||Historically, veal calves were fed until slaughter weight with only milk replacer and, in absence of solid feed the physiological development of the forestomachs was limited. In 1997, a new EU legislation stipulated that a minimum amount of solid feed (fiber) has to be provided for the welfare of the calves (solid feed supply reduce abnormal oral behaviors in veal calves; Van Putten, 1982; Veissier et al., 1998); however, no specifications were made about the type and source of feed.Rumen development is triggered by the production of VFA resulting from fermentation ofOMin the rumen (Flatt et al., 1958). Butyrate, and to a lesser extent propionate stimulate the development of the rumen mucosa; mostly because of their use as energy sources by the rumen epithelium (Sander et al., 1959; Tamate et al., 1962). In rearing calves, information on rumen fermentation of different sources of dietary carbohydrates is relatively well documented (Davis and Drackley, 1998; Lesmeister and Heinrichs, 2004) but only a few experiments have been conducted in veal calves.Based on earlier research (Blokhuis et al., 2000) it was hypothesized that stimulating early rumen development in veal calves would be beneficial to their subsequent performance and health. Therefore in vivo experiments were designed to establish the effects of stimulating an early rumen development in veal calves, aiming to optimize nutrient utilization from rumen fermentation and to prevent health problems in the lower gastrointestinal tract (e.g. ulcers in abomasum). In addition the potential interactions of feeding solid feed with a milk replacer based diet were investigated. Finally but not least, the development and evaluation (comparison) of techniques for estimating fermentation characteristics of different substrates, to facilitate the choice of feed ingredient for veal calves diets was also carried out. Chapter 2:This study was conducted to evaluate the effects of concentrate intake, differing in carbohydrates composition in addition to a milk replacer, on growth performance and rumen fermentation characteristics in veal calves. Accordingly, 160 Holstein Friesian x Dutch Friesian male calves, were fed with one of the following treatments: 1) milk replacer control (CONTROL), 2) pectin-based concentrate (PECTIN), 3) neutral detergent fiber (NDF) based concentrate, 4) starch-based concentrate (STARCH) and 5) mixed concentrate (MIXED) (equal amounts of concentrates of treatments 2, 3, and 4). Concentrate diets were provided in addition to a commercial milk replacer Results: Calves showed an ADG varying between 0.70 and 0.78 kg/d, with a rumen fermentation in concentrates fed calves characterized by a low pH (4.9 - 5.2), relatively low VFA concentrations between 100 and 121 mmol/L and high concentrations of reducing sugars (33-66 g/kg DM). Calves fed the CONTROL diet had higher lactate concentration (21mmol/L) than concentrate fed calves (between 5 and 11 mmol/L). Results indicated that the carbohydrate source can influence intake, growth rate and rumen fermentation in young veal calves.Chapter 3:This experiment aimed to gain an insight into the effects of age (calves were euthanized either at the end of 8 or 12 weeks of age) and concentrate supplementation, differing in carbohydrates composition, on rumen development in young veal calves. Moreover, some selected plasma metabolites as predictors of rumen development were evaluated. Diets treatments correspond to those described in Chapter 2. Results: Feeding concentrates differing in carbohydrate composition to veal calves promoted rumen development compared with calves fed milk replacer only. In most calves, a poorly developed rumen mucosa was observed. Coalescing rumen papillae with embedded hair, feed particles and cell debris were found in all calves fed concentrate diets. Calves fed concentrates had significantly heavier rumens than calves fed CONTROL. Although the variation in carbohydrate composition caused variation in rumen development, the latter was generally small. In the dorsal location of the rumen, calves fed concentrate diets showed an increased ratio of mucosa to serosa length (RMSL) than calves fed CONTROL. Mucosa thickness (MCT) and muscle thickness (MST) were bigger in the ventral and in the dorsal locations of the rumen, respectively.At 8 weeks, calves fed concentrate diets had higher plasma acetate concentrations than calves on the CONTROL treatment. However, at 12 weeks, only NDF fed calves showed significantly higher plasma acetate concentrations. For plasma BHBA concentration no differences were observed among treatments at 12 weeks. Results from a principal component analysis indicated that veal calves, in addition to rumen volatile fatty acids (VFA) concentrations, other factors are likely to affect rumen development. Chapter 4:This experiment was designed based on the results obtained in experiment 1 (Chapter 2 and 3) where in concentrate-fed veal calves a rumen environment, characterized by a sub clinical acidosis (pH< 5.2), relatively low VFA concentrations (100-120 mmol/L) and a rumen mucosa characterized by poorly shaped papillae with feed and cell debris embedded between them (referred as plaque formation) were observed. Feeding only roughage to young calves generally does not promote rapid papillae development (Nocek and Kesler, 1980); however, roughage consumption and its inherent coarseness stimulate the development of the rumen wall (Tamate et al., 1962) and rumination (Hodgson, 1971) and the healthiness of the rumen mucosa (Haskins et al., 1969). Information concerning the effects of roughage intake on veal calves performance and rumen development is scarce (Blokhuis et al., 2000; Cozzi et al., 2002). Therefore, it was hypothesized that adding small amount of roughage to a concentrate diet will improve and stimulates the development of the rumen wall, without having negative effects on calf's performance. Sixty four male Holstein Friesian x Dutch Friesian veal calves (46 kg ± 3.0 kg), were fed on of the following diets: 1) C100= concentrate only, 2) C70-S30= concentrate (70%) with straw (30%), 3) C70-G30= concentrate (70%) with dried grass (30%), 4) C70-G15-S15= concentrate (70%) with dried grass (15%) and straw (15%), 5) C70-CS30= concentrate (70%) with corn silage (30%), 6) C40-CS60= concentrate (40%) with corn silage (60%), 7) C70-CS30-AL= concentrate (70%) with corn silage (30%) ad libitum, 8) C70-G15-S15-AL= concentrate (70%) with dried grass (15%) and straw (15%) ad libitum. All dietary treatments were provided in addition to a commercial milk replacer. Concentrate was provided as pellets and roughage was chopped. Results: Roughage and intake level affects rumen fermentation and rumen development of veal calves. Substitution of part of the concentrate by roughage did not affect DMI and ADG, but among roughage sources feeding straw reduced DMI and ADG. The addition of roughage did not affect rumen pH (pH >5.3). Rumen fermentation was characterized by high total volatile fatty acids (VFA) and reducing sugars (RS) concentrations. Cobalt recovery, as an indication of milk leakage was found in the rumen, varying between 20.5 and 34.9 %, but it was not affected by dietary treatments. Roughage addition decreased the incidence of plaque formation and the incidence of calves with poorly developed rumen mucosa. However, morphometric parameters of the rumen wall were hardly influenced by the type and level of roughage. Results indicated that in veal calves, the addition of roughage to concentrate diets did not affect growth performance and positively influenced the macroscopic appearance of the rumen wall.Chapter 5:Several methodologies have been developed to characterize feedstuffs in terms of digestibility and degradability, comprising in vivo , in situ and in vitro methods.The gas production technique (GPT) provides gas production profiles that give an indication of the fermentative characteristics of the feed. The objective of this experiment was to estimate fermentation kinetic parameters of various solid feeds supplied to veal calves using the GPT, and to study the effect of adaptation of the rumen microflora to these solid feeds on their subsequent fermentation patterns. Thus, from the in vivo experiment described in Chapter 2 and 3; three out of five dietary treatments were selected as inoculum sources: pectin ( PECTIN ), neutral detergent fiber ( NDF ), and starch ( STARCH ). Sugar beet pulp ( SBP ), sugar beet pectin ( SBPec) , native corn starch ( NCS ), soy bean hulls ( SBH ) and crystalline cellulose ( AVICEL) were selected as substrates. For the second in vitro experiment, three out of eight dietary treatments (from the in vivo experiment described in Chapter 4) were selected as inocula. The selected diet treatments were: C100= concentrate only, C70-S30 = concentrate (70%) with straw (30%) and C70-CS30 = concentrate (70%) with corn silage (30%). For this gas production experiment, straw ( STRAW ), soy bean hulls ( SBH ), native corn starch ( NCS ) and sugar beet pectin ( SBPec ) were chosen as in vitro substrates.For both in vivo experiments, cumulative gas production was measured over time (72 h) as an indicator of the kinetics of fermentation. Fermentation end-products, including volatile fatty acids and ammonia, and organic matter loss, were also measured. Results : In both experiments significant differences between the inoculum sources, in terms of both fermentation kinetics characteristics and end-products of fermentation were observed. Similarly, significant effects were also observed for substrate compositions. Differences between the fermentation characteristics of NCS, SBPec and SBH, were consistent for both experiments. The total VFA production was not different among these substrates in both experiments. Finally, for both experiments, there was a significant inocula and substrate interaction which may indicate differences in the microbial activity occurring between the calves. Therefore, it was concluded that rumen inoculum from adapted animals should be used to obtain a more accurate assessment of feed ingredients in veal calf diets.Chapter 6 (General discussion) focused in four points: a) Factors influencing rumen development in rearing and veal calves; b) The importance of ruminal drinking in veal calves fed solids feeds; c) Effects of feeding strategies on ruminal pH and buffering capacity of rumen contents in veal calves; d) comparative analysis of the results obtained from the GPT (results presented in Chapter 5) and the PDE activities (results presented in Chapter 2 and 4).