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Assessment of tree fodder preference by cattle using chemical composition, in vitro gas production and in situ degradability
Animal Feed Science and Technology 123–124 (2005) 277–289
Assessment of tree fodder preference by cattle using chemical composition, in vitro gas production and in situ degradability Carlos A. Sandoval-Castro ∗ , Henry L. Lizarraga-Sanchez, Francisco J. Solorio-Sanchez Facultad de Medicina Veterinaria y Zootecnia, Universidad Aut´onoma de Yucat´an, Apdo. 4-116 Itzimn´a, M´erida, Yucat´an 97100, M´exico
Abstract Short term preference for five tree fodders by cattle was assessed by their chemical composition, as well as in situ and in vitro gas production, in two studies. In study 1, five heifers (341 ± 36 kg liveweight) were offered Brosimun alicastrun (BA), Piscidia piscipula (PP), Leucaena leucocephala (LL), Lysiloma latisiliquum (TL) and Guazuma ulmifolia (GU) in a 6 h ‘cafeteria’ study over 5 consecutive days. In study 2, the same 5 heifers and tree fodders were used, but each heifer was offered a single tree fodder and fresh Taiwan grass (Pennisetum purpureum) ad libitum for 6 h. After 6 h, refusals were weighed and, for the rest of the day, only Taiwan grass was offered ad libitum. Forages offered in both studies were analyzed for dry matter (DM), crude protein, ash, neutral detergent fibre, acid detergent fibre, lignin, total polyphenols and condensed tannins, as well as for in situ DM degradation (0, 3, 6, 12, 24, 48, 72 and 96 h) and in vitro gas production (3, 6, 9, 12, 15, 18, 21, 24, 30, 36, 48, 60, 72, 96 and 120 h). In situ degradability was fitted to the equation p = a + b(1−e−ct ), while the equation gas = GV(1 + (B/t)C−1 ) was used for in vitro gas production. Residues from in vitro gas production were used to estimate in vitro DM and organic matter digestibility. Relationship between tree fodder intake, chemical composition, and in situ and in vitro digestibility was assessed using Pearson correlation analysis. In study 1, tree fodder intake (g DM/kg LW0.75 ) was: PP 5.41, TL 5.62, LL 15.62, GU 17.31 and BA 55.36 (S.E.D. 1.63). Intake was correlated with lignin (Pearson coefficient, −0.898,
Abbreviations: ADF, acid detergent fibre; C, condensed tannins; TP, total phenols; DM, dry matter; DMI, DM intake; IVOMD, in vitro apparent DM digestibility; IVOMD, in vitro apparent OM digestibility; IVGP, in vitro gas production; NDF, neutral detergent fibre; OM, organic matter ∗ Corresponding author. Fax: +52 999 9 42 32 05. E-mail address: [email protected] (C.A. Sandoval-Castro). 0377-8401/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.anifeedsci.2005.04.057
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P=0.039), IVDMD (r = 0.916, P=0.029) IVOMD (r = 0.902, P=0.036), parameter ‘b’ and ‘c’ from in situ degradability (r = 0.926, P=0.034 and r = 0.926, P=0.024, respectively) and with the 12, 24, 48, 72 and 96 h measurements, as well as parameter GV from in vitro gas production (r = 0.873, P=0.053) and 12, 15, 18, 21, 24, 30, 36 and 48 h cumulative gas readings. In study 2, tree fodder intake (g DM/kg LW0.75 ) was: PP 17.74, TL4.08, LL 22.18, GU 13.47 and BA 38.56 (S.E.D. 0.52). Grass intake (g DM/kg LW0.75 ) during the same time was: 79.41, 89.64, 87.69, 86.30 and 77.43 (S.E.D. 0.83) when PP, TL, LL, GU and BA, respectively, was offered. Total DM intake (in 6 h) increased (P<0.05) when LL and BA were offered. Tree fodder intake was related to IVDMD (r = 0.950, P=0.013), IVOMD (r = 0.942, P=0.017), parameter ‘c’ from in situ degradability (r = 0.908, P=0.033) and 12 and 24 h measurements, as well as parameter GV from in vitro gas production (r = 0.910, P=0.032) and 3, 6, 9, 12, 15, 18, 21, and 24 h cumulative gas readings. As feed preference is a short term response, lignin might be an indicator of intake preference. © 2005 Elsevier B.V. All rights reserved. Keywords: Tree fodder; Preference; Cattle; In vitro; In situ; Chemical composition
1. Introduction Voluntary intake of tropical grasses can be a major constraint to ruminant performance in tropical countries. Use of tree fodders as feed supplements can improve the rumen environment, leading to increased forage intake (Umunna et al., 1995). Thus, the tree fodders Leucaena leucocephala (Shelton and Brewbaker, 1994), Erythrina poeppigiana (Snaola and Rios, 1994), and Brosimun alicastrum (Sandoval-Castro et al., 2001) have been evaluated in order to effectively incorporate them into ruminant diets. Although most studies have focused on single tree fodder evaluation, reports relating to tropical tree fodder preference by cattle are limited (Nieto Marin et al., 2001). However, in tropical cattle production systems, a common practice is to offer a variety of tree fodders as supplements, either separately or with grasses. Information on effects of these practices on intake of forages is not readily available. In order to develop strategies for management of fodder trees, a fast and inexpensive method to assess short term preference and potential short term intake is needed if cattle are to be offered a supplement of several tree fodders in a ‘cafeteria’, or when the basal diet of grass is supplemented with a single tree fodder. The in vitro gas production technique (Theodorou et al., 1994) has been used to predict tree fodder preference (Sandoval-Castro et al., 2000b), as well as voluntary intake of single grasses (e.g., Khazaal et al., 1995; Bl¨ummel and Bullerdieck, 1997; Mendoza-Nazar and Sandoval-Castro, 2003), but with little success. However, when in situ and chemical composition data were included, predictions improved (i.e., Khazaal et al., 1995; Bl¨ummel and Bullerdieck, 1997; Mendoza-Nazar and Sandoval-Castro, 2003), although most of these experiments evaluated single feeds and not feed preference. Therefore, more information is needed to assess the value of these parameters as predictors of forage preference, and hence their value in designing feed management strategies. The objectives were to assess short term preference of cattle to 5 tree fodders in a cafeteria study, to evaluate short term intake of grass and an individual tree fodder when offered simultaneously and estimate the value of chemical composition, in vitro gas production
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and in situ degradability, as predictors of preference and intake of tree fodders. Preliminary results have been presented (Lizarraga S´anchez et al., 2001a,b,c).
2. Materials and methods 2.1. Preference study 1 Five heifers (341 ± 36 kg liveweight) were allocated to individual pens and fed fresh Taiwan grass (Pennisetum purpureum) ad libitum (15% more than the previous days intake). Simultaneously, 2 kg of fresh forage from each of B. alicastrun, Piscidia piscipula, L. leucocephala, Lysiloma latisiliquum and Guazuma ulmifolia was offered in a 10 d adaptation. After adaptation, 5 more days were used to measure intake in a ‘cafeteria’ study as suggested by (Borman et al., 1991). Tree leaves were harvested from branches with a 0–3 cm. All tree fodders were offered fresh, ad libitum, in separate containers for 6 h to each heifer. Care was taken to ensure that feeders had between 3 and 5 kg of fresh tree fodder at all times during the 6 h period. Each day the position of the tree fodder was changed to avoid conditioning and learning effects. After 6 h, refusals were weighed and Taiwan grass was offered as usual. In addition, intake was measured hourly to determine intake patterns. The 5 day experimental period was repeated thrice. 2.1.1. Chemical analysis Samples of the forages were analyzed according to AOAC (1980) procedures for dry matter (DM; 7.007), crude protein (CP; 2.057), ash (7.009). Neutral detergent fibre (NDF) was analyzed with sulphite and without amylase (Van Soest et al., 1991), and both NDF and acid detergent fibre (ADF) were not ash corrected, Total phenols (TP) (Price and Butler, 1977) and condensed tannin (CT) (Price et al., 1978) were also determined. 2.2. Preference study 2 The same 5 heifers from study 1 (347 ± 33 kg liveweight) were allocated to individual pens and fed fresh Taiwan grass (Pennisetum purpureum) ad libitum. Simultaneously, 2 kg of fresh forage of each of the same 5 tree fodders used in preference study 1 were offered for a 10 d adaptation period. After adaptation, a fresh tree fodder was offered for 6 h, ad libitum, at least 20% over the previous days grass DM intake (DMI), simultaneous (in a separate container) to Taiwan grass. A different tree fodder was fed to each heifer so that all 5 tree fodders were evaluated in each period. Assignment of tree fodders to individual heifers within periods was within a Latin square design, adaptated from Borman et al. (1991), to avoid confounding and carryover effects. After 6 h, refusals were weighed and Taiwan grass was offered ad libitum. There were 3 d of grass feeding between each day of tree fodder evaluation, to avoid residual effects on intake due to acceptability of the previous forage, until each heifer was offered all tree fodders. The procedure was repeated thrice. Samples of the forages were collected for chemical analyses as described in study 1.
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2.3. In situ ruminal DM degradation Ruminal degradation was determined by incubating approximately 5 g (DM) of sample in nylon bags (Ørskov et al., 1980) suspended in the rumen of Bos indicus cattle receiving 700 g/kg freshly cut forage (P. purpureum) and 300 g/kg concentrate with 180 g/kg CP. This diet was chosen to ensure that cellulolysis was optimum and that feed potentials were expressed under similar conditions. Nylon bags (Bar Diamond Inc, Parma, ID, USA), measuring 10 cm×20 cm (53 m pore size), were inserted into the rumen of the steers prior to the morning feeding. Bags were incubated in duplicate for 3, 6, 12, 24, 48, 72 and 96 h. At each incubation time, bags were removed from the rumen, immediately rinsed in cold water, and frozen. After completing all incubations, bags were thawed and washed for 5 min in a washing machine until the water was clear. Washed bags and contents were dried at 60 ◦ C for 48 h and weighed. Samples were incubated twice using different cows. Differences between the weight of DM initially placed in the bag and the weight of residual DM after incubation was used as the amount of DM disappearing. The organic matter (OM) disappearance in situ was calculated from DM disappearance and analysis of the residues at each incubation time. Zero-time washing losses were estimated by soaking 2 bags/sample in water for 10 min, washing with water until it became clear, filtering on previously weighed filter paper, followed by washing and drying as before. In situ degradation profiles were obtained after fitting data to the exponential equation of McDonald (1981) P = a + b(1−e−ct−TL ), using the GraphPad Prism 2.0 (1994/1995), where ‘P’ is the degradation at time ‘t’ and ‘a’, ‘b’ and ‘c’ are parameters of the equation. Thus ‘a + b’ is the potential degradability of the material and ‘TL’ an initial digestion lag phase.
2.4. In vitro gas production Rumen samples were obtained from two crossbred cows (Bos indicus×B. taurus) receiving 700 g/kg freshly cut forage (P. purpureum) and 300 g/kg concentrate with 180 g/kg CP. It was collected before the morning feeding, placed in a container, sealed immediately and transported to the laboratory that was 100 m distant. Preparation of N rich media and rumen liquor was as described by Menke and Steingass (1988). The method used for gas production measurements was as described by Theodorou et al. (1994). In vitro gas production (IVGP) profiles were determined in four 100 ml capacity serum bottles with 1 g sample/incubation/bottle in a single run. Readings were made at 3, 6, 9, 12, 15, 18, 21, 24, 30, 36, 48, 60, 72, 96 and 120 h post-infusion. The IVGP profiles were fitted to the monophasic equation of Groot et al. (1996). At the end of the incubation period, contents of each serum bottle were filtered into previously weighed filter paper, and the DM loss was calculated as the difference between the DM weight of the sample at the start of the incubation and the weight of DM remaining at the end of the incubation, after correcting for the blank. Residues were then ashed at 550 ◦ C for 24 h and OM fermentation calculated.
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2.5. Statistical analysis Preference, measured as DMI, was analyzed using the general linear means procedure of Minitab (1997), and an appropriate least significant difference test was used to determine differences among means. 2.5.1. Preference study 1 Dry matter intake data was analyzed according in repeated Latin square design, where heifers are the squares, columns are the position in pen, rows are the days, and treatments are the each individual tree fodder. As the procedure was repeated thrice, the number of valid observations for each tree fodder, according to Borman et al. (1991), was 15. 2.5.2. Preference study 2 Dry matter intake data was analyzed as repeated Latin square design where squares is the each one of the 3 full procedures, columns are the heifers (5), rows are the observation days (5), and treatments are the each of the 5 tree fodders. As the procedure was repeated thrice (squares), the number of valid observations for each tree fodder was 15. 2.5.3. Correlation analyses Pearson correlation analyses was used to assess relationships between short term tree fodder preference (i.e., studies 1 and 2) and chemical composition, in situ degradation, IVGP, IVDMD and IVOMD. 3. Results 3.1. Preference study 1 B. alicastrum had the highest IVDMD and IVOMD and also the highest DMI in the 6 h cafeteria study (Table 1). It also had the highest in situ potential DM degradability and the fastest rate (i.e., c) of all forages (Table 2). L. Leucocephala and G. ulmifolia were higher in CP content, but only had moderate intake (Table 1), in situ DM degradability and IVGP (Table 2). The TP and CT contents were high in L. latisiliquum which, together with P. piscipula, had the lowest intakes (Table 1). L. latisiliquum also had the lowest gas production and in situ digestion rate c (Table 2). Dry matter intake in 6 h was correlated (Table 3) with lignin (r = −0.898, P=0.039), IVDMD (r = 0.916, P=0.029) IVOMD (r = 0.902, P=0.036), parameter b and c from in situ degradability (r = 0.906, P=0.034 and r = 0.926, P=0.024, respectively) and with 12, 24, 48, 72 and 96 h measurements (Table 4), as well as parameter GV from IVGP (r = 0.873, P=0.053) (Table 3) and 12, 15, 18, 21, 24, 30, 36 and 48 h cumulative gas readings (Table 4). Intake of tree fodders differed from the first hour (Fig. 1) and cumulative intake patterns did not change in trend during the 6 h period. 3.2. Preference study 2 All forages had a different DMI, with B. alicastrum the highest and L. latisiliquum the lowest (P<0.05) during the 6 h period, although intakes were lower than those achieved
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Table 1 Chemical composition, digestible in vitro dry matter (IVDMD) and organic matter (IVOMD) content (g/kg DM), and short term intake (g DM/kgLW0.75 ) in a 6 h cafeteria study of five tree fodders in preference study 1
DM CP NDF ADF Lignin Ash TP CT IVDMD IVOMD Intakeb S.E.D. (intake)
BAa
PPa
LLa
TLa
GUa
418 169 360 288 68 116 17 7 695 755 55.36 a
385 185 481 289 148 126 18 7 479 538 5.41 c
349 267 395 239 108 79 24 12 536 570 15.62 b 1.63
471 213 418 212 116 79 37 12 373 404 5.62 c
324 155 426 259 107 109 14 18 535 590 17.31 b
Means with different letters (a, b, c) differ P<0.05. a BA, B. alicastrum; PP, P. piscipula; LL, L. Leucocephala; TL, L. Latisiliquum; GU, G. ulmifolia; TP, total phenols; CT, condensed tannins. b Intake = cafeteria study (n = 15 each tree).
when the tree fodders were fed alone. Intake of grass was lower (P<0.05) when the companion forage B. alicastrum or P. piscipula (P<0.05) was offered, and total DMI was higher (P<0.05) with B. alicastrum and L. leucocephala (Table 5). Tree fodder intake was related to IVDMD (r = 0.950, P=0.013), IVOMD (r = 0.942, P=0.017), parameter c (r = 0.908, P=0.033) (Table 3) and 12 and 24 h measurements of in situ degradability (Table 4), as well as parameter GV of IVGP (r = 0.910, P=0.032) (Table 3) and 3, 6, 9, 12, 15, 18, 21 and 24 h cumulative gas readings (Table 4).
Table 2 In situ rumen degradation and in vitro gas production profiles of five tree fodders In situ
Aa
ba
ca
Lag
T0
Sx.y
B. alicastrum P. piscipula L. leucocephala L. latisiliquum G. ulmifolia
28.50 c (2.65) 40.35 b (1.94) 40.20 b (1.48) 43.72 a (1.00) 29.25 c (2.02)
60.72 a (2.55) 25.20 d (2.05) 35.62 c (1.43) 34.16 c (15.14) 49.11 b (1.955)
0.082 a (0.007) 0.036 c (0.010) 0.054 b (0.006) 0.008 d (0.005) 0.055 b (0.006)
2.29 0.27 0.25 0 0.09
39.0 40.6 40.7 38.2 29.5
2.96 3.31 2.55 1.92 2.66
In vitro
GVb
Bb
Cb
Sx.y
B. alicastrum P. piscipula L. leucocephala L. latisiliquum G. ulmifolia
216.6 a (3.63) 145.7 c (7.44) 155.1 c (6.87) 96.45 d (24.25) 169.7 b (3.81)
40.97 c (1.07) 57.60 b (5.21) 54.41 b (3.79) 82.61 a (31.09) 51.36 b (1.70)
1.66 a (0.037) 1.27 c (0.053) 1.51 b (0.064) 1.39 bc (0.214) 1.68 a (0.041)
3.16 2.91 3.06 5.92 2.44
Values in the same column with different letters (a, b, c, d) differ (P<0.05). a In situ parameters a, b and c according to the equation P = a + b(1 − e−ct ). b In vitro parameters ‘GV’, ‘B’ and ‘C’ according to the equation: ml gas = GV(1 + (B/t)C−1 ).
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Table 3 Pearson correlation coefficients of five tree fodders intake when offered in cafeteria or simultaneously with grass (during 6 h periods) and in vitro dry matter (IVDMD) and organic matter (IVOMD) digestibility, chemical composition, in situ rumen DM degradability and in vitro gas production parameters Study 1
Study 2
Grass DMI IVDMD IVOMD
0.883 (0.047)a 0.916 (0.029) 0.902 (0.036)
– 0.950 (0.013) 0.942 (0.017)
Chemical composition DM CP NDF ADF Ash Total phenols Condensed tannins Lignin
0.053 (0.932) −0.339 (0.576) −0.790 (0.112) 0.498 (9.393) 0.312 (0.609) −0.415 (0.488) −0.297 (0.627) −0.898 (0.039)
−0.142 (0.819) 0.141 (0.821) −0.588 (0.297) 0.698 (0.190) 0.431 (0.469) −0.562 (0.324) −0.492 (0.400) 0.653 (0.232)
In situ degradabilityb A B C Lag
−0.705 (0.184) 0.926 (0.034) 0.926 (0.024) 0.354 (0.559)
−0.359 (0.553) 0.704 (0.184) 0.908 (0.033) 0.044 (0.944)
In vitro gas productionc GV B C ml/gDM fermented
0.873 (0.053) −0.722 (0.168) 0.697 (0.190) 0.684 (0.203)
0.910 (0.032) −0.859 (0.062) 0.454 (0.442) 0.727 (0.164)
a b c
Correlation (P value). In situ parameters a, b and c according to the equation P = a + b(1 − e−ct ). In vitro parameters ‘GV’, ‘B’ and ‘C’ according to the equation: ml gas = GV(1 + (B/t)C−1 ).
Fig. 1. Intake pattern of five tree fodders in a cafeteria study (each point; mean ± S.E., n = 15).
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Table 4 Pearson correlation coefficients of tree fodders intake when offered in cafeteria or simultaneously with grass (during 6 h periods) and in situ rumen degradability and in vitro gas production time measurements Study 1
Study 2
In situ (h) 3 6 12 24 48 72 96
−0.251 (0.683)a 0.571 (0.314) 0.895 (0.040) 0.917 (0.029) 0.926 (0.024) 0.909 (0.033) 0.897 (0.039)
−0.191 (0.759) 0.762 (0.134) 0.897 (0.039) 0.910 (0.032) 0.870 (0.055) 0.866 (0.058) 0.825 (0.086)
In vitro (h) 3 6 9 12 15 18 21 24 30 36 48 60 72 96
0.829 (0.083) 0.775 (0.124) 0.785 (0.116) 0.833 (0.047) 0.887 (0.045) 0.887 (0.045) 0.887 (0.045) 0.911 (0.031) 0.873 (0.053) 0.911 (0.031) 0.895 (0.040) 0.847 (0.070) 0.847 (0.070) 0.359 (0.553)
0.900 (0.037) 0.884 (0.047) 0.909 (0.032) 0.976 (0.004) 0.984 (0.003) 0.984 (0.003) 0.984 (0.003) 0.953 (0.012) 0.862 (0.060) 0.867 (0.057) 0.803 (0.102) 0.773 (0.125) 0.773 (0.125) 0.662 (0.263)
a
Correlation (P value).
Table 5 Voluntary intake (DM) of tree fodders when offered simultaneously with Taiwan grass for a period of 6 h
L. latisiliquum G. ulmifolia P. piscipula L. leucocephala B. alicastrum S.E.D.
Tree (kg)
Tree (g/kgLW0.75 )
Grass (kg)
Grass (g/kgLW0.75 )
Grass + tree (kg)
Grass + tree (g/kgLW0.75 )
0.031a 1.037b 1.368c 1.732d 2.981e 0.04
4.08a 13.47b 17.74c 22.18d 38.56e 0.52
6.83a 6.90a 6.21b 6.80a 5.90c 0.06
89.64a 86.30b 79.41c 87.69ab 77.43c 0.83
6.86a 7.94b 7.58c 8.53d 8.88e 0.08
93.72a 99.77b 97.16b 109.87c 116.00d 0.92
n = 15 for each tree. Values with different superscripts (a, b, c, d, e) in columns differ (P<0.05).
4. Discussion The chemical composition, IVDMD, IVOMD, in situ DM degradability and IVGP profiles of the tree fodders were within ranges commonly reported for the region (Sandoval Castro et al., 2000a; Bobadilla Hernandez, 2001; Pinto Ruiz et al., 2002a,b) and elsewhere (Solorio Sanchez and Solorio Sanchez, 2001). This confirms that tree fodder
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leaves have low variation in chemical composition, which results in consistent nutritional values. Prediction of DMI of individual forages has been attempted with in vitro digestibility, IVGP and in situ degradation techniques, with relatively good results (Hovell et al., 1986; Bl¨ummel and Ørskov, 1993; Khazaal et al., 1993, 1995; Shem et al., 1995; Bl¨ummel and Bullerdieck, 1997; Fernandez-Rivera, 1998; Chenost et al., 2001; Mendoza-Nazar and Sandoval-Castro, 2003). Similarly, results here (Table 4) confirm that short term measurement is a valuable option for intake prediction (Bl¨ummel and Bullerdieck, 1997; FernandezRivera, 1998; Chenost et al., 2001). 4.1. Preference study 1 During the 6 h period, there was a marked preference towards B. alicastrum, the tree fodder with the highest digestibility both in vitro and in situ. The preference was not affected by CP, probably because N was not limiting in these heifers (Van Soest, 1994) as all forages had CP contents above 150 g/kg DM. It is accepted that plant morphology and structure can influence preference and intake of forages by ruminants (Ortega and Provenza, 1993; Burns et al., 2001). However, in the present study none of the tree fodders had thorns and their leaves were equally accessible, suggesting that these factors were not the cause of the difference which favoured B. alicastrum (Fig. 1). This was confirmed by Nieto Marin et al. (2001), who observed that intake rate of these tree fodders, when offered ad libitum as single feeds, was similar at about 20 g DM/min. The main cause of the differences in intake was the effective time the heifer spent eating each tree fodder. In contrast, Sosa Rubio et al. (2004) found, in studies with sheep, that difference in intake of 5 tree fodders was due to intake rate (from 4 to 34 g/min), although it is not clear if intake rate was estimated using effective intake time, as in Nieto Marin et al. (2001). Nevertheless, results suggest that heifers may detect, at least in short term intake, foliage characteristics related to feed quality before they are detected as post-ingestive feedback signals (Provenza, 1995). The correlation between preference and lignin indicates that it may be detected orally as an indication of a hard, more difficult to comminute, material. Thus, by selecting ‘soft’ forages, the animal is also selecting highly and rapidly digestible material. This hypothesis will also accommodate the correlation between preference and IVDMD, IVOMD, IVGP and the c fraction of the in situ profile. Thus, the degree of lignification might be related to the c fraction (r = −0.818, P=0.09), and the ‘c’ fraction is highly correlated with both IVDMD (r = 0.988, P=0.033) and IVOMD (r = 0.975, P=0.005), thereby acting as a feedback mechanism during chewing to signal the animal about potential physical ruminal load of the feed. Similarly, Chenost et al. (2001) suggested that prediction of intake of single forages should be related to the in situ c fraction of the feeds, because it reflects physical regulation of voluntary intake. 4.2. Preference study 2 A similar pattern of preference in DMI occurred, as in preference study 1, with the exception of P. piscipula intake, which was proportionately higher. This might indicate
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that when the possibility of the animal to select is reduced, it might increase intake of forage which was previously not well accepted. Similar results have been reported by Camacho Morfin (2003) with sheep using tree fodders from central Mexico. In contrast, L. latisiliquum, which was not well accepted in both studies, also had the lowest digestion rate and gas production. This might result from its TP and CT content, as it has been reported that its tannins possess high biological activity amongst tree fodders of the region (Armend´ariz and Ya˜nez, 1998). In this particular case, the biological activity of CT might be detected even during short periods of intake, allowing the animal to discriminate for feed with limited nutritional potential due to tannins (McSweeney et al., 2001, 2003). Beneficial effects on DMI of including a tree fodder on the diet (Umunna et al., 1995) was only apparent with B. alicastrum and L. leucocephala, and there was no clear relationship with any chemical feed component. Degradation profiles and CT contents indicate that ruminants might prefer a rapidly rumen degradable forage (as discussed above), with low TP and CT. Although in the case of B. alicastrum, the increased intake also resulted in reduction of basal grass intake, essentially a substitution effect. The correlation between in vitro digestibility and IVGP with in situ feed characteristics has been previously reported (i.e., Camacho Morfin, 2003; Fernandez-Rivera, 1998; Khazaal et al., 1993, 1995). As both in situ and IVGP are evaluating activity of rumen microbes, and the potential of the feed to supply energy and N, a strong relationship has always been expected. However, it is noteworthy that they also correlate with short term intake (i.e., preference), supporting the hypothesis suggested above that, while eating, the animal might be detecting orally (possibly by feed hardiness and resistance to comminution), the potential of any feed to supply nutrients, which could be a selection characteristic learned by association with post-ingestive feedback effects from previous feeds (Provenza, 1995). Secondary compounds, such as tannins, thus being a secondary signal to intake regulation during short term studies, as their post-ingestive effects need variable time to develop, require time to be sensed and associated with the feeds by the animal. As shown previously (i.e., Fernandez-Rivera, 1998; Chenost et al., 2001), both IVGP and in situ time measurement were correlated with IVDMD and IVOMD from 12 h onwards in the in situ degradation measurements and from 3 to 72 h for IVGP measurements. However, as they also appear correlated with preference, it suggests that short term incubation studies might be a useful tool to predict cattle forage preference.
5. Conclusions Cattle are able to differentially select among tree fodder species if given the opportunity. The selection mechanism seems to be independent of post-ingestive feedback signals, suggesting that this ability must be taken into account when designing feeding strategies which include mixtures of tree fodders. However, grass was preferred over tree fodder when heifers were given the opportunity to select, another behavior to be taken into account in the design of feeding strategies that include tree fodders. Assessment of tree fodder preference can be done by employing in situ degradation and IVGP techniques, which would be complimentary to traditional chemical measurements.
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Acknowledgement This work was partially financed by IFS-Sweden. Grant B/2351-2.
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Assessment of tree fodder preference by cattle using chemical composition, in vitro gas production and in situ degradability Carlos A. Sandoval-Castro ∗ , Henry L. Lizarraga-Sanchez, Francisco J. Solorio-Sanchez Facultad de Medicina Veterinaria y Zootecnia, Universidad Aut´onoma de Yucat´an, Apdo. 4-116 Itzimn´a, M´erida, Yucat´an 97100, M´exico
Abstract Short term preference for five tree fodders by cattle was assessed by their chemical composition, as well as in situ and in vitro gas production, in two studies. In study 1, five heifers (341 ± 36 kg liveweight) were offered Brosimun alicastrun (BA), Piscidia piscipula (PP), Leucaena leucocephala (LL), Lysiloma latisiliquum (TL) and Guazuma ulmifolia (GU) in a 6 h ‘cafeteria’ study over 5 consecutive days. In study 2, the same 5 heifers and tree fodders were used, but each heifer was offered a single tree fodder and fresh Taiwan grass (Pennisetum purpureum) ad libitum for 6 h. After 6 h, refusals were weighed and, for the rest of the day, only Taiwan grass was offered ad libitum. Forages offered in both studies were analyzed for dry matter (DM), crude protein, ash, neutral detergent fibre, acid detergent fibre, lignin, total polyphenols and condensed tannins, as well as for in situ DM degradation (0, 3, 6, 12, 24, 48, 72 and 96 h) and in vitro gas production (3, 6, 9, 12, 15, 18, 21, 24, 30, 36, 48, 60, 72, 96 and 120 h). In situ degradability was fitted to the equation p = a + b(1−e−ct ), while the equation gas = GV(1 + (B/t)C−1 ) was used for in vitro gas production. Residues from in vitro gas production were used to estimate in vitro DM and organic matter digestibility. Relationship between tree fodder intake, chemical composition, and in situ and in vitro digestibility was assessed using Pearson correlation analysis. In study 1, tree fodder intake (g DM/kg LW0.75 ) was: PP 5.41, TL 5.62, LL 15.62, GU 17.31 and BA 55.36 (S.E.D. 1.63). Intake was correlated with lignin (Pearson coefficient, −0.898,
Abbreviations: ADF, acid detergent fibre; C, condensed tannins; TP, total phenols; DM, dry matter; DMI, DM intake; IVOMD, in vitro apparent DM digestibility; IVOMD, in vitro apparent OM digestibility; IVGP, in vitro gas production; NDF, neutral detergent fibre; OM, organic matter ∗ Corresponding author. Fax: +52 999 9 42 32 05. E-mail address: [email protected] (C.A. Sandoval-Castro). 0377-8401/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.anifeedsci.2005.04.057
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P=0.039), IVDMD (r = 0.916, P=0.029) IVOMD (r = 0.902, P=0.036), parameter ‘b’ and ‘c’ from in situ degradability (r = 0.926, P=0.034 and r = 0.926, P=0.024, respectively) and with the 12, 24, 48, 72 and 96 h measurements, as well as parameter GV from in vitro gas production (r = 0.873, P=0.053) and 12, 15, 18, 21, 24, 30, 36 and 48 h cumulative gas readings. In study 2, tree fodder intake (g DM/kg LW0.75 ) was: PP 17.74, TL4.08, LL 22.18, GU 13.47 and BA 38.56 (S.E.D. 0.52). Grass intake (g DM/kg LW0.75 ) during the same time was: 79.41, 89.64, 87.69, 86.30 and 77.43 (S.E.D. 0.83) when PP, TL, LL, GU and BA, respectively, was offered. Total DM intake (in 6 h) increased (P<0.05) when LL and BA were offered. Tree fodder intake was related to IVDMD (r = 0.950, P=0.013), IVOMD (r = 0.942, P=0.017), parameter ‘c’ from in situ degradability (r = 0.908, P=0.033) and 12 and 24 h measurements, as well as parameter GV from in vitro gas production (r = 0.910, P=0.032) and 3, 6, 9, 12, 15, 18, 21, and 24 h cumulative gas readings. As feed preference is a short term response, lignin might be an indicator of intake preference. © 2005 Elsevier B.V. All rights reserved. Keywords: Tree fodder; Preference; Cattle; In vitro; In situ; Chemical composition
1. Introduction Voluntary intake of tropical grasses can be a major constraint to ruminant performance in tropical countries. Use of tree fodders as feed supplements can improve the rumen environment, leading to increased forage intake (Umunna et al., 1995). Thus, the tree fodders Leucaena leucocephala (Shelton and Brewbaker, 1994), Erythrina poeppigiana (Snaola and Rios, 1994), and Brosimun alicastrum (Sandoval-Castro et al., 2001) have been evaluated in order to effectively incorporate them into ruminant diets. Although most studies have focused on single tree fodder evaluation, reports relating to tropical tree fodder preference by cattle are limited (Nieto Marin et al., 2001). However, in tropical cattle production systems, a common practice is to offer a variety of tree fodders as supplements, either separately or with grasses. Information on effects of these practices on intake of forages is not readily available. In order to develop strategies for management of fodder trees, a fast and inexpensive method to assess short term preference and potential short term intake is needed if cattle are to be offered a supplement of several tree fodders in a ‘cafeteria’, or when the basal diet of grass is supplemented with a single tree fodder. The in vitro gas production technique (Theodorou et al., 1994) has been used to predict tree fodder preference (Sandoval-Castro et al., 2000b), as well as voluntary intake of single grasses (e.g., Khazaal et al., 1995; Bl¨ummel and Bullerdieck, 1997; Mendoza-Nazar and Sandoval-Castro, 2003), but with little success. However, when in situ and chemical composition data were included, predictions improved (i.e., Khazaal et al., 1995; Bl¨ummel and Bullerdieck, 1997; Mendoza-Nazar and Sandoval-Castro, 2003), although most of these experiments evaluated single feeds and not feed preference. Therefore, more information is needed to assess the value of these parameters as predictors of forage preference, and hence their value in designing feed management strategies. The objectives were to assess short term preference of cattle to 5 tree fodders in a cafeteria study, to evaluate short term intake of grass and an individual tree fodder when offered simultaneously and estimate the value of chemical composition, in vitro gas production
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and in situ degradability, as predictors of preference and intake of tree fodders. Preliminary results have been presented (Lizarraga S´anchez et al., 2001a,b,c).
2. Materials and methods 2.1. Preference study 1 Five heifers (341 ± 36 kg liveweight) were allocated to individual pens and fed fresh Taiwan grass (Pennisetum purpureum) ad libitum (15% more than the previous days intake). Simultaneously, 2 kg of fresh forage from each of B. alicastrun, Piscidia piscipula, L. leucocephala, Lysiloma latisiliquum and Guazuma ulmifolia was offered in a 10 d adaptation. After adaptation, 5 more days were used to measure intake in a ‘cafeteria’ study as suggested by (Borman et al., 1991). Tree leaves were harvested from branches with a 0–3 cm. All tree fodders were offered fresh, ad libitum, in separate containers for 6 h to each heifer. Care was taken to ensure that feeders had between 3 and 5 kg of fresh tree fodder at all times during the 6 h period. Each day the position of the tree fodder was changed to avoid conditioning and learning effects. After 6 h, refusals were weighed and Taiwan grass was offered as usual. In addition, intake was measured hourly to determine intake patterns. The 5 day experimental period was repeated thrice. 2.1.1. Chemical analysis Samples of the forages were analyzed according to AOAC (1980) procedures for dry matter (DM; 7.007), crude protein (CP; 2.057), ash (7.009). Neutral detergent fibre (NDF) was analyzed with sulphite and without amylase (Van Soest et al., 1991), and both NDF and acid detergent fibre (ADF) were not ash corrected, Total phenols (TP) (Price and Butler, 1977) and condensed tannin (CT) (Price et al., 1978) were also determined. 2.2. Preference study 2 The same 5 heifers from study 1 (347 ± 33 kg liveweight) were allocated to individual pens and fed fresh Taiwan grass (Pennisetum purpureum) ad libitum. Simultaneously, 2 kg of fresh forage of each of the same 5 tree fodders used in preference study 1 were offered for a 10 d adaptation period. After adaptation, a fresh tree fodder was offered for 6 h, ad libitum, at least 20% over the previous days grass DM intake (DMI), simultaneous (in a separate container) to Taiwan grass. A different tree fodder was fed to each heifer so that all 5 tree fodders were evaluated in each period. Assignment of tree fodders to individual heifers within periods was within a Latin square design, adaptated from Borman et al. (1991), to avoid confounding and carryover effects. After 6 h, refusals were weighed and Taiwan grass was offered ad libitum. There were 3 d of grass feeding between each day of tree fodder evaluation, to avoid residual effects on intake due to acceptability of the previous forage, until each heifer was offered all tree fodders. The procedure was repeated thrice. Samples of the forages were collected for chemical analyses as described in study 1.
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2.3. In situ ruminal DM degradation Ruminal degradation was determined by incubating approximately 5 g (DM) of sample in nylon bags (Ørskov et al., 1980) suspended in the rumen of Bos indicus cattle receiving 700 g/kg freshly cut forage (P. purpureum) and 300 g/kg concentrate with 180 g/kg CP. This diet was chosen to ensure that cellulolysis was optimum and that feed potentials were expressed under similar conditions. Nylon bags (Bar Diamond Inc, Parma, ID, USA), measuring 10 cm×20 cm (53 m pore size), were inserted into the rumen of the steers prior to the morning feeding. Bags were incubated in duplicate for 3, 6, 12, 24, 48, 72 and 96 h. At each incubation time, bags were removed from the rumen, immediately rinsed in cold water, and frozen. After completing all incubations, bags were thawed and washed for 5 min in a washing machine until the water was clear. Washed bags and contents were dried at 60 ◦ C for 48 h and weighed. Samples were incubated twice using different cows. Differences between the weight of DM initially placed in the bag and the weight of residual DM after incubation was used as the amount of DM disappearing. The organic matter (OM) disappearance in situ was calculated from DM disappearance and analysis of the residues at each incubation time. Zero-time washing losses were estimated by soaking 2 bags/sample in water for 10 min, washing with water until it became clear, filtering on previously weighed filter paper, followed by washing and drying as before. In situ degradation profiles were obtained after fitting data to the exponential equation of McDonald (1981) P = a + b(1−e−ct−TL ), using the GraphPad Prism 2.0 (1994/1995), where ‘P’ is the degradation at time ‘t’ and ‘a’, ‘b’ and ‘c’ are parameters of the equation. Thus ‘a + b’ is the potential degradability of the material and ‘TL’ an initial digestion lag phase.
2.4. In vitro gas production Rumen samples were obtained from two crossbred cows (Bos indicus×B. taurus) receiving 700 g/kg freshly cut forage (P. purpureum) and 300 g/kg concentrate with 180 g/kg CP. It was collected before the morning feeding, placed in a container, sealed immediately and transported to the laboratory that was 100 m distant. Preparation of N rich media and rumen liquor was as described by Menke and Steingass (1988). The method used for gas production measurements was as described by Theodorou et al. (1994). In vitro gas production (IVGP) profiles were determined in four 100 ml capacity serum bottles with 1 g sample/incubation/bottle in a single run. Readings were made at 3, 6, 9, 12, 15, 18, 21, 24, 30, 36, 48, 60, 72, 96 and 120 h post-infusion. The IVGP profiles were fitted to the monophasic equation of Groot et al. (1996). At the end of the incubation period, contents of each serum bottle were filtered into previously weighed filter paper, and the DM loss was calculated as the difference between the DM weight of the sample at the start of the incubation and the weight of DM remaining at the end of the incubation, after correcting for the blank. Residues were then ashed at 550 ◦ C for 24 h and OM fermentation calculated.
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2.5. Statistical analysis Preference, measured as DMI, was analyzed using the general linear means procedure of Minitab (1997), and an appropriate least significant difference test was used to determine differences among means. 2.5.1. Preference study 1 Dry matter intake data was analyzed according in repeated Latin square design, where heifers are the squares, columns are the position in pen, rows are the days, and treatments are the each individual tree fodder. As the procedure was repeated thrice, the number of valid observations for each tree fodder, according to Borman et al. (1991), was 15. 2.5.2. Preference study 2 Dry matter intake data was analyzed as repeated Latin square design where squares is the each one of the 3 full procedures, columns are the heifers (5), rows are the observation days (5), and treatments are the each of the 5 tree fodders. As the procedure was repeated thrice (squares), the number of valid observations for each tree fodder was 15. 2.5.3. Correlation analyses Pearson correlation analyses was used to assess relationships between short term tree fodder preference (i.e., studies 1 and 2) and chemical composition, in situ degradation, IVGP, IVDMD and IVOMD. 3. Results 3.1. Preference study 1 B. alicastrum had the highest IVDMD and IVOMD and also the highest DMI in the 6 h cafeteria study (Table 1). It also had the highest in situ potential DM degradability and the fastest rate (i.e., c) of all forages (Table 2). L. Leucocephala and G. ulmifolia were higher in CP content, but only had moderate intake (Table 1), in situ DM degradability and IVGP (Table 2). The TP and CT contents were high in L. latisiliquum which, together with P. piscipula, had the lowest intakes (Table 1). L. latisiliquum also had the lowest gas production and in situ digestion rate c (Table 2). Dry matter intake in 6 h was correlated (Table 3) with lignin (r = −0.898, P=0.039), IVDMD (r = 0.916, P=0.029) IVOMD (r = 0.902, P=0.036), parameter b and c from in situ degradability (r = 0.906, P=0.034 and r = 0.926, P=0.024, respectively) and with 12, 24, 48, 72 and 96 h measurements (Table 4), as well as parameter GV from IVGP (r = 0.873, P=0.053) (Table 3) and 12, 15, 18, 21, 24, 30, 36 and 48 h cumulative gas readings (Table 4). Intake of tree fodders differed from the first hour (Fig. 1) and cumulative intake patterns did not change in trend during the 6 h period. 3.2. Preference study 2 All forages had a different DMI, with B. alicastrum the highest and L. latisiliquum the lowest (P<0.05) during the 6 h period, although intakes were lower than those achieved
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Table 1 Chemical composition, digestible in vitro dry matter (IVDMD) and organic matter (IVOMD) content (g/kg DM), and short term intake (g DM/kgLW0.75 ) in a 6 h cafeteria study of five tree fodders in preference study 1
DM CP NDF ADF Lignin Ash TP CT IVDMD IVOMD Intakeb S.E.D. (intake)
BAa
PPa
LLa
TLa
GUa
418 169 360 288 68 116 17 7 695 755 55.36 a
385 185 481 289 148 126 18 7 479 538 5.41 c
349 267 395 239 108 79 24 12 536 570 15.62 b 1.63
471 213 418 212 116 79 37 12 373 404 5.62 c
324 155 426 259 107 109 14 18 535 590 17.31 b
Means with different letters (a, b, c) differ P<0.05. a BA, B. alicastrum; PP, P. piscipula; LL, L. Leucocephala; TL, L. Latisiliquum; GU, G. ulmifolia; TP, total phenols; CT, condensed tannins. b Intake = cafeteria study (n = 15 each tree).
when the tree fodders were fed alone. Intake of grass was lower (P<0.05) when the companion forage B. alicastrum or P. piscipula (P<0.05) was offered, and total DMI was higher (P<0.05) with B. alicastrum and L. leucocephala (Table 5). Tree fodder intake was related to IVDMD (r = 0.950, P=0.013), IVOMD (r = 0.942, P=0.017), parameter c (r = 0.908, P=0.033) (Table 3) and 12 and 24 h measurements of in situ degradability (Table 4), as well as parameter GV of IVGP (r = 0.910, P=0.032) (Table 3) and 3, 6, 9, 12, 15, 18, 21 and 24 h cumulative gas readings (Table 4).
Table 2 In situ rumen degradation and in vitro gas production profiles of five tree fodders In situ
Aa
ba
ca
Lag
T0
Sx.y
B. alicastrum P. piscipula L. leucocephala L. latisiliquum G. ulmifolia
28.50 c (2.65) 40.35 b (1.94) 40.20 b (1.48) 43.72 a (1.00) 29.25 c (2.02)
60.72 a (2.55) 25.20 d (2.05) 35.62 c (1.43) 34.16 c (15.14) 49.11 b (1.955)
0.082 a (0.007) 0.036 c (0.010) 0.054 b (0.006) 0.008 d (0.005) 0.055 b (0.006)
2.29 0.27 0.25 0 0.09
39.0 40.6 40.7 38.2 29.5
2.96 3.31 2.55 1.92 2.66
In vitro
GVb
Bb
Cb
Sx.y
B. alicastrum P. piscipula L. leucocephala L. latisiliquum G. ulmifolia
216.6 a (3.63) 145.7 c (7.44) 155.1 c (6.87) 96.45 d (24.25) 169.7 b (3.81)
40.97 c (1.07) 57.60 b (5.21) 54.41 b (3.79) 82.61 a (31.09) 51.36 b (1.70)
1.66 a (0.037) 1.27 c (0.053) 1.51 b (0.064) 1.39 bc (0.214) 1.68 a (0.041)
3.16 2.91 3.06 5.92 2.44
Values in the same column with different letters (a, b, c, d) differ (P<0.05). a In situ parameters a, b and c according to the equation P = a + b(1 − e−ct ). b In vitro parameters ‘GV’, ‘B’ and ‘C’ according to the equation: ml gas = GV(1 + (B/t)C−1 ).
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Table 3 Pearson correlation coefficients of five tree fodders intake when offered in cafeteria or simultaneously with grass (during 6 h periods) and in vitro dry matter (IVDMD) and organic matter (IVOMD) digestibility, chemical composition, in situ rumen DM degradability and in vitro gas production parameters Study 1
Study 2
Grass DMI IVDMD IVOMD
0.883 (0.047)a 0.916 (0.029) 0.902 (0.036)
– 0.950 (0.013) 0.942 (0.017)
Chemical composition DM CP NDF ADF Ash Total phenols Condensed tannins Lignin
0.053 (0.932) −0.339 (0.576) −0.790 (0.112) 0.498 (9.393) 0.312 (0.609) −0.415 (0.488) −0.297 (0.627) −0.898 (0.039)
−0.142 (0.819) 0.141 (0.821) −0.588 (0.297) 0.698 (0.190) 0.431 (0.469) −0.562 (0.324) −0.492 (0.400) 0.653 (0.232)
In situ degradabilityb A B C Lag
−0.705 (0.184) 0.926 (0.034) 0.926 (0.024) 0.354 (0.559)
−0.359 (0.553) 0.704 (0.184) 0.908 (0.033) 0.044 (0.944)
In vitro gas productionc GV B C ml/gDM fermented
0.873 (0.053) −0.722 (0.168) 0.697 (0.190) 0.684 (0.203)
0.910 (0.032) −0.859 (0.062) 0.454 (0.442) 0.727 (0.164)
a b c
Correlation (P value). In situ parameters a, b and c according to the equation P = a + b(1 − e−ct ). In vitro parameters ‘GV’, ‘B’ and ‘C’ according to the equation: ml gas = GV(1 + (B/t)C−1 ).
Fig. 1. Intake pattern of five tree fodders in a cafeteria study (each point; mean ± S.E., n = 15).
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Table 4 Pearson correlation coefficients of tree fodders intake when offered in cafeteria or simultaneously with grass (during 6 h periods) and in situ rumen degradability and in vitro gas production time measurements Study 1
Study 2
In situ (h) 3 6 12 24 48 72 96
−0.251 (0.683)a 0.571 (0.314) 0.895 (0.040) 0.917 (0.029) 0.926 (0.024) 0.909 (0.033) 0.897 (0.039)
−0.191 (0.759) 0.762 (0.134) 0.897 (0.039) 0.910 (0.032) 0.870 (0.055) 0.866 (0.058) 0.825 (0.086)
In vitro (h) 3 6 9 12 15 18 21 24 30 36 48 60 72 96
0.829 (0.083) 0.775 (0.124) 0.785 (0.116) 0.833 (0.047) 0.887 (0.045) 0.887 (0.045) 0.887 (0.045) 0.911 (0.031) 0.873 (0.053) 0.911 (0.031) 0.895 (0.040) 0.847 (0.070) 0.847 (0.070) 0.359 (0.553)
0.900 (0.037) 0.884 (0.047) 0.909 (0.032) 0.976 (0.004) 0.984 (0.003) 0.984 (0.003) 0.984 (0.003) 0.953 (0.012) 0.862 (0.060) 0.867 (0.057) 0.803 (0.102) 0.773 (0.125) 0.773 (0.125) 0.662 (0.263)
a
Correlation (P value).
Table 5 Voluntary intake (DM) of tree fodders when offered simultaneously with Taiwan grass for a period of 6 h
L. latisiliquum G. ulmifolia P. piscipula L. leucocephala B. alicastrum S.E.D.
Tree (kg)
Tree (g/kgLW0.75 )
Grass (kg)
Grass (g/kgLW0.75 )
Grass + tree (kg)
Grass + tree (g/kgLW0.75 )
0.031a 1.037b 1.368c 1.732d 2.981e 0.04
4.08a 13.47b 17.74c 22.18d 38.56e 0.52
6.83a 6.90a 6.21b 6.80a 5.90c 0.06
89.64a 86.30b 79.41c 87.69ab 77.43c 0.83
6.86a 7.94b 7.58c 8.53d 8.88e 0.08
93.72a 99.77b 97.16b 109.87c 116.00d 0.92
n = 15 for each tree. Values with different superscripts (a, b, c, d, e) in columns differ (P<0.05).
4. Discussion The chemical composition, IVDMD, IVOMD, in situ DM degradability and IVGP profiles of the tree fodders were within ranges commonly reported for the region (Sandoval Castro et al., 2000a; Bobadilla Hernandez, 2001; Pinto Ruiz et al., 2002a,b) and elsewhere (Solorio Sanchez and Solorio Sanchez, 2001). This confirms that tree fodder
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leaves have low variation in chemical composition, which results in consistent nutritional values. Prediction of DMI of individual forages has been attempted with in vitro digestibility, IVGP and in situ degradation techniques, with relatively good results (Hovell et al., 1986; Bl¨ummel and Ørskov, 1993; Khazaal et al., 1993, 1995; Shem et al., 1995; Bl¨ummel and Bullerdieck, 1997; Fernandez-Rivera, 1998; Chenost et al., 2001; Mendoza-Nazar and Sandoval-Castro, 2003). Similarly, results here (Table 4) confirm that short term measurement is a valuable option for intake prediction (Bl¨ummel and Bullerdieck, 1997; FernandezRivera, 1998; Chenost et al., 2001). 4.1. Preference study 1 During the 6 h period, there was a marked preference towards B. alicastrum, the tree fodder with the highest digestibility both in vitro and in situ. The preference was not affected by CP, probably because N was not limiting in these heifers (Van Soest, 1994) as all forages had CP contents above 150 g/kg DM. It is accepted that plant morphology and structure can influence preference and intake of forages by ruminants (Ortega and Provenza, 1993; Burns et al., 2001). However, in the present study none of the tree fodders had thorns and their leaves were equally accessible, suggesting that these factors were not the cause of the difference which favoured B. alicastrum (Fig. 1). This was confirmed by Nieto Marin et al. (2001), who observed that intake rate of these tree fodders, when offered ad libitum as single feeds, was similar at about 20 g DM/min. The main cause of the differences in intake was the effective time the heifer spent eating each tree fodder. In contrast, Sosa Rubio et al. (2004) found, in studies with sheep, that difference in intake of 5 tree fodders was due to intake rate (from 4 to 34 g/min), although it is not clear if intake rate was estimated using effective intake time, as in Nieto Marin et al. (2001). Nevertheless, results suggest that heifers may detect, at least in short term intake, foliage characteristics related to feed quality before they are detected as post-ingestive feedback signals (Provenza, 1995). The correlation between preference and lignin indicates that it may be detected orally as an indication of a hard, more difficult to comminute, material. Thus, by selecting ‘soft’ forages, the animal is also selecting highly and rapidly digestible material. This hypothesis will also accommodate the correlation between preference and IVDMD, IVOMD, IVGP and the c fraction of the in situ profile. Thus, the degree of lignification might be related to the c fraction (r = −0.818, P=0.09), and the ‘c’ fraction is highly correlated with both IVDMD (r = 0.988, P=0.033) and IVOMD (r = 0.975, P=0.005), thereby acting as a feedback mechanism during chewing to signal the animal about potential physical ruminal load of the feed. Similarly, Chenost et al. (2001) suggested that prediction of intake of single forages should be related to the in situ c fraction of the feeds, because it reflects physical regulation of voluntary intake. 4.2. Preference study 2 A similar pattern of preference in DMI occurred, as in preference study 1, with the exception of P. piscipula intake, which was proportionately higher. This might indicate
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that when the possibility of the animal to select is reduced, it might increase intake of forage which was previously not well accepted. Similar results have been reported by Camacho Morfin (2003) with sheep using tree fodders from central Mexico. In contrast, L. latisiliquum, which was not well accepted in both studies, also had the lowest digestion rate and gas production. This might result from its TP and CT content, as it has been reported that its tannins possess high biological activity amongst tree fodders of the region (Armend´ariz and Ya˜nez, 1998). In this particular case, the biological activity of CT might be detected even during short periods of intake, allowing the animal to discriminate for feed with limited nutritional potential due to tannins (McSweeney et al., 2001, 2003). Beneficial effects on DMI of including a tree fodder on the diet (Umunna et al., 1995) was only apparent with B. alicastrum and L. leucocephala, and there was no clear relationship with any chemical feed component. Degradation profiles and CT contents indicate that ruminants might prefer a rapidly rumen degradable forage (as discussed above), with low TP and CT. Although in the case of B. alicastrum, the increased intake also resulted in reduction of basal grass intake, essentially a substitution effect. The correlation between in vitro digestibility and IVGP with in situ feed characteristics has been previously reported (i.e., Camacho Morfin, 2003; Fernandez-Rivera, 1998; Khazaal et al., 1993, 1995). As both in situ and IVGP are evaluating activity of rumen microbes, and the potential of the feed to supply energy and N, a strong relationship has always been expected. However, it is noteworthy that they also correlate with short term intake (i.e., preference), supporting the hypothesis suggested above that, while eating, the animal might be detecting orally (possibly by feed hardiness and resistance to comminution), the potential of any feed to supply nutrients, which could be a selection characteristic learned by association with post-ingestive feedback effects from previous feeds (Provenza, 1995). Secondary compounds, such as tannins, thus being a secondary signal to intake regulation during short term studies, as their post-ingestive effects need variable time to develop, require time to be sensed and associated with the feeds by the animal. As shown previously (i.e., Fernandez-Rivera, 1998; Chenost et al., 2001), both IVGP and in situ time measurement were correlated with IVDMD and IVOMD from 12 h onwards in the in situ degradation measurements and from 3 to 72 h for IVGP measurements. However, as they also appear correlated with preference, it suggests that short term incubation studies might be a useful tool to predict cattle forage preference.
5. Conclusions Cattle are able to differentially select among tree fodder species if given the opportunity. The selection mechanism seems to be independent of post-ingestive feedback signals, suggesting that this ability must be taken into account when designing feeding strategies which include mixtures of tree fodders. However, grass was preferred over tree fodder when heifers were given the opportunity to select, another behavior to be taken into account in the design of feeding strategies that include tree fodders. Assessment of tree fodder preference can be done by employing in situ degradation and IVGP techniques, which would be complimentary to traditional chemical measurements.
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Acknowledgement This work was partially financed by IFS-Sweden. Grant B/2351-2.
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