Evaluation of the effects of oral colostrum supplementation during the first fourteen days on the health and performance of preweaned calves

Diagnosis and Treatment of Infectious Enteritis in Neonatal and Juvenile Ruminants

Common causes of infectious enteritis in neonate and juvenile ruminants include viral, bacterial, and protozoal pathogens. The most common presenting sign in ruminants with infectious enteritis is diarrhea. Diagnosis of the cause of enteritis has important zoonotic and herd health implications. Severity of clinical signs with similar pathogens may differ between calves and small ruminants. Treatment of enteritis involves supportive care to correct fluid and electrolyte imbalances, provision of nutritional support for neonates, prevention and treatment of endotoxemia or sepsis, and pathogen-specific treatments when relevant and available.

Concerning Increase in Antimicrobial Resistance in Shiga Toxin-Producing Escherichia coli Isolated from Young Animals during 1980-2016

This study was conducted in order to assess the antimicrobial resistance patterns of E. coli isolated from young animals affected between 1980 and 2016. The selected isolates for this study (n=175) carried stx₁/stx₂ genes and the most prevalent type of pathogenic E. coli found belonged to serogroup O101, antigen (K99)–F41 positive. All STEC-positive isolates were tested for susceptibility to 11 antimicrobials. Multidrug resistance (MDR) increased from 11% during the 1980s to 40% between 2000 and 2016. Resistance to tetracycline and streptomycin was the most frequent co-resistance phenotype (37%). Co-resistance to tetracycline and sulfonamide was found in 21% of E. coli isolates, while the MDR pattern to tetracycline, sulfonamide, and streptomycin was observed in 12% of the strains tested. Only 8% of isolates were co-resistant to tetracycline, ampicillin, streptomycin, and sulfonamide. The most common resistance genes found were those encoding for tetracycline, sulphonamides, and streptomycin, with 54% (n=95) of the tested isolates containing at least one of the genes encoding tetracycline resistance. A total of 87% of E. coli that tested positive for tetracycline (tetA, tetB, and tetC) and sulphonamide (sul1) resistance genes were isolated between 2000 and 2016. A large number of isolates (n=21) carried int1 and a nucleotide sequence analysis revealed that all class 1 integron gene cassettes carried sul1, tet, and dfrA1 resistance genes. An increase was observed in the level of resistance to antimicrobials in Romania, highlighting the urgent need for a surveillance and prevention system for antimicrobial resistance in livestock in Eastern Europe.

Bovine colostrum enriched with lyophilized bovine colostrum stimulates intestinal epithelium renewal of Holstein calves in the first days of life

Consumption of a second meal of colostrum with high quality could contribute to the intestinal epithelium development, especially if there is poor supply of colostrum just after birth. The effect of a second colostrum meal was evaluated on histomorphometry of the intestinal mucosa of newborn Holstein calves fed with high- and low-quality first colostrum. Seventy-two calves were fed with a first colostrum meal with high (HFM, close to 100 mg/ml) or low (LFM, close to 30 mg/ml) IgG concentration. At 12 hr of life, three treatments of second colostrum feeding were applied to the calves either fed high or low first colostrum: calves fed with low (LOW-close to 30 mg/ml) or high (HIGH-close to 100 mg/ml) IgG concentration; and colostrum enriched with lyophilized bovine colostrum with high IgG concentration (ENRICHED-higher than 120 mg/ml), resulting in six groups. Intestinal samples were collected after 24 and 72 hr of life. In the distal jejunum and ileum, LOW showed higher villus height than ENRICHED (p < .05). In the distal jejunum, greater villus perimeter was observed in the LOW compared to ENRICHED at 24 hr (p < .05). In ileum, LFM showed higher villus perimeter compared to HFM (p < .05). LOW showed the highest villus height-to-crypt depth ratio in the medium and distal jejunum and ileum, p < .05. ENRICHED and HFM showed decreased muscle layer thickness in the proximal and distal jejunum respectively (p < .05). The results reveal that the high concentration of total solids, crude protein, IgG and IGF-I of colostrum with high quality worsened the absorptive area, but may have stimulated the activity of cell division in intestinal crypts. Considering the present results, bovine colostrum enriched with lyophilized bovine colostrum stimulates intestinal epithelium renewal of Holstein calves in the first days of life.

Effect of quality of colostrum on health, growth and immunoglobulin G concentration in Holstein calves in a hot environment

The aim of this study was to determine the effect of ingestion of pasteurized and subsequently frozen–thawed pooled colostrum (≥50 mg Ig/mL) with different bacterial counts and immunoglobulin concentration (IgC) on the occurrence of diarrhea and pneumonia in 306 neonatal Holstein calves in a hot environment. Calves were assigned to be fed colostrum with total bacterial counts (TBC) lower or greater than 100 000 colony‐forming units (cfu)/mL, total coliform counts (TCC) greater or lower than 10 000 cfu/mL, and IgC lower or higher than 85 mg Ig/mL. Calves fed colostrum with TBC ≥100 000 cfu/mL were more likely (risk ratio 1.34, confidence interval 1.05–1.71; P < 0.05) to present pneumonia than calves receiving colostrum with lower TBC (incidence 53.2 vs. 39.8%). Calves fed colostrum with high TCC had increased chances of suffering pneumonia (51.4 vs. 42.1%; P < 0.05) than calves fed colostrum with lower TCC. Calves fed colostrum with ≥85 mg Ig/mL tended to present higher daily weight gain (505 ± 113 vs. 484 ± 126 g; P = 0.09). TBC and TCC in colostrum did not influence the incidence rate of diarrhea. It was concluded that under the conditions of the present study, heavy contamination of on‐farm pasteurized frozen–thawed colostrum is seemingly unavoidable and this contamination poses a threat for pneumonia, but not for diarrhea.

Risk for the development of Antimicrobial Resistance (AMR) due to feeding of calves with milk containing residues of antibiotics

EFSA was requested to: 1) assess the risk for the development of antimicrobial resistance (AMR) due to feeding on farm of calves with colostrum potentially containing residues of antibiotics; 2) assess the risk for the development of AMR due to feeding on farm of calves with milk of cows treated during lactation with an antibiotic and milked during the withdrawal period, and 3) propose possible options to mitigate the risk for the development of AMR derived from such practices. Treatment of dairy cows during the dry period and during lactation is common in the EU Member States. Penicillins, alone or in combination with aminoglycosides, and cephalosporins are most commonly used. Residue levels of antimicrobials decrease with the length of the dry period. When the interval from the start of the drying-off treatment until calving is as long as or longer than the minimum specified in the Summary of Product Characteristics of the antimicrobial, faecal shedding of antimicrobial-resistant bacteria will not increase when calves are fed colostrum from treated cows. Milk from cows receiving antimicrobial treatment during lactation contains substantial residues during the treatment and withdrawal period. Consumption of such milk will lead to increased faecal shedding of antimicrobial-resistant bacteria by calves. A range of possible options exist for restricting the feeding of such milk to calves, which could be targeting the highest priority critically important antimicrobials. β-Lactamases can reduce the concentration of β-lactams which are the most frequently used antimicrobials in milking cows. Options to mitigate the presence of resistant bacteria in raw milk or colostrum are mainly based on thermal inactivation.

Evaluation of the effects of colostrum replacer supplementation of the milk replacer ration on the occurrence of disease, antibiotic therapy, and performance of pre-weaned dairy calves

The objective of this study was to evaluate the effects of colostrum supplementation of the milk replacer ration on disease occurrence, antibiotic therapy, and performance of pre-weaned dairy calves with adequate transfer of passive immunity. Two hundred and two 1-d-old Holstein dairy calves were assigned to 1 of 2 groups after arrival to a dairy calf rearing facility. Calves assigned to the control group (n = 100) received milk replacer (28% crude protein and 20% crude fat) without colostrum inclusion twice daily. Calves assigned to the treatment group (n = 102) received 150 g of supplemental colostrum replacer powder added to their milk replacer twice daily for the first 14 d of life. Before group assignment, serum samples were collected from all calves to confirm transfer of passive immunity. Calves were evaluated daily until weaning (56 d of life) for signs of clinical disease as well as any treatment with antibiotics. Presentation of clinical disease and antibiotic treatment was recorded daily by personnel blinded to treatment allocation. Adequate transfer of passive immunity was confirmed in all calves at the start of the study and mean serum IgG values were similar among calves from treatment and control groups. The odds ratios of having abnormal feces and abnormal respiration during the pre-weaning period for calves from the treatment group were 0.15 and 0.46 the odds ratios of calves from the control group, respectively. The odds ratios of receiving antibiotic therapy during the pre-weaning period for calves from the treatment group were 0.09 the odds ratios of calves from the control group. Mean body weight and average daily gain at weaning were not significantly different among calves from the treatment and control groups. Colostrum replacer supplementation of the milk replacer ration was effective in reducing antibiotic therapy and occurrence of disease during the pre-weaning period.

The gut microbiome as a virtual endocrine organ with implications for farm and domestic animal endocrinology

The gut microbiome exerts a marked influence on host physiology, and manipulation of its composition has repeatedly been shown to influence host metabolism and body composition. This virtual endocrine organ also has a role in the regulation of the plasma concentrations of tryptophan, an essential amino acid and precursor to serotonin, a key neurotransmitter within both the enteric and central nervous systems. Control over the hypothalamic-pituitary-adrenal axis also appears to be under the influence of the gut microbiota. This is clear from studies in microbiota-deficient germ-free animals with exaggerated responses to psychological stress that can be normalized by monocolonization with certain bacterial species including Bifidobacterium infantis. Therapeutic targeting of the gut microbiota may thus be useful in treating or preventing stress-related microbiome-gut-brain axis disorders and metabolic diseases, much the same way as redirections of metabolopathies can be achieved through more traditional endocrine hormone-based interventions. Moreover, the implications of these findings need to be considered in the context of farm and domestic animal physiology, behavior, and food safety.

Prediction of respiratory disease and diarrhea in veal calves based on immunoglobulin levels and the serostatus for respiratory pathogens measured at arrival

Failure of passive transfer is a common problem in calves destined for veal production. At present it is unknown whether the risk for respiratory disease (BRD) or neonatal calf diarrhea (NCD) in the veal herd is associated with total immunoglobulin (Ig) and/or on the serostatus for respiratory pathogens measured at arrival. Therefore, the first objective of this prospective longitudinal cohort study was to determine associations between serum protein fractions as determined by routine electrophoresis (total protein, albumin, alpha-1 and -2 globulins, beta-globulins and Ig's) at arrival and BRD and NCD in the first 3 weeks of the production cycle. The second objective was to determine whether the serostatus (seropositive/seronegative) of seven respiratory pathogens (bovine respiratory syncytial virus (BRSV), parainfluenzavirus-3, bovine coronavirus (BCV), bovine herpesvirus-1, bovine viral diarrhea virus, Mannheimia haemolytica and Mycoplasma bovis) of these arrival serum samples could be associated with the risk of having BRD. The third objective was to determine which of the electrophoresis proteins and respiratory serostatuses were associated with average daily gain (ADG) in the study period. The study population consisted of 150 rosé veal calves housed in a single air-space. The study period ended at day 18 post arrival, when BRD incidence was judged to be too high to further postpone a group treatment. A Cox regression model was used to determine the effect of the studied protein fractions and antibodies on the time to BRD and NCD occurrence. The effect of the studied predictors on ADG was determined by linear regression. Calves with Ig levels under 7.5g/L had an increased BRD hazard (hazard ratio (HR)=1.9 (95% confidence interval (CI)=1.2-3.0)). NCD was only positively associated with the alpha-2 globulin concentration. Calves with a negative serostatus for BCV (HR=1.7 (95% CI=1.0-2.8)) or BRSV (HR=2.0 (95% CI=1.0-3.9)) had an increased BRD hazard. Average daily gain (ADG) was 0.242kg/day (SD=0.142) and was not related to the occurrence of BRD or NCD. Calves with Ig's below 7.5g/L and with increased levels of alpha-2 globulins showed a decrease in ADG. This study showed the importance of providing sufficient colostrum to veal calves and the potential benefit of the presence of BCV and BRSV antibodies at arrival to reduce the BRD hazard in the first 3 weeks.

A perspective on Cryptosporidium and Giardia, with an emphasis on bovines and recent epidemiological findings

Cryptosporidium and Giardia are two common aetiological agents of infectious enteritis in humans and animals worldwide. These parasitic protists are usually transmitted by the faecal-oral route, following the ingestion of infective stages (oocysts or cysts). An essential component of the control of these parasitic infections, from a public health perspective, is an understanding of the sources and routes of transmission in different geographical regions. Bovines are considered potential sources of infection for humans, because species and genotypes of Cryptosporidium and Giardia infecting humans have also been isolated from cattle in molecular parasitological studies. However, species and genotypes of Cryptosporidium and Giardia of bovids, and the extent of zoonotic transmission in different geographical regions in the world, are still relatively poorly understood. The purpose of this article is to (1) provide a brief background on Cryptosporidium and Giardia, (2) review some key aspects of the molecular epidemiology of cryptosporidiosis and giardiasis in animals, with an emphasis on bovines, (3) summarize research of Cryptosporidium and Giardia from cattle and water buffaloes in parts of Australasia and Sri Lanka, considering public health aspects and (4) provide a perspective on future avenues of study. Recent studies reinforce that bovines harbour Cryptosporidium and Giardia that likely pose a human health risk and highlight the need for future investigations of the biology, population genetics and transmission dynamics of Cryptosporidium and Giardia in cattle, water buffaloes and other ruminants in different geographical regions, the fate and transport of infective stages following their release into the environment, as well as for improved strategies for the control and prevention of cryptosporidiosis and giardiasis, guided by molecular epidemiological studies.

Advances in prevention and therapy of neonatal dairy calf diarrhoea: a systematical review with emphasis on colostrum management and fluid therapy

Neonatal calf diarrhoea remains the most common cause of morbidity and mortality in preweaned dairy calves worldwide. This complex disease can be triggered by both infectious and non-infectious causes. The four most important enteropathogens leading to neonatal dairy calf diarrhoea are Escherichia coli, rota- and coronavirus, and Cryptosporidium parvum. Besides treating diarrhoeic neonatal dairy calves, the veterinarian is the most obvious person to advise the dairy farmer on prevention and treatment of this disease. This review deals with prevention and treatment of neonatal dairy calf diarrhoea focusing on the importance of a good colostrum management and a correct fluid therapy.

Effect of bovine colostrum intake on growth, reproductive parameters and survival in red kids

The aim of this study is to evaluate the efficacy of frozen Azawak colostrum supplementation on body weight (BW), average daily gain (ADG), reproductive parameters (mean age at first parturition, fertility, fecundity, prolificacy) and mortality rate among red kids. The study was conducted at the goat farm secondary centre of Maradi in Niger from September 2010 to September 2011. The control animals (n = 20) were left with their mother, while the treatment animals (n = 20) received in addition 50 ml/animal/day of bovine colostrum at birth and 15 ml/animal/day from d2 to d15. Weight was measured weekly from birth to d365. Mortalities were also recorded over the same period. For reproductive parameters, observations began at weaning (d197). Growth rate was higher (p < 0.001) in supplemented animal, and the treatment effects on ADG were observed up to 150 day after the end of supplementation. A similar long-lasting trend was also observed in relation to the mortality rate (25% for ColG vs. 55% for ConG; p = 0.05). The age at first kidding tended to be lower in the treated group (13.8 ± 0.7 vs. 14.1 ± 0.8 month; p < 0.1). In conclusion, mild bovine colostrum supplementation induces a long-lasting positive impact on growth rate and to a lower extent on reproduction parameters and survival rate.

Effect of feeding colostrum at different volumes and subsequent number of transition milk feeds on the serum immunoglobulin G concentration and health status of dairy calves

Transfer of sufficient IgG to the newborn calf via colostrum is vital to provide it with adequate immunological protection and resistance to disease. The objectives of the present study were to compare serum IgG concentration and health parameters of calves (1) fed different volumes of colostrum [7, 8.5, or 10% of body weight (BW)] within 2h of birth and (2) given 0, 2, or 4 subsequent feedings of transition milk (i.e., milkings 2 to 6 postcalving). Ninety-nine dairy calves were fed 7, 8.5, or 10% of BW in colostrum within 2h of birth and given 0, 2, or 4 subsequent feedings of transition milk. The concentration of IgG in the serum of calves was measured at 24, 48, 72, and 642 h of age by an ELISA. The apparent efficiency of absorption for IgG was determined. Health scores were assigned to calves twice per week and all episodes of disease were recorded. The effect of experimental treatment on calf serum IgG concentration differed by the age of the calf. Calves fed 8.5% of BW in colostrum had a greater mean serum IgG concentration than calves fed 7 or 10% of BW at 24, 48, and 72 h of age. At 642 h of age, serum IgG concentrations of calves fed 8.5% of BW (24.2g/L) and calves fed 10% of BW (21.6g/L) did not differ, although the serum IgG concentration of calves fed 8.5% of BW was still greater than that of calves fed 7% of BW (20.7 g/L). No difference in serum IgG concentration existed between calves fed 7% of BW and those fed 10% of BW at any age. No significant effect of number of subsequent feedings of transition milk on calf serum IgG concentration was detected. The apparent efficiency of absorption of calves fed 8.5% of BW in colostrum (38%) was greater than calves fed 7% of BW in colostrum (26%) and tended to be greater than in calves fed 10% of BW (29%). Calves fed further feedings of transition milk after the initial feeding of colostrum had a lower odds (0.62; 95% confidence interval: 0.41 to 0.93) of being assigned a worse eye/ear score (i.e., a more copious ocular discharge or pronounced ear droop) and a lower odds (0.5; 95% confidence interval: 0.32 to 0.79) of being assigned a worse nasal score (i.e., a more copious and purulent nasal discharge) during the study period relative to calves that received no further feedings of transition milk. In conclusion, calves fed 8.5% of BW in colostrum within 2h of birth achieved a greater concentration of IgG in serum in the first 3 d of life than calves fed either 7 or 10% of BW. Feeding calves transition milk subsequently reduced their odds of being assigned a worse eye/ear and nasal score.

Commensal microbiome effects on mucosal immune system development in the ruminant gastrointestinal tract

Commensal microflora play many roles within the mammalian gastrointestinal tract (GIT) that benefit host physiology by way of direct or indirect interactions with mucosal surfaces. Commensal flora comprises members across all microbial phyla, although predominantly bacterial, with population dynamics varying with host species, genotype, and environmental factors. Little is known, however, about the complex mechanisms regulating host-commensal interactions that underlie this mutually beneficial relationship and how alterations in the microbiome may influence host development and susceptibility to infection. Research into the gut microbiome has intensified as it becomes increasingly evident that symbiont-host interactions have a significant impact on mucosal immunity and health. Furthermore, evidence that microbial populations vary significantly throughout the GIT suggest that regional differences in the microbiome may also influence immune function within distinct compartments of the GIT. Postpartum colonization of the GIT has been shown to have a direct effect on mucosal immune system development, but information is limited regarding regional effects of the microbiome on the development, activation, and maturation of the mucosal immune system. This review discusses factors influencing the colonization and establishment of the microbiome throughout the GIT of newborn calves and the evidence that regional differences in the microbiome influence mucosal immune system development and maturation. The implications of this complex interaction are also discussed in terms of possible effects on responses to enteric pathogens and vaccines.

An alkalinizing oral rehydration solution containing lecithin-coated citrus fiber is superior to a nonalkalinizing solution in treating 360 calves with naturally acquired diarrhea

The aim of this field study was to compare the efficacy and cost of 2 commercially available oral rehydration therapy (ORT) solutions in treating dairy calves with naturally acquired diarrhea. A total of 1,349 newborn Holstein-Friesian calves were prospectively enrolled in the study. Calves were housed in individual hutches and fed a mixture of pasteurized hospital milk and an all-milk protein milk replacer twice per day. Calves were monitored twice each day from d 2 of life until 30 d of age for the presence or absence of diarrhea, and were assigned a fecal score and a hydration score at each examination. Calves that developed mild to severe diarrhea that did not need intravenous fluids and did not have clinical evidence of concurrent disease (n = 360) were assigned randomly to receive 1 of 2 commercial ORT solutions: a hypertonic alkalinizing ORT containing lecithin-coated citrus fibers (Diaque, group D, n = 180; Boehringer Ingelheim, Ingelheim, Germany), and an isotonic nonalkalinizing ORT (RE-SORB, group R, n = 180; Pfizer Animal Health, New York, NY) for 2 to 8d; the duration of treatment depended on whether diarrhea was still present. No significant differences were observed in mortality rates or treatment failure rates between the 2 treatment groups. Fecal consistency returned to normal more quickly in group D calves than in group R calves; consequently, group D calves were treated for 1d less than were group R calves. The increase in body weight after 4d of treatment was larger in group D than in group R. The average daily gain from birth to weaning in calves that did not develop concurrent disease (such as pneumonia) during the study period tended to be higher in group D calves (0.53±0.11 kg/d) than in group R calves (0.51±0.09 kg/d). The smaller number of treatments at a lower cost per treatment produced a cost advantage of $4.82 per treated calf in group D calves compared with group R calves. Our findings support the concept that milk should continue to be fed to diarrheic calves that are being administered an ORT solution in order to maintain growth.

Scientific Opinion on the welfare of cattle kept for beef production and the welfare in intensive calf farming systems

Information given in previous Opinions "Welfare of cattle kept for beef production" (SCAHAW, 2001) and "The risks of poor welfare in intensive calf farming systems" (EFSA, 2006) is updated and recent scientific evidence on the topics reviewed. Risks of poor welfare are identified using a structured analysis, and issues not identified in the SCAHAW (2001) beef Opinion, especially effects of housing and management on enteric and respiratory diseases are reviewed. The Opinion covers all systems of beef production, although the welfare of suckler cows or breeding bulls is not considered. The Chapter on beef cattle presents new evidence and recommendations in relation to heat and cold stress, mutilations and pain management, digestive disorders linked to high concentrate feeds and respiratory disorders linked to overstocking, inadequate ventilation, mixing of animals and failure of early diagnosis and treatment. Major welfare problems in cattle kept for beef production, as identified by risk assessment, were respiratory diseases linked to overstocking, inadequate ventilation, mixing of animals and failure of early diagnosis and treatment, digestive disorders linked to intensive concentrate feeding, lack of physically effective fibre in the diet, and behavioural disorders linked to inadequate floor space, and co-mingling in the feedlot. Major hazards for white veal calves were considered to be iron-deficiency anaemia, a direct consequence of dietary iron restriction, enteric diseases linked to high intakes of liquid feed and inadequate intake of physically effective fibre, discomfort and behavioural disorders linked to inadequate floors and floor space.

Excretion of antibiotic resistance genes by dairy calves fed milk replacers with varying doses of antibiotics

Elevated levels of antibiotic resistance genes (ARGs) in soil and water have been linked to livestock farms and in some cases feed antibiotics may select for antibiotic resistant gut microbiota. The purpose of this study was to examine the establishment of ARGs in the feces of calves receiving milk replacer containing no antibiotics versus subtherapeutic or therapeutic doses of tetracycline and neomycin. The effect of antibiotics on calf health was also of interest. Twenty-eight male and female dairy calves were assigned to one of the three antibiotic treatment groups at birth and fecal samples were collected at weeks 6, 7 (prior to weaning), and 12 (5 weeks after weaning). ARGs corresponding to the tetracycline (tetC, tetG, tetO, tetW, and tetX), macrolide (ermB, ermF), and sulfonamide (sul1, sul2) classes of antibiotics along with the class I integron gene, intI1, were monitored by quantitative polymerase chain reaction as potential indicators of direct selection, co-selection, or horizontal gene transfer of ARGs. Surprisingly, there was no significant effect of antibiotic treatment on the absolute abundance (gene copies per gram wet manure) of any of the ARGs except ermF, which was lower in the antibiotic-treated calf manure, presumably because a significant portion of host bacterial cells carrying ermF were not resistant to tetracycline or neomycin. However, relative abundance (gene copies normalized to 16S rRNA genes) of tetO was higher in calves fed the highest dose of antibiotic than in the other treatments. All genes, except tetC and intI1, were detectable in feces from 6 weeks onward, and tetW and tetG significantly increased (P < 0.10), even in control calves. Overall, the results provide new insight into the colonization of calf gut flora with ARGs in the early weeks. Although feed antibiotics exerted little effect on the ARGs monitored in this study, the fact that they also provided no health benefit suggests that the greater than conventional nutritional intake applied in this study overrides previously reported health benefits of antibiotics. The results suggest potential benefit of broader management strategies, and that cost and risk may be avoided by minimizing incorporation of antibiotics in milk replacer.

Calf health from birth to weaning. I. General aspects of disease prevention

Calfhood diseases have a major impact on the economic viability of cattle operations. This is the first in a three part review series on calf health from birth to weaning, focusing on preventive measures. The review considers both pre- and periparturient management factors influencing calf health, colostrum management in beef and dairy calves and further nutrition and weaning in dairy calves.