Diarrhoea is a serious disturbance of normal gut function, characterized by an excessive water excretion in faeces. The derangement is in particular observed in young, milk-fed calves. In the literature a distiction is usually made between scouring from nutritional factors and that caused by pathogenic infections. The former derangement in calves is investigated in our work.In practice it is well known that the new-born calf is highly susceptible to nutritional diarrhoea. Its prevention requires a number of precautions in calf nutrition and management. Nowadays these requirements also impose severe restrictions upon the type and amount of feed components suitable for use in milk replacers for these animals. Nevertheless their importance in calves, the role of diet composition and intake in this disturbance has not been systematically investigated. The lack of information prompted an investigation of the role of diet composition and intake in scouring disturbances in milk-fed calves. The work included experiments, where the main objective was to classify common diet components according to their diarrhoeic properties. The changes in the digestive processes were also measured. Associated changes in the microbial population in the lower intestine, and in electrolyte and water metabolism in body tissues were not investigated.The literature on the physiological changes responsible for nutritional diarrhoea in calves, reviewed in chapter 2, is rather scarce and generally refers to research in infants and man. According to these investigations proteins and carbohydrates are considered to have the greatest potential to induce scouring. Exceeding the digestive capacity of these components in the small intestine may result in either proteo-saccharolytic or saccharolytic fermentation in the hind gut. Both conditions inhibit water absorption in the colon, either because of an increased amount of components with osmotic activity in the digesta, or because of toxic properties in the end products of fermentation. Certain products of proteo-saccharolytic fermentation in particular are considered to be toxic. The derangements may lead to either putrefactive diarrhoea when dietary protein is involved, or saccharolytic diarrhoea in case of excessive carbohydrate intake. These two types of scouring are among others characterized by changes in faecal pH, being higher or lower, respectively, than in normal faeces. The literature on these effects in milk-fed calves is less clear. It provided no insight into the different diarrhoeic properties of the individual organic components, protein, fat or carbohydrates, in the diet. Another question arising was the role of abomasal clotting of the diet in scouring. This was by several authors considered to be important in putrefactive diarrhoea.Five experiments were carried out to investigate these aspects in young, milk-fed calves. In the first experiment (Exp. 1, section 3.3) the effect of milk composition and daily allowance on faecal characteristics (visual score, pH and DM content) was investigated in twelve subsequent periods, each lasting three days. A commercial milk replacer served as control diet in six calves, providing approximately 4.5 g protein, 3.5 g crude fat and 8 g Hex. Eq.*) lactose per kg BW per day. In the separate experimental periods either lactose, sucrose, gelatinized or raw starch, casein or fat was added in addition to that diet. The extra lactose addition to the control diet varied up to 125 % of control Hex. Eq. intake, sucrose and starch to 75 %, casein to 140 % of control crude protein intake and fat to 60 % of control crude fat intake. In one period the daily offer of the control diet was increased to 175 % of the normal intake.The carbohydrate additions resulted in a rapid fall of faecal consistency and pH (table 2). Sucrose and the starches proved to be more critical in this respect than lactose, although the response to starch was slightly delayed. Dietary lactose induced quickly fermentative scouring when fed in excess of 10 g Hex. Eq. per kg BW per day. Calf`s response to lactose was not affected by animal's age in this experiment, contrary to that reported by several authors. High concentrations of well homogenized fats of high nutritional quality seemed to become critical too, although the effect was less pronounced than with similar increases of carbohydrates (table 3). Scouring was also observed on high milk intakes (table 4). The response of faecal pH and DM content in this period suggested an interaction in scouring effects between the dietary components. The experiment did not allow any conclusion to be drawn as to whether or not the interaction interfered with the digestion or the microbial fermentation of the undigested dietary residues in the lower intestine. Casein did not affect faecal characteristics, irrespective of the level of intake.Although faecal responses to the individual treatments were evident, they were less informative about the quantitative differences in diarrhoeic properties of the individual components tested. Faecal pH was usually closely related to visual score when increasing carbohydrate intake. This parameter, however, gave less information on the effect of high fat or protein intake. A diagnosis based on DM content alone might lead to misinterpretation of the severity of scours, as was demonstrated in the starch treatments. This criterion does not always reflect the increased water excretion in diarrhoeic faeces. It may be affected simultaneously by a higher excretion of components exerting no osmotic effects, e.g. mucous substances.The absence of scouring in the high casein treatments seemed to conflict with the results cited in the literature. These postulate the risk of putrefactive scouring induced by dietary protein, in particular, when milk clotting in the abomasum is insufficient. The role of abomasal clotting in diarrhoea was therefore investigated in Exp. 2 (section 3.4). For that purpose a liquid commercial milk replacer was directly infused into the proximal duodenum, replacing quantitatively the digesta collected at that site. The effect of this treatment on faecal characteristics, visual score and pH, was measured in two experimental periods lasting 4 x 12 h and 4 x 24 h, respectively.The milk infusate increased substantially the firmness of the faeces, especially in the latter period (table 5). Faeces became so firm that the animals had obviously problems in defaecating. This unexpected, and yet still unexplainable, result clearly shows that no negative relation exists between abomasal clotting of milk protein and scouring.The importance of undigested dietary components in the large intestine digesta in the scouring derangement was more closely investigated in Exp. 3, 4 and 5 with fifteen ileal fistulated calves (section 3.5). The higher inflow of dietary components into the colon was simulated by an infusion of graded levels casein, fat or lactose into the distal end of the ileum. Casein was infused at 5 and 10 % of oral crude protein intake. The fat infusion was 5 % of crude fat intake and those of lactose 5, 10 and 20 % of oral NFE intake in the control milk replacer. Faecal response (visual score, pH and DM content) was measured, as well as the ileal and faecal apparent digestibility.The effect of the infusates on faecal characteristics (table 6) merely confirmed those observed in Exp. 1, although faecal responses were rather mild. The casein and fat treatments did not change faecal characteristics. Only the highest level of lactose infusion, 20 % of oral NFE intake, affected all faecal parameters. The control diet was highly digestible in these experiments (table 7). The digestion and absorption of most dietary components was almost completed at the distal end of the small intestine. Only a minor part of N and NFE disappeared in the lower intestine together with a substantial amount of Na. Faecal digestibility was hardly affected by the casein and fat infusates (table 8). N and NFE excretion in the faeces increased, however, when the lactose infusion exceeded 5 % of oral NFE intake. Faecal N excretion increased by 15-16 ing per g lactose infused in addition to the 5 % treatment.The results of Exp. 1-5 indicate that high intakes of milk protein of standard quality, i.e. casein, are not detrimental to young calves. High intake of fats or carbohydrates may result in nutritional scouring. The fat concentration of the diet is, however, presumably more limited by technological capabilities than by the level tolerated by these animals. It was therefore decided to undertake further work on the effect of carbohydrates. As lactose is the most important carbohydrate in milk replacers, this sugar received our main attention.The literature on the digestion and absorption of carbohydrates (chapter 4) indicates that milk-fed calves have an impressive ability to digest lactose and to absorb glucose and galactose. Consequently their levels of intake permitted in these animals are rather high. The enzymatic activity for digestion of other carbohydrates is much less well developed or even absent. Their intake is therefore strictly limited or even not tolerated at all. The literature is less informative about the digestive changes in young calves responsible for the nutritionally induced scouring. Neither have the consequences of the derangement for animals' health been investigated systematically. Four important questions in this respect needed further clarification. They concerned: (1) the maximal limit of lactose permitted in milk replacers for young calves, (2) the effect of excessive lactose intake on the digestion and absorption in the small intestine, (3) the significance of the quantitative and qualitative characteristics of ileal digesta in scouring and (4) the effect of fermentative diarrhoea on the water and mineral excretion. These aspects were investigated in ten experiments (Exp. 6-15, Chpt. 5).Exp. 6 and 7 were designed to investigate the relationship between lactose intake, carbohydrate digestion and absorption, using jugular vein blood sugar as a parameter, and scouring in young calves. In the former experiment graded levels of dietary lactose were fed to thirty calves, allotted to five groups of six animals. The individual faecal responses were measured and related to the individual blood sugar responses. Lactose was offered in three treatments, A, B and C, supplying 10, 13.5 and 17 g Hex. Eq. per kg BW per day, respectively. All treatments were fed for three successive days in each period. In Exp. 7 treatment A and C were offered to nine calves. The experimental periods were prolonged in this trial from three to seven days in order to test the possible effect of adaptation to high lactose intakes.The results in both experiments confirm that fermentative scouring quickly occurs, when the daily lactose intake exceeds 10 g Hex. Eq. per kg BW in calves aged at least four weeks. The response in blood sugar level provided evidence that the digestion and absorption of lactose increase with the daily intake. Treatment C resulted in significantly higher blood sugar levels than treatment B; those were in both treatments significantly higher than in control treatment A (table 14, 16). The blood sugar responses seemed also to indicate that calves, responding in a less pronounced way regarding the blood sugar curve in treatments B and C, suffer scouring more seriously. No adaptive response in the blood sugar curve occurred when the high lactose treatment was prolonged to seven days. However, calves adapt quickly to high lactose intake in their first four weeks of life. No effect of age was demonstrated from 4-12 weeks of age on the limit of lactose tolerance. The slope of the blood sugar curve, however, becomes steeper as animals grow older.The effect of dietary lactose on the digestive processes in the small intestine was further investigated in six experiments. Digesta flow rate*), transit time*) and digesta osmolality were the main parameters investigated in Exp. 9, 10 and 11 (section 5.4. 1). In Exp. 9 sixteen calves were used for this purpose, either fitted with re-entrant duodenal or ileal cannulae, or not fistulated. The effect of dietary lactose was tested in treatments A and C, offering 10 and 17 g Hex. Eq. lactose per kg BW per day, respectively. The duodenal or ileal flow rates of wet digesta, N and reducing substances were measured in the fistulated calves. The non-fistulated animals served as a 'control group' to test the scouring effect of the dietary treatments in this experiment. The flow rates of ileal wet digesta, measured in Exp. 10, 11 and 13, closely agreed with those observed in Exp. 9. In these trials treatment A and C were tested and also treatment D, offering 8 g Hex. Eq. lactose and 3 g Hex. Eq. sucrose per kg BW per day.The scour-inducing level of lactose (treatment Q neither affects abomasal transit time or duodenal flow rates of wet digesta, nor that of its components, N and reducing substances (table 19). Neither the digesta transit time, nor N flow rates are different at the end of the ileum for either treatment (table 29). The flow rates of wet digesta and reducing substances are, however, substantially higher when daily lactose intake increases from ca. 9- 10 to ca. 16-17 g Hex. Eq. per kg BW. Treatment D, supplying sucrose, basically acts in scouring in a similar way as lactose, although the calves respond more seriously to this treatment (table 21). Compared with the lactose diets, wet digesta flow rates into the lower intestine are significantly enhanced and digesta transit time in the abomasurn plus small intestine is reduced in the sucrose treatment.The effect of high lactose intake on the apparent digestibility of dietary components in the small intestine was measured in Exp. 12 (section 5.4.2) On account of the interaction observed in Exp. 1 the experiment was designed to investigate also the influence of diet composition on the digestion. Fifteen calves, fitted with re-entrant ileal cannulae, were allotted to three groups, receiving either diet A, B or C in five experimental periods of five days each. In P 1 and P 5 the apparent digestibility of the diets, offered in amounts equal to those usually given in treatments A, B and C, was investigated. In the other periods the animals received equal amounts of lactose with the three diets; 15.3, 12.2 and 9 g Hex. Eq. per kg BW per day in P 2 , P 3 and P 4 , respectively.The feed intake and digestive ability of the calves declined when the experiment lasted longer. This prevented an accurate determination of the apparent digestibility. Covariance analyses provided much evidence that the apparent digestibility coefficients of the main dietary components in the small intestine are hardly affected by lactose intake. Only the carbohydrate fraction is relatively less efficiently digested when lactose intake increases (table 22). It also seems to reduce the apparent digestibility of crude protein. It confirmed our view that, although the lactose digestion and absorption improves in the high lactose treatments, the improvement is insufficient to prevent greater amounts of carbohydrates flowing into the hind gut. The apparent digestibility in the small intestine is not substantially affected by diet composition. The differences in Exp. 12 were too small to be responsible for the large effect of diet composition on faecal characteristics on high milk intake in Exp. 1. These latter differences were obviously caused by differences in microbial fermentation in the lower intestine. The level of lactose that can be tolerated is obviously an absolute amount depending upon body weight, not on the percentage composition of the diet.In two subsequent trials, Exp. 13 and 14, the significance of lactase activity in the maximal limit of lactose intake was investigated (section 5.4.3). The previous experiments give little information on that aspect, especially because of a discrepancy between the results obtained in analysis of reducing substances and of the individual sugars, glucose, galactose and lactose, according to a standard enzymatic method. Exp. 13 was designed to get more information on that point and to determine more accurately the individual sugar content in ileal digesta. For that purpose diets A and C were fed to five calves, fitted with re-entrant ileal cannulae. Ileal digesta and faeces were quantitatively collected in separate experimental periods.The results (table 24, 25 and 26) suggest that glucose is efficiently absorbed, mainly in the small intestine. In agreement with the literature, absorption of galactose is reduced in that region, in particular when lactose intake increases. Lactose recovery in ileal digesta and faeces is low compared with galactose recovery. The analyses on carbohydrate composition strongly suggested that the major part of galactose in ileal digesta was a component of other oligosaccharides, containing approximately 4.5 to 4.6 times as much galactose as glucose molecules. These compounds would explain the observed discrepancies between reducing substances and the individual sugars analysed, and also the lower efficiency of P- galactosidase in the 'lactose' digestion in ileal and faecal samples. The fact that these compounds were not found in the milk diet or duodenal digesta strongly suggested that they were synthesized in the small intestine during lactose digestion.Whether or not lactose is converted by lactase into oligosaccharides in calves, was furher investigated in Exp. 14. In this trial the high lactose diet C and a high hexose diet C' were fed. Both diets offered daily equal amounts of Hex. Eq.; 16 to 17 g per kg BW. The diets were fed to ten calves, fitted with re-entrant ileal cannulae and to eight non-fistulated calves. Ileal digesta and faeces of the fistulated animals were quantitatively collected and analysed by gas chromatography for sugar content. The non-fistulated calves served as a 'control group ' to measure the scouring response of the dietary treatments more accurately.Both diets induced scours in the calves (table 27). Treatment C' seemed to be slightly more conducive to diarrhoea and resulted in a weak condition of the calves, probably resulting from lesions in the intestinal mucosa. These adverse effects still carried over when this treatment had been stopped for three weeks and replaced by control diet A. The difference between both treatments was also reflected in the flow rates of ileal wet digesta, being higher in treatment C' than in the lactose treatment C (table 28).The sugar analyses in ileal digesta, collected in treatment C, proved that substantial amounts of hexoses were bound as oligosaccharides, i.e. lactose, maltose and other di- and trisaccharides, mainly consisting of galactose molecules. The samples collected in treatment C' contained more free and total glucose and galactose, but hardly any oligosaccharides. The recovery of total glucose and galactose in both treatments confirms the preferential absorption of hydrolysed hexoses over free hexoses in the intestinal lumen.Free glucose and galactose were the main sugars excreted in the faeces (table 29). Traces of lactose and other disaccharides were only found in faeces collected in treatment C, but not in treatment C'. Trisaccharides were not excreted in the faeces. The recovery of total glucose and galactose in the faeces, although almost negligible in both treatments, was slightly higher when lactose was fed.The results in Exp. 13 and 14 confirm the conversion of lactose into other oligosaccharides by lactase in the small intestine of calves. These compounds are not responsible for the lower galactose absorption in relation to glucose absorption. The apparent digestibility of galactose in the small intestine is slightly higher when lactose is fed instead of the individual hexoses, glucose and galactose. They also confirm that the limits put on galactose absorption are mainly responsible for maximal lactose digestion and absorption. Although some lactose is recovered at the end of the small intestine, this quantity is much lower than the total galactose recovery. Moreover, the lactose in ileal digesta may to some extent originate from transgalactosylation. The total galactose content is the main factor responsible for the changes in ileal digesta, when diets high in lactose are given to young calves.The effect of the quantitative and qualitative characteristics of ileal digesta in scouring was investigated in two trials (Exp. 8 and 11). In the latter experiment ileal digesta were quantitatively collected in calves, receiving the sucrose treatment D, and infused into the distal ileal cannula of calves, receiving control diet A, and vice versa. The three calves showed severe scouring when infused with ileal digesta from diet D (table 30). The other three calves, receiving the diet A infusate, responded with normal faecal characteristics despite the sucrose intake. The results confirmed that the changes in ileal digesta, when feeding an excess of carbohydrates, are responsible for the scouring phenomenon.The carbohydrate content in ileal digesta is generally presumed to be the main factor in scouring. However, the response of calves to carbohydrate infusions at that site were rather mild in Exp. 5 in relation to those usually observed in treatment C. It prompted us to repeat that trial, infusing lactose and galactose in amounts equal to 40 % of oral Hex. Eq. intake (Exp. 8). All sugars were infused as an aqueous solution, isotonic with ileal digesta (312 m osmol per L). An isotonic NaCl solution served as a control treatment in this experiment.The introduction of carbohydrates into the colonic lumen quickly reduced faecal consistency, pH and DM content (table 31).The responses were almost similar to those in calves severely suffering scours on high dietary lactoseintakes and indicate that the carbohydrates in ileal digesta are those primarily responsible for the scouring.The effect of lactose induced scouring on the water excretion of the calves was investigated in Exp. 9, 14 and 15, measuring quantitatively the urine and the water excretion in faeces in treatments A, B and C (section 5.6). The results suggest that lactose induced scouring in milk-fed calves does not necessarily change the total water excretion (table 32). The extra faecal water loss is compensated for by a lower urine excretion. The carbohydrate excretion in the urine is, however, significantly increased in high dietary lactose intake (table 33).The effect of dietary milk sugar in diets A, B and C on the absorption and excretion of macro minerals in milk-fed calves was investigated in Exp. 15. These diets were fed in three successive periods of five days each to twenty calves, ten of them fitted with re-entrant ileal cannulae. The absorption of Na, K, Cl, Ca, P and Mg in the small intestine was investigated in the fistulated calves. The nonfistulated animals served to measure the faecal apparent absorption and the retention of these minerals.The apparent absorption of the minerals in the small intestine in treatment A was rather high (table 34). The results in that treatment agree with those cited in the literature. The absorption of most elements is almost completed in the small intestine, except of Na, which is absorbed in considerable amounts in the large intestine. The absorption in the small intestine of K, Cl, Ca and in particular of Na decreases when dietary lactose intake increases. This effect is also reflected in the apparent faecal absorption and in the retention of these minerals, but only the changes in the absorption and retention of Na and K were significant in this experiment (table 35). Most animals did not suffer net electrolyte losses in treatment B or C. However, that did occur with Na in some individual calves, that responded severely to the lactose treatments (table 36).The results obtained in these fifteen experiments prove that lactose is the only carbohydrate tolerated in considerable amounts in milk replacer diets by young calves. The maximal limit of lactose, ca. 10 g Hex. Eq. per kg BW per day, seems to be slightly higher than that of glucose and galactose. It is not yet clear, whether or not the oligosaccharides in the ileal digesta, originating from the conversion during lactose digestion, may have some benefits in this respect. When, however, the lactose intake exceeds the limit of hexose, and in particular that of galactose absorption, scouring occurs. Related changes in intestinal digesta and faecal characteristics are basically similar as those observed in infants and man, suffering hypolactasia.