Diet selection by growing lambs offered whole barley and a protein supplement, free choice: Effects on performance and digestion

Livestock Science 101 (2006) 81 – 93 www.elsevier.com/locate/livsci

Diet selection by growing lambs offered whole barley and a protein supplement, free choice: Effects on performance and digestion A.R. Askar a,1, J.A. Guada a,*, J.M. Gonza´lez b, A. de Vega a, C. Castrillo a a

Departamento de Produccio´n Animal y Ciencia de los Alimentos, Universidad de Zaragoza. Miguel Servet 177, 50013 Zaragoza, Spain b Gabinete Te´cnico Veterinario S.L., Isla de Conejera s/n, 50014 Zaragoza, Spain Received 3 March 2005; received in revised form 13 September 2005; accepted 23 September 2005

Abstract Feeding whole barley and a protein supplement at choice to early weaned lambs (15 kg initial weight) was studied in four experiments. In a first experiment whole barley and a pelleted soybean meal-based supplement (348 g crude protein (CP)/kg dry matter (DM)) were given either at choice (CH) or mixed in a 75 / 25 ratio (M) and compared to a pelleted compound feed containing the same ingredients and supplemented with barley straw (C). The effect of allowing free access to straw in the CH treatment (CHS) was tested in a second experiment. Rumen fermentation was also studied in six lambs per treatment slaughtered after the growing period, and digestibility and allantoin excretion were determined in other four lambs per treatment. In Experiments 3 and 4 supplements containing 253–264 (L), 341–329 (M) and 424–427 (H) g CP/kg DM were offered at choice with whole barley to lambs penned individually (Experiment 3) or in a group (Experiment 4) and diet selection and lamb performance were recorded. The protein supplement was selected in proportions of 0.28 and 0.34 (S.E. 0.021) of concentrate intake with treatments M and CH (Experiment 1) and 0.45 and 0.52 (S.E. 0.033) with treatments CH and CHS (Experiment 2). The concentration of rumen ammonia increased ( P b 0.05) to 28 and 56 (S.E. 8.6) mg/l compared to 12 mg/l in treatment C. Rumen pH also increased ( P b 0.001) from 4.95 to 5.48 and 5.60 (S.E. 0.080) with treatments CH and CHS while the concentration of volatile fatty acids and the acetate to propionate ratio tended to decrease ( P b 0.1). Organic matter digestibility and allantoin excretion were higher with whole barley feeding (0.82 vs. 0.75 (S.E. 0.18) and 13.1 vs. 7.7 (S.E. 1.06) mmol/d) but decreased with straw supplementation (0.78 and 10.2 mmol/d). Daily weight gain of male lambs (but not of females) tended ( P = 0.08) to be higher with whole barley than with the compound feed due to higher concentrate intake ( P b 0.05), which was also depressed by straw supplementation of whole barley reducing the rate of gain ( P = 0.06). Incidence of hyperparakeratosis and ulceration of rumen wall was high with all treatments (0.80 and 0.36) but papillary clogging was less frequent ( P b 0.01) with whole barley (0.16) than with the compound feed (0.70) and tended ( P = 0.01) to be reduced by straw supplementation. In Experiments 3 and 4 the proportion of protein supplements selected ranked in reverse order

* Corresponding author. Tel.: +34 976761659; fax: +34 976761612. E-mail address: [email protected] (J.A. Guada). 1 Present address: Animal and Poultry Nutrition Department, Desert Research Center, El-Matareya, Cairo, Egypt. 0301-6226/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.livprodsci.2005.09.012

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to their CP content, resulting in diets of similar CP concentration (180–200 g/kg DM). It is concluded that choice feeding of whole barley may be a practical alternative that offers some advantages over conventional compound feeding of growing lambs. D 2005 Elsevier B.V. All rights reserved. Keywords: Whole barley; Protein supplement; Choice feeding; Diet selection; Lambs

1. Introduction Whole barley grain has been successfully used in fattening lambs when given in a loose mixture with a pelleted supplement providing protein, minerals and vitamins (Fraser and Ørskov, 1974; Economides et al., 1990). Apart from reducing the cost of feed manufacturing, the slower rate of fermentation of unprocessed grain in the rumen (Koenig et al., 2003) and the increase in rumination time avoid acidosis, fat softening (Ørskov et al., 1974a; Cazes et al., 1990) and rumenitis (Ørskov, 1973) without negative effects on lamb performance (Economides et al., 1990; Hadjipanayiotou, 1990). This effect could avoid the need for supplementary roughage to prevent digestive disorders, resulting in higher energy diets and improved lamb performance and feed conversion ratio (Castrillo et al., 1989). Furthermore a healthy rumen is of great relevance in female lambs for replacement in view of the long-term detrimental effect of acidosis on acetate absorption (Krehbiel et al., 1995). However, the selection of ingredients observed in individual feeding trials (Ørskov et al., 1974b; Castrillo et al., 1989) and labour requirements involved in handling feed mixtures have constrained the widespread use of this feeding system in intensive programs. Growing sheep are able to select a balanced diet when feeds varying in energy (Cooper and Kyriazakis, 1993) and protein content (Kyriazakis and Oldham, 1993) are offered at choice. Also, choice feeding of calves and goats at different physiological stages has resulted in the selection of diets according to energy and protein requirements (Atwood et al., 2001; Fedele et al., 2002). Therefore, if young lambs are able to select a diet of an adequate composition then the unrestricted feeding of whole barley grain and a protein supplement at choice would be feasible, simplifying feeding management and allowing the match between intake of nutrients and growth potential.

This paper reports the results of four feeding trials, the aim of which was to study diet selection when whole barley and a protein supplement were offered at choice to young lambs, and its effects on animal performance, rumen fermentation, digestibility and allantoin excretion as an index of microbial protein synthesis.

2. Material and methods 2.1. Experiment 1 2.1.1. Animals Fifty-four Rasa Aragonesa weaned lambs were blocked by weight and sex and randomly distributed in penned groups of 6 animals (3 males and 3 females) to one of three dietary treatments according to a complete randomised block design with three replicates (pens) by treatment. 2.1.2. Treatments A protein supplement containing, on a fresh basis, 0.70 soybean meal, 0.10 corn meal, 0.02 fat-enriched (0.6) whey powder, 0.04 sodium bicarbonate, 0.04 feed binder and 0.10 of mineral mix containing 0.6 calcium carbonate, 0.2 salt and 0.2 of a micro mineral and vitamin premix, was offered either as pellet (30 mm diameter), together with whole barley grain for choice feeding, or incorporated with barley meal into a compound pelleted concentrate (C) in a proportion of 25 : 75. The whole grain and the protein supplement were offered either at choice in separate troughs (CH) or mixed in the ratio of 75 : 25 (M). Free access to barley straw was allowed to animals in treatment C. All diets were offered ad libitum in initial amounts of 104 g dry matter (DM)/kg LW0.75 (1.3 times the estimated intake suggested by INRA (1978) for 15 kg lambs). Actual feed offered was adjusted at each control period

A.R. Askar et al. / Livestock Science 101 (2006) 81–93 Table 1 Chemical composition of ingredients and diets

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criteria of feed allowance described in Experiment 1 and their composition is shown in Table 1.

DM

OM

CP

NDF

ADF

Experiment 1 Barley grain Supplement Compound feed Barley straw

91.2 92.4 92.0 84.5

97.7 83.2 92.9 94.0

11.5 34.8 17.4 4.4

22.4 18.3 17.5 81.6

5.6 8.8 6.9 49.7

Experiment 2 Barley grain Supplement Compound feed Barley straw

91.7 90.6 90.3 91.2

97.8 83.7 94.8 93.3

12.0 34.5 17.8 4.6

20.2 17.5 18.4 79.0

5.9 8.0 6.3 51.5

Experiment 3 Barley grain Supplement H Supplement M Supplement L

89.9 90.9 91.6 91.2

97.6 84.6 85.9 91.3

12.5 42.4 34.1 25.3

20.7 15.2 14.5 15.6

6.0 8.5 7.7 7.0

Experiment 4 Barley grain Supplement H Supplement M Supplement L

91.3 92.2 91.9 92.1

97.3 84.8 85.7 92.8

10.2 42.7 32.9 26.4

19.3 16.4 15.5 18.6

6.2 10.6 8.3 7.9

2.3. Experiment 3 2.3.1. Animals In this experiment, 18 male lambs of the same breed and age as those in Experiment 1 were individually penned and randomly distributed to three experimental treatments. 2.3.2. Treatments A protein supplement containing 0.86 of soybean meal, 0.12 of the same mineral mix used in Experiment 1 and 0.02 of feed binder was given either alone or diluted with 0.25 or 0.50 of barley meal in order to achieve CP concentrations of 420 (H), 350 (M) and 270 (L) g/kg DM. The three supplements were pelleted and offered at choice with whole barley grain without access to supplementary roughage. The chemical composition of supplements and barley grain is shown in Table 1. 2.4. Experiment 4

according to intake recorded during the previous period to allow for 0.3 of refusals. Chemical composition of barley grain and straw, protein supplement and the compound feed is shown in Table 1. 2.2. Experiment 2 2.2.1. Animals Seventy-two lambs of the same breed and age as those used in Experiment 1 were blocked by weight and sex. Lambs from each block were randomly allocated by sex to one of three treatments according to a complete randomised block design with two replicates (pens) of six lambs by sex and treatment. 2.2.2. Treatments A pelleted protein supplement and whole barley grain of similar composition to those used in Experiment 1 were offered either at choice, with (CHS) or without free access to barley straw (CH), or incorporated together with barley meal into a compound pelleted feed in a proportion of 25 : 75 (C). Free access to barley straw was also allowed in this latter treatment. Diets were offered ad libitum following the

2.4.1. Animals Sixty lambs of the same breed and age as those in previous experiments, half males and half females, were blocked by weight and sex and randomly allocated by sex to one of three experimental treatments, according to a complete randomised block design with 2 replicates (pens) of five lambs by sex and treatment. 2.4.2. Treatments Three pelleted protein supplements formulated with the same ingredients as those used in Experiment 3, except the feed binder that was not included, were each offered at choice with whole barley grain without access to supplementary roughage. Chemical composition of barley grain and supplements is shown in Table 1. 2.5. Experimental procedures In all experiments, lambs were weaned at five weeks of age and immediately allocated to the experimental treatments, allowing a week for adaptation to diets

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before starting the experimental records, except in Experiment 3 that started one week later to allow for a longer adaptation of the individually penned lambs. Lambs were removed for slaughtering when females and males reached 23 and 24 kg live weight, respectively, or in any case after eight weeks of fattening. In all experiments, lambs were group-fed in slatted-floor pens of 2.6  1.3 m, except in Experiment 3 in which they were individually penned (1.3  1.3 m). Free access to water was ensured in all cases and animals were handled according to criteria for care and use of laboratory animals in research, being the experimental protocol approved by the Ethical Committee for Animal Research of Zaragoza University. Feed offered and refusals were recorded twice a week (Tuesday and Friday) and sampled to determine their DM concentration by drying at 65 8C during 48 h. A subsample was also preserved and pooled across weeks for further analysis. The proportion of grain and protein supplement in the refusal from mixed diets (treatment M, Experiment 1) was calculated from the crude protein concentration of refusals compared to that of the grain and the protein supplement. In all experiments, lambs were weighed twice a week in the morning before feeding. In Experiments 1 and 2 the reticulo-rumen was recovered from the slaughter house and lesions of rumen epithelium were evaluated by visual inspection. Also in Experiment 2 the intestinal tract of six male lambs per treatment was sectioned and the rumen, caecum and rectum contents were recorded and sampled. The pH of rumen fluid and caecum contents was also recorded and two aliquots of 10 and 4 ml of rumen fluid were preserved frozen for NH3 and volatile fatty acid analysis (VFA) following addition of 10 ml of HCl 0.1N and 1 ml of 0.2% (w / v) mercuric chloride and 2% (v / v) orthophosphoric acid solution, respectively. After 24 h of chilling, the carcass weight was also recorded. Five days before slaughtering these animals were moved into individual pens and fed at a restricted level close to the average treatment total intake recorded in the previous control, to avoid large fluctuations. During this period the protein supplement was also fixed to 0.30 of total feed offered. Another four male lambs from each treatment were moved into metabolic crates at the end of the fattening period (24 kg live weight) and faeces and urine collected for 7 days after a 10-day period of

adaptation to the cages. Feed was offered daily after previous-day orts removal, and whole barley grain and protein supplement were administered in the same proportions as in treatment C (75 : 25). Intake was restricted in order to maintain this proportion constant. Faeces were collected daily and two aliquots (0.1) taken from each animal. One was dried at 65 8C for DM analysis and the other stored at  18 8C to obtain a composite sample per animal that was later freezedried and milled through a 1 mm screen for further analyses. Urine was collected daily in 200 ml of sulphuric acid solution (0.1 v / v) and after recording weight and specific gravity a proportionate aliquot (0.1) was stored at  18 8C and pooled on an individual animal basis. A ground sample (1 mm screen) of the pelleted concentrate was labelled with Yb acetate (7 mg Yb/g DM) by immersion in acetate buffer (acetic acid 0.1 M with pH adjusted to 6 with ammonium hydroxide) following the procedures described by de Vega and Poppi (1997) and 8 g DM of the labelled material was dosed to each lamb before morning feeding on the second day of the digestibility balance. Faecal samples were subsequently collected at 6, 9, 12, 21, 30, 39 48, 60, 72, 96 and 120 h to calculate the slow fractional outflow rate of labelled particles (k 1) according to the procedures described by Grovum and Williams (1973), stored at  18 8C and then freeze-dried for Yb analysis. 2.6. Analytical procedures Dry matter concentration of feed and faeces samples was determined by oven drying at 105 8C for 24 h, organic matter (OM) by ashing at 550 8C for 8 h, CP by the Kjeldahl method using Se as catalyst and ash-free neutral detergent fibre (NDF) and acid detergent fibre (ADF) as described by Van Soest et al. (1991). Rumen fluid VFA were determined by gas chromatography (Jouany, 1982) and NH3 concentration by the colorimetric method described by Chaney and Marbach (1962). Concentration of Yb in faeces was determined by atomic absorption spectrometry after digestion of 0.2 g DM in 15 ml of a 5 : 1 mixture of nitric (0.6 v / v) and perchloric (0.7 v / v) acids according to de Vega and Poppi (1997). Purine derivatives (PD) and creatinine in urine were analysed by high-performance liquid chroma-

A.R. Askar et al. / Livestock Science 101 (2006) 81–93

tography as described by Balcells et al. (1992) and purine bases content of rectal faeces according to Balcells et al. (1991) with the modification proposed by Martı´n-Oru´e et al. (1995).

Table 2 Animal performance and feed intake of lambs consuming whole barley and a pelleted protein supplement at choice (CH), mixed in 75 : 25 proportions (M) or incorporated into a pelleted compound feed with free acess to straw (C) in Experiment 1 Treatment

2.7. Statistical analysis Data on intake and performance from Experiments 1, 2 and 4 were subjected to analysis of variance using a randomised block design, with the penned group as the experimental unit, and treatment means were compared by the least significant difference. In Experiment 2 the treatment sum of squares was partitioned into two orthogonal contrasts: whole vs. ground barley (CH and CHS vs. C) and whole barley vs. whole barley plus straw (CH vs. CHS). Data from Experiment 3 and digestibility, purine derivatives and rumen metabolism data from Experiment 2 were analysed as a completely randomised design and discrete data from visual inspection of rumen epithelium by the Chi square test. All analyses were made with the help of the SAS statistical package (version 8.01).

3. Results 3.1. Experiment 1 One male lamb that died from pasteurellosis in treatment M was not considered in the analysis and one female lamb with an injured leg in treatment CH had to be removed from the experiment at 21.6 kg live weight but its data were not excluded from the analysis because its live weight at removal was close to the set final weight. Average DM intake and animal performance are presented in Table 2. Daily weight gain did not differ significantly between treatments although it was slightly higher in M and CH than in C. This difference rose to 20% in male lambs, with treatment effect reaching a statistical trend ( P = 0.08) when the average pen daily gain was considered independently for each sex. No differences were recorded for females. Total DM intake was similar in all treatments averaging 811 (S.E. 5.2) g/d. Straw consumption amounted to 78 (S.E. 6.6) g/kg of total DM intake in treatment C, depressing, in a similar proportion, the

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M Live weight (kg) Initial Final Weight daily gain (g/d) Average Males Females Dry matter intake (g/d) Barley straw Concentrate Protein supplement / concentrate Concentrate feed / gain Days of fattening

S.E.M. Significance

CH

14.9 24.1

311 342 281

15.1 23.9

314 342 285

C 15.1 23.9

280 286 273

– – 64 805 809 753 0.28 0.34 – 2.60 30.5

2.58 29.5

2.70 31.6

0.19 0.07

11.3 13.9 15.7

Ns Ns

Ns P = 0.08 Ns

4.3 Ns 8.5 * 0.021 Ns 0.091 Ns 1.61

Ns

*P b 0.05; Ns: not significant.

intake of concentrate, that was lower ( P b 0.05) in this treatment. The conversion ratio of concentrate feed to gain was also similar for all treatments and the proportion of protein supplement selected amounted to 0.28 and 0.34 (S.E. 0.021) in treatments M and CH. Visual inspection of the rumen epithelium revealed that 0.80 and 0.36 of lambs developed a considerable degree of hyperparakeratosis and ulceration of rumen wall, respectively, without differences being found between whole and ground barley diets, although papillary clogging was less frequent ( P b 0.001) with whole barley (0.16) than the compound feed (0.70). 3.2. Experiment 2 3.2.1. Lamb performance During this experiment, one female lamb died from pasteurellosis in treatment C and another female in the same treatment plus two male lambs in treatment CHS had to be removed due to leg lesions and their growth data were excluded from the analysis. Average feed intake and animal performance are given in Table 3. Daily weight gain was higher ( P b 0.01) in males than females (333 vs. 290 (S.E.

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Table 3 Animal performance and feed intake of lambs receiving whole barley and a protein supplement at choice (CH), the same ingredients plus free access to barley straw (CHS) or incorporated into a pelleted compound feed with free acess to straw (C) in Experiment 2 Sex

Live weight (kg) Initial Final Weight gain (g/d) Dry matter intake (g/d) Barley straw Concentrate Protein supplement / concentrate Concentrate feed /gain Days of fattening

Treatments

S.E.

CH

CHS

C

M F M F M F

13.4 13.6 24.3 23.2 357 292

13.5 13.6 23.7 23.4 324 283

13.4 13.5 24.4 23.1 318 294

M F M F M F M F M F

– – 780 768 0.45 0.42 2.20 2.63 32.4 34.9

32 33 731 800 0.52 0.52 2.26 2.82 33.4 35.8

55 45 776 729 – – 2.45 2.49 35.5 34.4

Significance Sex

C1

C2

0.21

Ns

0.29

*

Ns Ns Ns Ns Ns Ns

Ns Ns Ns Ns P = 0.09 Ns

**. ** Ns * – – P = 0.06 * Ns Ns

– – * Ns Ns Ns Ns Ns Ns Ns

11.4

**.

2.6

Ns

13.2

Ns

0.033

Ns

0.072

**

1.78

Ns

C1 = C vs. CH and CHS (except for straw intake, where C1 = C vs. CHS); C2 = CH vs. CHS; *P b 0.05; **P b 0.01; Ns: not significant.

6.6) g/d) with concentrate feed / gain ratios of 2.30 and 2.65 (S.E. 0.042; P b 0.01). Although weight gain was not significantly affected by grain processing or straw supplementation when all animals were considered together, it tended ( P = 0.09) to decrease with straw supplementation of whole barley in male lambs (357 vs. 324 (S.E. 11.4 g/d) due to a reduction of 49 g/d in the concentrate intake (C2: P b 0.05). Straw intake increased with grain processing (C1: P b 0.01) which, in females, depressed the consumption of compound feed (C1: P b 0.05) resulting in a lower conversion ratio of the concentrate feed (C1: P b 0.05). The concentrate conversion ratio in male lambs, however, tended to be higher with treatment C (C1: P = 0.06) reflecting differences in weight gain and intake with treatments CH and CHS. The amount of protein supplement selected, when given at choice with whole grain, accounted for 0.44 and 0.52 (S.E. 0.023) of the concentrate intake in treatments CH and CHS ( P = 0.08) regardless of sex, increasing the CP concentration of the diets from 170 g/kg DM for the compound feed to 216 and 227 (S.E. 3.9) g/kg DM in treatments CH and CHS. The empty weight of the reticulo-rumen averaged 25.3 (S.E. 1.08) g/kg empty body weight and did not

differ between treatments but its wet content was higher ( P b 0.01) in lambs fed whole grain than in those consuming the compound feed (106 vs. 81 (S.E.D. 6.4) g/kg live weight) regardless of straw supplementation. This difference was reflected in the killing out percentage, that averaged 47.6%, 46.5% and 45.4% (S.E. 0.48) in treatments C, CH and CHS, respectively. The empty weight of the caecum and its wet content were however higher ( P b 0.05) in lambs fed on the compound feed than in those that received whole barley, without any significant effect of straw supplementation: 2.62 vs. 1.82 (S.E.D. 0.289) g/kg empty body weight and 7.62 vs. 5.37 (S.E.D. 0.774)) g/kg live weight. The pH of caecal content was similar in treatments CH and C averaging 6.05 (S.E. 0.137) but decreased non-significantly to 5.79 when the whole barley was supplemented with straw, while the faecal concentration of purine bases increased ( P b 0.05) from 31 to 40 and 54 (S.E. 4.9) mmol/kg DM in treatments C, CH and CHS, respectively. Examination of rumen lesions confirmed the results of Experiment 1. Extensive hyperparakeratosis and ulceration of rumen wall was recorded in 0.73 and 0.35 of lambs regardless of dietary treatment, but the proportion of papillary clogging was lower ( P b 0.01)

A.R. Askar et al. / Livestock Science 101 (2006) 81–93

in lambs fed whole barley (0.21) than in those fed the compound feed (0.50) and tended to decrease ( P = 0.10) with straw supplementation of whole barley (0.05). 3.2.2. Rumen fermentation Total DM intake during the five days previous to slaughtering averaged 978 (S.E. 52.7) g/d and did not differ between treatments. However straw intake tended ( P = 0.06) to decrease with whole barley feeding compared to the compound feed (19 vs. 40 (S.E. 7.1) g/d), reducing ( P b 0.05) the proportion of straw in the diet from 0.041 to 0.019 (S.E. 0.0065). The contribution of supplement consumed to concentrate DM intake averaged 0.28 and 0.31 (S.E. 0.015) in treatments CH and CHS, increasing ( P b 0.05) the CP content of diets from 173 g/kg in treatment C to 181 and 186 (S.E. 2.7) g/kg in treatments CH and CHS. In spite of the high dietary CP content rumen ammonia concentration was low in all treatments, as shown in Table 4, although it increased significantly in lambs fed whole barley (C1: P b 0.05) particularly when it was supplemented with straw (C2: P b 0.05). Rumen fluid pH was below 6 in all treatments but increased significantly ( P b 0.001) when lambs consumed whole barley grain compared to the compound feed while total VFA tended to be higher ( P = 0.08) with this latter treatment. Molar proportions of acetate and butyrate did not differ between treatments but the Table 4 Rumen fermentation characteristics of lambs receiving whole barley and a protein supplement at choice (CH), the same ingredients plus access to barley straw (CHS) or incorporated into a pelleted compound feed with free acess to straw (C) in Experiment 2 Treatment CH pH NH3-N (mg/l) Total VFA (mM/l) VFA (mol/100 mol) Acetate Propionate Butyrate Valerate Isobutyrate Isovalerate Acetate : propionate ratio

S.E.M. Significance

CHS C

C1

C2

5.48 5.60 4.95 0.08 27.8 56.7 12.0 8.64 82 85 103 8.4

*** Ns * * P = 0.08 Ns

46.6 40.1 8.39 2.41 1.30 1.20 1.18

Ns Ns Ns Ns ** ** P = 0.08

49.7 51.7 2.74 34.1 31.7 3.04 10.8 12.0 1.52 2.86 3.14 0.340 1.31 0.80 0.121 1.19 0.67 0.130 1.48 1.97 0.278

Ns Ns Ns Ns Ns Ns Ns

C1 = C vs. CH and CHS; C2 = CH vs. CHS; *P b 0.05; **P b 0.01; ***P b 0.001, Ns: not significant.

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acetate propionate ratio tended to be lower ( P = 0.08) in lambs fed whole barley than in those consuming the compound feed. Also higher ( P b 0.01) molar proportions of branched-chain VFA (isobutyrate and isovalerate) were observed in animals fed whole barley. Straw supplementation had no effect on rumen pH and VFA concentrations or their molar proportions. 3.2.3. Diet digestibility and urinary excretion of purine derivatives During the digestibility trial total DM intake averaged 690, 638 and 638 (S.E. 26.1) g/d in treatments CH, CHS and C but did not differ significantly. Due to the contribution of straw to total DM intake (0.07 S.E. 0.008) that of concentrate was higher ( P b 0.05) in treatment CH (690 g/d) than in treatments CHS and C (596 and 593 (S.E. 24.8) g/d). The proportion of protein supplement consumed in both choice feeding treatments amounted to 0.25 (S.E. 0.001) of concentrate intake giving, as a result, dietary CP contents of 174 and 166 (S.E. 0.9) g/kg DM in treatments CH, CHS ( P b 0.001). The last value being similar to treatment C (169 g/kg DM) which also included straw. Mean digestibility coefficients and urinary excretion of creatinine and purine derivatives are shown in Table 5. Apparent digestibility of OM tended to be lower in treatment C compared with CH and CHS (C1: P = 0.06), which was mainly due to straw intake since supplementation of whole grain with straw also reduced digestibility, although not significantly. An accumulative effect of both factors, straw intake and grain processing, is suggested by the NDF digestibility as it was additively reduced by 9 and 13 per cent units by straw supplementation and processing of barley grain, respectively. Neither the form of administration of barley grain nor its supplementation with straw had a significant effect on k 1, which averaged 5.78% h 1 (S.E. 0.296). The urinary excretion of allantoin was higher in lambs fed whole barley than in those consuming the compound feed (C1: P b 0.01) but straw supplementation of whole grain tended to reduce it by 22% ( P = 0.09). This response was reflected in the allantoin to creatinine ratio, since the excretion of creatinine was constant, averaging 0.42 (S.E. 0.052) mmol/kg LW0.75. The excretion of total purine derivatives also followed the same pattern, although it was partially

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Table 5 Digestibility coefficients of dry matter (DMD), organic matter (OMD) and neutral detergent fibre (NDFD), intake of digestible organic matter (DOMI), particles fractional outflow rate (k 1) and urinary excretion of purine derivatives by lambs receiving whole barley and a protein supplement at choice (CH), the same ingredients plus free access to straw (CHS) or incorporated into a pelleted compound feed with free access to straw (C) in Experiment 2 Treatment

DMD OMD NDFD DOMI k 1 (%/h) Purine derivatives (PD; mmol/d) Allantoin Total Creatinine (mmol/d) Alantoin / creatinine Allantoin/DOMI (mmol/kg) Total PD/DOMI (mmol/kg)

S.E.M.

Significance C1

C2

0.74 0.75 0.39 455 5.43

0.018 0.018 0.041 21.0 0.513

Ns P = 0.06 ** Ns Ns

Ns Ns Ns P = 0.06 Ns

7.68 9.96 4.42 1.76 16.7 21.7

1.06 1.40 0.53 0.143 2.11 2.78

** * Ns *** * Ns

P = 0.09 Ns Ns P = 0.06 Ns Ns

CH

CHS

C

0.80 0.82 0.61 533 6.57

0.76 0.78 0.52 468 5.34

13.1 14.9 4.31 3.03 24.6 28.0

10.2 12.5 3.98 2.60 22.0 26.7

C1 = C vs. CH and CHS; C2 = CH vs. CHS; *P b 0.05; **P b 0.01; ***P b 0.001; Ns: not significant.

masked by the constant excretion of hypoxanthine, xanthine and uric acid (1.15 (S.E. 0.106), 0.25 (S.E. 0.081) and 0.71 (S.E. 0.083) mmol/d, respectively). When allantoin or total PD excretion were expressed in relation to digestible OM intake, the differences between treatments were lower, although there was still a significant increase in allantoin excretion when whole barley was fed ( P b 0.05), suggesting an improved efficiency of microbial synthesis.

supplement than in treatment H ( P b 0.05) was selected, resulting in similar concentrations of CP in the diet (190 and 205 (S.E. 4.0) g/kg DM), the proportion of supplement consumed with treatment L was similar to that of treatment M, decreasing the diet CP concentration to 162 g/kg DM. This response did not differ between males and females. The lower DM intake and protein concentration of diet selected with treatment L was reflected in lamb weight gain, decreasing in males by 55 ( P b 0.05) and

3.3. Experiments 3 and 4 As shown in Table 6, total DM intake was similar in all treatments from Experiment 3, but the proportions of grain and protein supplement selected varied, increasing the contribution of barley, since the CP content of supplement was raised. Currently selected diets were estimated to contain 187, 200 and 176 (S.E. 9.2) g CP/kg DM with treatments H, M and L, respectively. Daily weight was similar with treatments L and M but decreased ( P b 0.05) with treatment H. When similar supplements were given to group replicates of males and females (Experiment 4), there was no interaction between treatment and sex and therefore only main effect means are presented in Table 7. Total DM intake was similar for treatments H and M but in treatment L it was 9% lower ( P b 0.01). While in treatment M a higher proportion of protein

Table 6 Animal performance and feed intake of lambs given whole barley at choice with a pelleted supplement of high (H), medium (M) or low (L) protein concentration in Experiment 3 Treatment H

M

S.E.M. Significance L

Live weight (kg) Initial 17.7 17.9 17.1 0.67 Ns Final 24.3 24.5 24.6 0.25 Ns Weight gain (g/d) 286 362 341 18.5 * Dry matter intake (g/d) Total 888 885 862 41.7 Ns Barley grain 688 561 492 31.3 ** Protein supplement 200 324 370 52.2 P = 0.09 Protein supplement / 0.214 0.354 0.418 0.0482* DM intake Feed / gain 3.12 2.48 2.54 0.112 ** Fattening days 23.4 19.2 22.7 2.06 Ns *P b 0.05; **P b 0.01, Ns: not significant.

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Table 7 Animal performance and feed intake of lambs given whole barley at choice with a pelleted supplement of high (H), medium (M) or low (L) protein concentration in Experiment 4 Treatment

Live weight (kg) Initial Final Weight gain (g/d) Dry matter intake (g/d) Total Barley grain Protein supplement Protein supplement / DM intake Feed / gain Fattening days

S.E.M.

H

M

L

15.0 23.2 281

14.9 23.3 309

14.8 22.4 243

793 541 252 0.32 2.85 29.0

805 492 313 0.39 2.66 27.9

731 460 271 0.37 3.03 31.8

Sex

S.E.M.

Male

Female

0.17 0.34 9.9

14.8 23.6 310

15.0 22.3 246

12.2 16.1 13.0 0.017 0.110 0.76

793 503 290 0.37 2.59 29.2

759 492 267 0.35 3.10 30.0

Significance Treatment

Sex

0.14 0.28 8.1

Ns Ns **

Ns * ***

10.0 13.2 10.6 0.014 0.090 0.62

** * * * Ns *

* Ns Ns Ns ** Ns

*P b 0.05; **P b 0.01; ***P b 0.001; Ns:not significant.

88 g/d ( P b 0.01) compared to treatments H and M, respectively. A similar response, although 50% lower (22 and 44 g/d), was recorded in females in spite of their lower growth rate compared to males (246 vs. 310 (S.E. 8.1) g/d, P b 0.001).

4. Discussion 4.1. Barley processing The results from Experiments 1 and 2 confirmed that lambs are able to utilize whole barley grain efficiently without adverse effect on intake, daily weight gain and digestibility in agreement with previous findings (Fraser and Ørskov, 1974; Economides et al., 1990; Castrillo et al., 1989). As previously reported (Ørskov et al., 1974a; Castrillo et al., 1989) lambs consuming whole barley plus protein supplement at choice show higher rumen pH and digestibility, particularly of NDF, than those on the pelleted compound feed. This suggests an increased cellulolytic activity in the rumen in agreement with Mann and Ørskov (1975) who recorded a higher number of cellulolytic bacteria in lambs fed whole instead of pelleted barley. In all treatments, the rumen liquor pH was lower than the values recorded by Ørskov et al. (1974a) and Hadjipanayiotou (1990) by intragastric tubing of lambs also fed whole or pelleted barley diets. The prevalence of higher acidotic conditions in this experiment was

confirmed by the frequent rumen lesions that were only partially alleviated by whole barley feeding. Ørskov et al. (1974a) recorded a more pronounced effect of whole barley feeding on rumen lesions and also higher differences in the rumen fermentation pattern. The increase in rumen pH with whole barley feeding was, in our case, associated with a lower acetate to propionate ratio and a decreased concentration of total VFA (Table 4), probably as a result of differential rates of acetate and propionate absorption at the low pH promoted by the processed diet (McLeod et al., 1984) together with the detrimental effect of low pH on overall rate of VFA absorption mediated by the increased osmotic pressure (Tabaru et al., 1990). These results suggest that severe acidosis caused by the processed diet was alleviated but not avoided by whole barley feeding. The higher concentration of branched-chain VFA and ammonia-N in rumen fluid of lambs fed whole barley would be in part related to the higher protein content of the diets consumed: 181, 186 and 173 (S.E. 2.7) g/kg DM with treatments CH, CHS and C, respectively. However, it is difficult to explain the low levels of rumen ammonia recorded with diets of this protein content although low concentrations have been also observed in growing cattle fed concentrate diets with a CP concentration of 0.13–0.17 (Ludden and Cecava, 1995; Devant et al., 2001). Protein degradability has been reported to be negatively affected by low rumen pH in vitro (Erfle et al., 1982) and in vivo (Loerch et al., 1983) and Lana et al. (1998) found a

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decreased rate of deamination at low rumen pH. The concentration of rumen ammonia with the pelleted compound diet (Table 4) was lower than the minimum requirements for microbial synthesis (Satter and Slyter, 1974), a fact that seems to agree with the lower allantoin excretion per unit of digestible OM recorded with this treatment, suggesting the shortage of degradable nitrogen in conditions of high availability of fermentable energy. This could explain the selection pressure towards higher protein diets but it must be considered that dietary proportions of protein supplement were fixed in the balance trial and protein intake per unit of digestible OM was similar with all treatments (225, 227 and 237 (S.E. 4.9) g/kg in CH, CHS and C, respectively). Also allantoin excretion data must be interpreted with caution since it is known that purine bases are not completely degraded in the rumen (Pe´rez et al., 1996; Vicente et al., 2004) and differences in allantoin excretion might result from variations in the extent of rumen purine degradation associated with barley processing (Askar et al., 2005). Furthermore, rumen ammonia levels may be highly variable throughout the day when ingredients that differ in their protein content are given at choice, since ruminants do not seem to select a balanced diet in the short term of a meal (Yeates et al., 2002). Therefore data recorded by spot sampling of rumen fluid did not necessarily reflect average daily levels and it is known that short term imbalances in energy and protein supply seem to have little effect on digestion or N utilisation in longer term (Ludden et al., 2002). However, this reasoning does not apply to the pelleted compound feed that resulted in the lower rumen ammonia level. 4.2. Straw supplementation Results from Experiment 2 indicate that straw supplementation of whole barley reduced concentrate intake and diet digestibility, depressing the growth rate of lambs, but it did not reduce the prevalence of rumen lesions, although papillary clogging tended to decrease. Characteristics of rumen fermentation were not modified either except for a two-fold increase in ammonia concentration that cannot be explained in terms of higher protein intake, suggesting higher deamination probably related in part to the slight increase in rumen pH (Erfle et al., 1982; Lana et al., 1998). Moreover, a

shift of OM fermentation from rumen to the hindgut could explain the higher ammonia concentration in response to straw supplementation of whole barley. The higher concentration of purine bases in the rectal content and the slight decrease in caecal pH, which were significantly correlated (r = 0.74), would support an increased hindgut fermentation as result of straw supplementation although it was not reflected in a higher fractional outflow from the rumen probably due to differences in size between labelled particles (1 mm screen) and those in the rumen digesta pool of lambs fed whole grain. An increased escape of starch from the rumen by roughage supplementation of high-concentrate diets has been suggested by Ørskov et al. (1969) but the higher digestibility of the processed grain in both the rumen (Beauchemin et al., 2001) and the small intestine (Owens et al., 1986) would explain why this effect was not reflected in higher hindgut fermentation of the compound feed. Since no significant improvements in diet utilization or rumen health were detected with straw supplementation when whole barley was offered at choice, feeding systems based on whole grain may be simplified by omitting the roughage supplementation, confirming previous results obtained with similar diets (Castrillo et al., 1989). 4.3. Diet selection Choice feeding may constitute an attractive system to achieve optimal nutrient intake according to breed and sex differences in growth potential if diets are selected by lambs according to requirements, as suggested by the results reported by Kyriazakis and Oldham (1993) and Fedele et al. (2002). The higher response in daily gain of males compared with females to choice or mixed feeding in Experiment 1 (treatments CH and M) seems to confirm this fact, in agreement with a similar finding by Economides et al. (1990). In a cafeteria assay with three-month old male lambs, Sahin et al. (2003) found a decreased pattern of protein intake with age, also suggesting selective intake according to requirements. Experiments 3 and 4 show that lambs tended to select diets of similar protein concentration when supplements of different protein content were offered with the exception of supplement L in Experiment 4 (Tables 6 and 7). Intake of this supplement was

A.R. Askar et al. / Livestock Science 101 (2006) 81–93

91

Supplement / DM intake

Exp.3. 0.50 0.40 0.30 0.20 0.10 0.00 0

5

10

15

20

25

30

Days

Supplement / DM intake

Exp.4. 0.50 0.40 0.30 0.20 0.10 0.00 0

5

10

15

20

25

30

Days Fig. 1. Proportion of supplement selected in Experiments 3 and 4 by lambs given whole barley at choice with high (D), medium (o) and low (5) protein supplements. The standard error of means is also presented (?).

probably constrained by the lower hardness of the pellet as a result of its high content of barley and the absence of pellet binder in this experiment. This detrimental effect of grain dust has also been observed by Hadjipanayiotou (1990) in kids fed rolled barley pellets and it is clearly appreciated in Fig. 1 which illustrates the proportion of protein supplement selected throughout the fattening period in Experiments 3 and 4. However, it is noteworthy that male and female lambs from Experiments 2 and 4 selected similar proportions of dietary protein in spite of differences in growth rate and requirements. Similar results have been reported by Rodrı´guez et al. (2003) when whole barley and a soybean-based protein supplement were offered at choice to male and female Awasi lambs. In addition, in Experiments 3 and 4 preference for supplements, other than L in Experiment 4, reached stable proportions of diet selected after 10 days of adaptation and these proportions remained stable throughout the fattening period, in contrast with the declining pattern of protein intake recorded by Sahin

et al. (2003). A similar adaptation period to feeds of different protein content was also observed in sheep by Kyriazakis and Oldham (1993). This preference for high protein diets irrespective of sex and age suggest that selection responded to factors other than net protein requirements. It is possible that protein may have been selected as a buffer (Phy and Provenza, 1998) or as a source of degradable N as suggested by the low rumen ammonia-N concentrations recorded in Experiment 2. The involvement of degradable N supply in feed selection has been invoked by Tolkamp et al. (1998) to explain the high protein intake observed in choice feeding trials with pregnant sheep (Cooper et al., 1994) and lactating cows (Tolkamp et al., 1998; Lawson et al., 2000), although James et al. (2001) failed to show that food selection by sheep is regulated on the basis of rumen degradable N supply. Over consumption of protein in a choice between pairs of feeds of low and high protein content may also result from the rejection of the lower protein feed when it is associated with detrimental properties

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(Kyriazakis and Oldham, 1993) as may happen with barley grain in an attempt to avoid acidosis. In this respect it is interesting to note that availability of barley straw in Experiment 2 led to a reduction in the intake of grain increasing the relative proportion of protein supplement in the diet selected.

5. Conclusions The above results suggest that choice feeding of whole barley and a protein supplement may be a practical alternative to conventional feeding of a straw supplemented compound feed, removing the need of roughage supplementation and encouraging greater concentrate intake, which allows male lambs to express their growth potential. It also improves OM digestibility and increases rumen pH, reducing the incidence of papillary clogging. In these feeding conditions lambs tend to select supplements in proportions that are inversely related to their protein content although a concentration higher than 340 g CP/kg DM may have adverse effects on performance. Diets selected contained 180–200 g CP/kg DM when soybean meal was used as a source of supplemental protein, but the reason for this preference for high protein diets and whether it depends on protein degradability is still to be fully understood. Acknowledgements Thanks are due to the Spanish Agency for International Cooperation (AECI) that provided a grant for Dr. A. R. Askar and to the Ministry for Science and Technology for funding the present work (Project PTR1995-0415-OP). References Askar, A.R., Guada, J.A., Balcells, J., de Vega, A., Castrillo, C., 2005. Validation of use of purine bases as a microbial marker by 15N labelling in growing lambs given high-concentrate diets: effects of grain processing, animal age and sampling site. Anim. Sci. 81, 57 – 65. Atwood, S.B., Provenza, F.D., Wiedmeier, R.D., Banner, R.E., 2001. Influence of free-choice vs. mixed-ration diets on food intake and performance of fattening calves. J. Anim. Sci. 79, 3034 – 3040.

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