barry, 1985); Pritchard et al., 1988; Silanikov et al., 1996a). Increases in food intake improve body weight when sheep eat species such as Ceratonia siliqua (Silanikov et al., 1994) and when goats eat species such as Quercus calliprinos or even the poorly preferred Pistacia lentiscus (Silanikove et al., 1996a). 1443

2 1444 Titus et al. Collectively, these studies demonstrate that PEG affects intake of foods high in tannins, but they do not portray how PEG might affect preferences for a variety of foods that vary in concentrations of macronutrients and tannins. Our objective was to explore how macronutrients, tannins, and PEG interact to affect food preferences of sheep. Materials and Methods Conditioning. Lambs (ewes and wethers) used in the study were 4-mo-old commercial crossbreds. The same lambs were used throughout the study, and they were allocated to treatments such that sex was not confounded with treatment. The lambs were divided into two groups (12 lambs/group) and placed in individual pens adjacent to one another. Lambs in Group 1 received 100 g of PEG (MW 3350, Sigma, St. Louis, MO) mixed with 50 g of ground barley from 0700 to 0715, whereas lambs in Group 2 received only 50 g of ground barley. Each group was then offered ground alfalfa ad libitum from 0715 to Lambs had free access to fresh water and trace mineral blocks, Intake of grain, PEG, and alfalfa were determined daily during the last 3 d of the 10-d conditioning period. Pretrial Period. In the pretrial period, we determined lambs preferences for foods that varied in energy and protein, and this served as a baseline for the trials that followed. Group 1 was given 50 g of PEG mixed with 50 g of ground barley from 0700 to 0715; Group 2 received 50 g of ground barley. During the pretrial period, and throughout all of the other trials, lambs readily ate all of the PEG and grain. Although others have successfully used 25 g of PEG as a supplement for animals eating foods high in tannins (Silanikove et al., 1994), we provided 50 g of PEG in an attempt to ensure that animals received enough PEG to attenuate the effects of quebracho tannin. All lambs were given a choice of three diets (mixtures of ground barley and ground alfalfa): high barley/low alfalfa (HB), low barley/high alfalfa (LB), and alfalfa (A) (Table 1). These diets differed in macronutrient concentrations (Table 1). Diets were fed for ad libitum consumption from 0715 to 1800 daily. The amount of food ingested was determined as the difference between the food offered at 0715 and the food weighed back at The pretrial period lasted for 5 d, but data were collected only during the last 2 d. A 2-d intertrial period was instituted after each trial (Trials 1 to 4) to again determine lambs preferences for HB, LB, and A in the absence of tannin in the diet. Trial 1. Trial 1 determined how preference for the high- and medium-energy diets was affected by the addition of tannin (quebracho tannin, Tannin Corp., Peabody, MA) (Table 1). Energy and tannin concentrations were varied such that the percentage of tannin was highest (10%) in the food with the greatest amount of barley (HB10T), moderate (5%) in the food with a low amount of barley (LB5T), and nil in alfalfa (A). The Table 1. Foods offered during the pretrial period and during Trials 1 to 4 of the experiment Barley content of foods Item High Low None Pretrial and intertrial diets Ingredients a 75B/25A 35B/65A 100A DE, Mcal/kg b CP, % c Trial 1 diets (d 3 to 8) Ingredients 75B/15A/10T 35B/60A/5T 100A DE, Mcal/kg CP, % Trial 2 diets (d 11 to 15) Ingredients 75B/5A/20T 35B/60A/5T 100A DE, Mcal/kg CP, % Trial 3 diets (d 18 to 22) Ingredients 75B/5A/20T 35B/55A/10T 100A DE, Mcal/kg CP, % Trial 4 diets (d 25 to 27) Ingredients 75B/5A/20T 35B/55A/10T 90A/10T DE, Mcal/kg CP, % a B, A, and T, represent, ground barley, ground alfalfa pellets, and quebracho tannin, respectively. b,c Values for DE and CP calculated from NRC (1985). procedures for this trial, as well as for Trials 2 to 4, were the same as those for the pretrial period. Trial 1 lasted 6 d. Trial 2. Trial 2 determined the effect of increased tannin levels on food ingestion. The amount of tannin in HB10T was increased to 20% (HB20T), and the tannin concentrations in LB (LB5T) and A were the same as in Trial 1 (Table 1). Due to the increased tannin content, HB20T and LB5T had similar concentrations of digestible energy (DE), but LB5T was higher in protein than HB20T. This trial lasted 5 d. Trial 3. In Trial 3, the concentration of tannin in HB20T (20%) and A (0%) remained the same, but the concentration of tannin in LB5T was increased to 10% (LB10T) (Table 1). The energy content of LB10T dropped so that HB20T was higher in DE than LB10T. The trial was 5 d. Trial 4. The objective of this trial was to determine how the lambs preferences for the foods changed when all three foods contained tannin (Table 1). The tannin levels in HB20T and LB10T were the same as in Trial 3, but A had 10% tannin (A10T). This trial lasted 3 d. Statistical Analysis. The repeated measures analyses of variance, which was performed separately for each trial, had two treatments (PEG-supplemented and unsupplemented). Lambs (12/treatment) were nested within treatments. Amount of food ingested was the dependent measure. Diet was a split-plot in the analysis. Day (Trial 1 = 6 d; Trials 2 and 3 = 5 d; and Trial 4 = 3 d) was the repeated measure. Least significant

3 Nutrients, tannins, and polyethylene glycol 1445 differences (LSD.05 ) were determined when F-ratios were significant (P <.05). Results Pretrial Period. In the pretrial period, the preference for HB, LB, and A were determined without tannin. There were no differences in food intake by PEG-supplemented and unsupplemented lambs (1,272 vs 1,206 g/ d; P =.59). Lambs in both groups preferred HB > LB > A (672 vs 325 vs 244 g/d; P =.0001; LSD.05 = 72) (Table 2). Nor did PEG-supplemented and unsupplemented lambs differ in intake of energy (3.642 vs Mcal/ d; P =.51) or protein (174 vs 165 g/d; P =.66). Treatment, food, and day did not interact (P =.61). Trial 1. In Trial 1, we determined how lambs preferences were affected by the addition of tannin to foods HB10T (10% tannin) and LB5T (5% tannin). There were no differences in intake by PEG-supplemented and unsupplemented lambs (1,642 vs 1,619 g/d; P =.813; Table 3). Lambs in both groups preferred HB10T = LB5T > A (600 vs 603 vs 427 g/d; P =.0001; LSD.05 = 81) (Table 2). Although the interaction between treatment, food, and day was significant (P =.004), there were no pronounced or recurrent patterns throughout the 6-d trial. Polyethylene glycol-supplemented and unsupplemented lambs did not differ in intake of energy (4.320 vs Mcal/d; P =.75), protein (210 vs 207 g/d; P =.96), or tannin (93 vs 87 g/d; P =.21) (Table 3). Although treatment, food, and day interacted for energy (P =.005) and protein (P =.002) intake, there were no recurrent patterns throughout the 6-d trial. Trial 2. The objective of this trial was to determine how lambs preferences were affected when the amount of tannin was doubled in HB20T (10 to 20%) but kept the same in LB5T (5%) and A (0%) as in Trial 1. There were no differences in food intake of PEG supplemented and unsupplemented lambs (1,651 vs 1,598 g/d; P =.56; Table 3). Lambs in both groups preferred LB5T > A > HB20T (667 vs 531 vs 425 g/d; P =.0001; LSD.05 = 65) (Table 2). Treatment, food, and day did not interact (P =.38). Polyethylene glycol-supplemented and unsupplemented lambs did not differ in intake of energy (4.173 vs Mcal/d; P =.54) or protein (210 vs 207 g/d; P =.84), but supplemented lambs ate more tannin (126 vs 111 g/d; P =.09) (Table 3). Treatment, food, and day did not interact for energy (P =.39), protein (P =.46), or tannin (P =.24) intake. Trial 3. In this trial, the concentration of tannin was doubled from 5 to 10% in LB (LB10T), and the concentrations of tannin in HB20T (20%) and A (0%) were as in Trial 2. Differences in food intake of PEG-supplemented and unsupplemented lambs became more pronounced (1,680 vs 1,504 g/d; P =.14; Table 3). Lambs in both groups preferred A > LB10T > HB20T (664 vs 559 vs 368 g/d; P =.0001; LSD.05 = 60) (Table 2). Treatment, food, and day did not interact (P =.63). Table 2. Percentage tannin in the three diets (HB, MB, A) and amounts of energy and protein in diets relative to the alfalfa a Pretrial Trial 1 Trial 2 Trial 3 Trial 4 Item HB LB A HB LB A HB LB A HB LB A HB LB A Tannin, % Energy Protien Protien:energy ratios Intake, g 672 x 325 y 244 z 600 x 603 x 427 y 425 x 667 y 531 z 368 x 559 y 664 z 395 x 618 y 600 y a Values were calculated based on information in Table 1. Intake values for each diet are averages for polyethylene glycol-supplemented and unsupplemented lambs. HB, high barley; LB, low barley; A, alfalfa. x,y,z Means lacking common superscripts, within a trial, differ (P <.05).

4 1446 Titus et al. Table 3. Mean intakes of energy, protein, and tannins by lambs supplemented or not supplemented with polyethylene glycol (PEG) during four trials Amount Ingested, g Diet Component Trial 1 (6 d) Trial 2 (5 d) Trial 3 (5 d) Trial 4 (3 d) Dry matter, g PEG 1,642 1,651 1,680 1,731 No PEG 1,619 1,598 1,504 1,498 SEM Probability Energy, Mcal/kg PEG No PEG SEM Probability Protein, g PEG No PEG SEM Probability Protien (g):energy (mcal) PEG NoPEG Tannin (g) PEG No PEG SEM Probability Differences between PEG-supplemented and unsupplemented lambs became more pronounced, compared with Trials 1 and 2, for energy (4.152 vs Mcal/d, P =.13), protein (213 vs 195 g/d, P =.20), and tannin (144 vs 117 g/d, P =.07) (Table 3). Treatment, food, and day did not interact for energy (P =.62), protein (P =.70), or tannin (P =.57) intake. Trial 4. This trial determined how lambs preferences changed when tannin was present in foods HB20T (20%), LB10T (10%), and A10T (10%). Polyethylene glycol-supplemented lambs ingested more food than unsupplemented lambs (1,731 vs 1,498 g/d; P =.03; Table 3). Lambs in both groups preferred LB10T = A10T > HB20T (618 vs 600 vs 395 g/d; P =.0001; LSD.05 = 51) (Table 2). Treatment, food, and day did not interact (P =.46). Polyethylene glycol-supplemented lambs ingested more than unsupplemented lambs of energy (4.122 vs Mcal/d, P =.03), protein (207 vs 183 g/d, P =.05), and tannin (219 vs 183 g/d, P =.02) (Table 3). There were no interactions among treatment, food, and day for energy (P =.43), protein (P =.61), or tannin (P =.29) intake. Intertrial Periods. The objective of the intertrial periods was to determine lambs preferences when tannin was removed from HB, LB, and A. There were no differences in food intake by PEG- supplemented and unsupplemented lambs between Trials 1 and 2 (1,731 vs 1,569 g/d; P =.17). Lambs in both groups preferred HB > LB > A (711 vs 564 vs 375 g/d; P =.0001; LSD.05 = 53). There were no interactions among treatment, food, and day (P =.83). Nor did PEG-supplemented and unsupplemented lambs differ in intake of energy (4.860 vs Mcal/d; P =.13) or protein (237 vs 216 g/d; P =.21). Polyethylene glycol-supplemented lambs ate more than unsupplemented lambs between Trials 2 and 3 (1,527 vs 1,284 g/d; P =.03). As a result, supplemented lambs also ingested more energy (4.308 vs Mcal/ d; P =.02) and protein (210 vs 177 g/d; P =.04) than unsupplemented lambs. Lambs in both groups preferred HB > LB > A (625 vs 444 vs 336 g/d; P =.0001; LSD.05 = 55). Treatment, food, and day did not interact (P =.24). Polyethylene glycol-supplemented and unsupplemented lambs did not differ in food intake between Trials 3 and 4 (1,740 vs 1,692 g/d; P =.74). Nor were there differences between supplemented and unsupplemented lambs in intake of energy (4.830 vs Mcal/ d; P =.68) or protein (243 vs 237 g/d; P =.78). Lambs in both groups preferred HB > LB = A (696 vs 500 vs 521 g/d; P =.0001; LSD.05 = 58). Treatment, food, and day did not interact (P =.77). PEG-supplemented and unsupplemented lambs did not differ in food intake after Trial 4 (1,962 vs 1,887 g/ d; P =.43). Nor did they differ energy (5.421 vs Mcal/d; P =.37) or protein (273 vs 264 g/d; P =.48) intakes. Lambs preferred HB > LB = A (713 vs 619 vs 592 g/d; P =.003; LSD.05 = 69). Treatment, food, and day did not interact (P =.69). Discussion Influence of PEG on Macronutrient Intake Macronutrients (energy and protein) influence food preference. Lambs prefer foods high in macronutrients,

5 Nutrients, tannins, and polyethylene glycol 1447 and they acquire preferences even for poorly nutritious foods like straw when straw is eaten before intraruminal infusions of energy (Villalba and Provenza, 1997a,c) or protein (Villalba and Provenza, 1997b). Lambs discriminate between the postingestive effects of energy and protein (Villalba and Provenza, 1999), and they maintain a relatively constant ratio of energy to protein in their diets when allowed to select from foods varying in energy and protein (Provenza et al., 1996; Wang and Provenza, 1996). Lambs in the present study maintained relatively constant ratios of energy to protein in their diets, and the ratios were in the range (47 to 53) reported in other studies (Provenza et al., 1996; Villalba and Provenza, 1999). Wang and Provenza (1996) found that lambs maintained a balance of energy:protein by ingesting a diet of 54% barley (3.26 Mcal/Kg DE, 8.5% DP) and 46% alfalfa (2.41 Mcal/Kg DE, 14.0% DP), when offered a choice between these two foods. Tannins reduce digestibility by binding to proteins and carbohydrates. The production of volatile fatty acids in vitro is positively related with the addition of PEG to high-tannin plants; conversely, addition of PEG to tannin-free plants does not increase production of volatile fatty acids (Makkar et al., 1995), in situ degradability (Silanikove et al., 1996b), or in vivo digestibility (Silanikove et al., 1996a). Thus, the positive effects of PEG on organic matter digestion are due to neutralizing the averse effects of tannins. As tannin levels increased from Trials 1 to 4, intake of macronutrients and tannin increasingly differed for PEG-supplemented and unsupplemented lambs (Table 3). The effects of PEG were greatest when tannin levels were high, and the differences in intake of dry matter, energy, protein, and tannin between supplemented and unsupplemented lambs became significant in Trial 4, when HB, LB, and A all contained tannin. For lambs not supplemented with PEG, energy intake declined by 16% and protein intake declined by 12% from Trial 1 to Trial 4. For lambs supplemented with PEG, energy intake declined by 5% and protein intake declined by 1%. The decline in intake of protein was not as great as energy because, as tannin levels increased in foods LB and HB, lambs ate more of food A, which had a higher concentration of protein and a lower concentration of energy than foods HB and LB. It is conceivable that lambs would ingest more protein (increase the protein:energy ratios) if a decrease in digestion of protein or energy was causing a deficit. However, there was no indication of an increase in intake of protein or energy by PEG-supplemented or unsupplemented lambs as tannin levels increased. Lambs maintained a constant ratio of protein to energy in their diets throughout the trials (Table 3), even though they could have increased protein:energy ratios from an average of 51 (Table 3) to as high as 63 (Table 2) by selecting exclusively for food A. Tannins differ in structure, which influences their effects on the body and on food intake (Clausen et al., 1990; Provenza et al., 1990). Quebracho tannin evidently caused averse effects in PEG-supplemented and unsupplemented lambs. The averse effects may be due to astringency in the mouth during food ingestion and postingestive lesions of gut mucosa and toxicity (Kumar and Singh, 1984; Reed, 1995). If tannins had merely decreased digestibility by binding to proteins or diluting the nutritional quality of the foods, we would have expected the lambs to increase their intake of the foods as tannin levels increased (Provenza et al., 1996). Instead, PEG-supplemented lambs maintained intake and unsupplemented lambs decreased intake. Influence of Tannin and PEG on Dietary Preference A high daily intake of energy is essential for survival, growth, and productivity. The energy content of food is important in regulating food intake by ruminants (Baile and Forbes, 1974; Forbes, 1996), and lambs preferred HB > LB > A in the absence of tannin. However, preferences changed as tannin levels were increased in foods HB and LB. Even lambs supplemented with PEG preferred alternative foods lower in tannins when they were available. In Trial 1, unsupplemented lambs preferred HB10T = LB5T > A, even though HB10T contained twice as much tannin as LB5T, which illustrates the influence of energy on preference. Adding tannin at 10% (HB) or 5% (LB) did not strongly deter lambs from eating HB10T or LB5T (Table 2). Alfalfa (A) was least preferred, evidently due to its low energy content. Nevertheless, HB10T and LB5T were equally preferred, which suggests that the 10% tannin in HB was aversive. If the tannin was just a diluent, then lambs should have preferred HB10T > LB5T > A. PEG-supplemented lambs also preferred HB10T = LB5T > A, which suggests that even with PEG supplementation they still experienced some aversive effects of tannins. In Trial 2, the DE levels in HB20T and LB5T were equal, but the tannin content of HB20T (20%) was four times that of LB5T (5%). All lambs preferred LB5T > A > HB20T. This reflects interactions between energy and tannin. As tannin concentrations increased in HB, lambs preference shifted to LB, which was higher in energy than A. In Trial 3, we expected lambs to avoid HB20T (20% tannin) and LB10T (10% tannin) because they contained twice as much tannin as in Trial 1 (10% and 5%, respectively). Polyethylene glycol-unsupplemented and supplemented lambs preferred A > LB10T > HB20T. Lambs preference for A > LB10T, as the tannin level in LB5T doubled, lends additional weight to the idea that quebracho tannin is aversive. During Trial 4, all lambs preferred LB = A > HB. Lambs preferred HB > LB > A before Trial 1, during the baseline period between Trials 1 and 2, and between Trials 2 and 3. However, between Trials 3 and 4 and after Trial 4 all lambs preferred HB > LB = A. The equal preference for LB and A may have been due to prolonged exposure to the high-tannin diets, which can

6 1448 adversely affect nitrogen availability and utilization (Barry, 1985; Robbins et al., 1987). Sheep respond to nitrogen deficits by increasing the intake of nitrogen (Cooper et al., 1993; Kyriazakis and Oldham, 1993; Kyriazakis et al., 1994), and greater intake of A as the trials progressed increased the amount of nitrogen in their diet. Protein also can neutralize the effects of tannins (Silanikove et al., 1997a), which may further explain the tendency to prefer high-protein foods when all of the alternatives contained tannins. Our findings are consistent with the hypothesis that PEG increases intake when diets are high in tannins, but supplemental PEG did not change the food preferences of lambs. Polyethylene glycol-supplemented lambs did not prefer tannin-rich foods, even though the high-tannin foods were also high in energy, unless the available alternatives all contained tannin. Thus, our results suggest that supplemental PEG will not inevitably lead to greater use of high-tannin plants when animals have a variety of alternatives from which to choose. The effectiveness of supplemental PEG may be low if alternative forages are equal or superior in nutritional quality and contain less metabolites with aversive effects. In such cases, animals would likely prefer alternatives to high-tannin foods. When alternatives are of equal or lower value, as occurs when goats eat twigs of blackbrush that differ in macronutrients and tannins, then supplemental PEG is likely to affect food preference (Titus et al., 2000). Finally, increasing the dosage of PEG may have changed our results. Lambs fed a 20% tannin diet ingest in excess of 200 g of PEG when PEG is offered free choice, and they can maintain intakes similar to lambs fed diets containing 10% tannin (Provenza et al., 2000). Implications Macronutrients, quebracho tannins, and polyethylene glycol (PEG) interacted along a continuum to affect food intake of lambs. As tannin concentrations increased in the preferred foods containing high and medium concentrations of barley, lambs supplemented with 50 g of PEG ingested more macronutrients and tannins than unsupplemented lambs. Supplemented lambs did not prefer tannin-rich foods, even though the high-tannin foods were also high in energy, unless the available alternatives all contained tannin. Thus, our results suggest that supplemental PEG will not lead inevitably to greater use of high-tannin plants when animals have a variety of alternatives from which to choose. Unless the alternatives are equal to or lower in nutritional quality than the high-tannin plants, animals may eat the alternatives, regardless of supplemental PEG. When alternatives are of equal or lower value, then supplemental PEG is likely to affect food preference. Such conditions are common in shrub-dominated rangelands worldwide. Titus et al. Literature Cited Aharoni, Y., N. Gilboa, Z. Nitsan and A. Perevolotsky Analysis of the suppressive effect of tannins on ruminal degradation by compartmental models. Anim. Feed Sci. Technol. 71: Baile, C. A., and J. M. Forbes Control of feed intake and regulation of energy balance in ruminants. Physiol. Rev. 54: Barry, T. N The role of condensed tannins in the nutritional value of Lotus pedunculatus for sheep. 3. Rates of body and wool growth. Br. J. Nutr. 54: Clausen, T. P., F. D. Provenza, E. A. Burritt, J. P. Bryant, and P. B. Reichardt Ecological implications of condensed tannin structure: a case study. J. Chem. Ecol. 16: Cooper, S. D. B., I. Kyriazakis, D. H. Anderson, and J. D. Oldham The effect of physiological state (late pregnancy) on the diet selection of ewes. Anim. Prod. 56:469A. Forbes, J. M Integration of regulatory signals controlling forage intake in ruminants. J. Anim. Sci. 74: Furstenburg, D., and W. Hoven Condensed tannin as antidefoliate agent against browsing giraffe (Giraffa camelopardalis) in the Kruger National Park. Comp. Biochem. Physiol. 107A: Gilboa, N The effects of PEG on the utilization of Mediterranean woody plants as forage for livestock. Ph.D. thesis. The Hebrew University, Jerusalem, Israel. Hannigan, N. A., and D. M. McNeill Cattle preference for two genotypes of fresh leucaena following the manipulation of their tannin content with polyethylene glycol. Proc. Anim. Soc. Anim. Prod. 22:401. Horigome, T., R. Kumar, and K. Okamato Effects of condensed tannins prepared from leaves of fodder plants on digestive enzymes in vitro and in the intestine of rats. Br. J. Nutr. 60: Kumar, R. and M. Singh Tannins, their adverse role in ruminant nutrition. J. Agric. Food Chem. 32: Kumar, R., and S. Vaithiyanathan Occurrence, nutritional significance and effect on animal productivity of tannins in tree leaves. Anim. Feed Sci. Technol. 30: Kyriazakis, I. and J. D. Oldham Diet selection in sheep: the ability of growing lambs to select a diet that meets their crude protein (nitrogen 6.25) requirements. Br. J. Nutr. 69: Kyriazakis, I., J. D. Oldham, R. L. Coop, and F. Jackson The effect of subclinical intestinal nematode infection on the diet selection of growing sheep. Br. J. Nutr. 72: Makkar, H. P. S., M. Blummel, and K. Becker Formation of complexes between polyvinyl pyrrolidones or polyethylene glycols and tannins, and their implications in gas production and true digestibility in in vitro techniques. Br. J. Nutr. 73: Nastis, A. S., and J. C. Malechek Digestion and utilization of nutrients in oak browse by goats. J. Anim. Sci. 53: NRC Nutrient requirements of sheep. National Academy Press, Washington, D.C. Pritchard, D. A., D. C. Stocks, B. M. O Sullivan, P. R. Martins, I. S. Hurwood, and P. K. O Rourke The effect of polyethylene glycol (PEG) on wool growth and live weight of sheep consuming a Mulga (Acacia aneura) diet. Proc. Aust. Soc. Anim. Prod. 17: Provenza, F. D., E. A. Burritt, T. P. Clausen, J. P. Bryant, P. B. Reichardt, and R. A. Distel Conditioned flavor aversion: a mechanism for goats to avoid condensed tannins in blackbrush. Am. Nat. 136: Provenza, F. D., E. A. Burritt, A. Perevolotsky, and N. Silanikove Self-regulation of intake of polyethylene glycol by sheep fed diets varying in tannin concentrations. J. Anim. Sci. 78: Provenza, F. D., L. Ortega-Reyes, C. B. Scott, J. J. Lynch, and E. A. Burritt Antiemetic drugs attenuate food aversions in sheep. J. Anim. Sci. 72:

7 Nutrients, tannins, and polyethylene glycol 1449 Provenza, F. D., C. B. Scott, T. S. Phy, and J. J. Lynch Preference of sheep for foods varying in flavors and nutrients. J. Anim. Sci. 74: Reed, J. D Nutritional toxicology of tannins and related polyphenols in forage legumes. J. Anim. Sci. 73: Robbins, C. T., T. A. Hanely, A. E. Hagerman, O. Hjeljord, D. L. Baker, C. C. Schwartz, and W. W. Mautz Role of tannins in defending plants against ruminants: Reduction in protein availability. Ecology 68: Silanikove, N., N. Gilboa, I. Nir, A. Perevolotsky, and Z. Nitsan. 1996a. Effect of a daily supplementation of polyethylene glycol on intake and digestion of tannin-containing leaves (Quercus calliprinos, Pistacia lentiscus and Ceratonia siliqua) by goats. J. Agric. Food Chem. 44: Silanikove, N., N. Gilboa, and Z. Nitsan. 1997a. Interactions among tannins, supplementation, and polyethylene glycol in goats fed oak leaves. Anim. Sci. 64: Silanikove, N., N. Gilboa, Z. Nitsan, and A. Perevolotsky. 1997b. Effects of foliage-tannins on feeding activity in goats. Mediterraneennes. 34(Series A): Silanikove, N., D. Shinder, N. Gilboa, M. Eyal, and Z. Nitsan. 1996b. Polyethylene glycol binding to plant samples as an assay for the biological effects of tannins: Predicting the negative effects of tannins in Medeterranean browse on rumen degradation. J. Agric. Chem. Food Sci. 44: Silanikove, N., Z. Nitsan, and A. Perevolotsky Effect of a daily supplementation of polyethylene glycol on intake and digestion of tannin-containing leaves (Ceratonia siliqua) by sheep. J. Agric. Food Chem. 42: Titus, C. H., F. D. Provenza, A. Perevolotsky, N. Silanikove, and J. Rogosic Food preferences of goats supplemented with polyethylene glycol while browsing blackbrush-dominated rangeland. J. Range Manage.(In press). Villalba, J. J., and F. D. Provenza Preference for flavored wheat straw by lambs conditioned with intraruminal administrations of sodium propionate. J. Anim. Sci. 74: Villalba, J. J., and F. D. Provenza. 1997a. Preference for flavored wheat straw by lambs conditioned with intraruminal infusions of acetate and propionate. J. Anim. Sci. 75: Villalba, J. J., and F. D. Provenza. 1997b. Preference for flavoured foods by lambs conditioned with intraruminal administration of nitrogen. Br. J. Nutr. 78: Villalba, J. J., and F. D. Provenza. 1997c. Preference for wheat straw by lambs conditioned with intraruminal infusions of starch. Br. J. Nutr. 77: Villalba, J. J. and F. D. Provenza Nutrient-specific preferences by lambs conditioned with intraruminal infusions of starch, casein, and water. J. Anim. Sci. 77: Wang, J., and F. D. Provenza Food preference and acceptance of novel foods by lambs depend on the composition of the basal diet. J. Anim. Sci. 74: