A calf’s susceptibility to morbidity and mortality may be mitigated or exacerbated by a number of management factors that are applied over three main developmental periods, namely the in utero, neonatal, and pre-weaning phases ^[1][2][10,23]. Poor management of any of these phases can have negative implications on calf health by increasing the incidences of disease, such as diarrhea and bovine respiratory disease ^[3][24]. The incidence of disease may be difficult to avoid if the dam or calf have been mismanaged during either of the first two phases. However, calf welfare during the pre-weaning phase is also considered to be one of the main factors associated with morbidity and mortality ^[4][25]. According to the globally accepted “five freedoms” ^[5][26] and the European Welfare Quality^® protocol, adequate calf welfare encompasses good health; comfort; adequate nutrition for maintenance and growth; the ability to express natural behavior; and the absence of pain, fear, and distress ^[6][27]. As previously acknowledged, calves are highly susceptible to diseases, in particular digestive disorders. Therefore during the pre-weaning phase, calves are constantly at risk of having a compromised welfare status due to digestive diseases and disorders ^[7][28].

However, digestive diseases can be mitigated by the nutritional management of the calf ^[8][29]. With this being said, from birth, the nutritional management of the calf includes fast diet transitions ^[9][30]. Therefore, one area of concern includes the management of the liquid diet of calves, i.e., colostral and post-colostral. This includes the provision of colostrum, utilization of waste milk, and restrictive feeding ^[10][31].

The most conspicuous factor influencing calf morbidity and mortality is the successful acquisition of passive immunity ^[11][12][32,33]. According to Lora et al. ^[13][34] the rate of failure of passive immunity (FPI) is likely to influence the occurrence of digestive diseases amongst pre-weaned calves. Calf studies within the last two decades report FPI rates ranging from 12.1 to 19.1% ^[14][15][35,36], as opposed to an older survey, which reported a rate of 40% ^[16][15]. However, Fischer et al. ^[8][29] suspect that the most recent statistics are not a true reflection of the actual rate of FPI because only healthy calves were evaluated. In reality, the rate of FPI may be much higher and might explain why there is still a high proportion of pre-weaned calf morbidities and mortalities caused by digestive diseases. This may motivate further investigation into colostrum management to ensure that digestive diseases are lowered.

It is already well known that feeding colostrum is associated with achieving successful passive immunity (SPI) ^[8][29]. After the ingestion of colostrum, immunoglobulins are actively absorbed from the small intestine and into blood circulation by pinocytosis ^[17][37]. The acquired immunoglobulins, predominantly immunoglobulin G (IgG), provide immunocompetence and protect the calf from disease ^[18][38]. Therefore, colostrum management may be regarded as the first step in preventing digestive diseases. Optimal IgG absorption encompasses the method and timing of colostrum feeding and the volume and quality of colostrum based on the IgG concentration and bacterial count ^[19][39]. Gut closure appears to be the greatest threat for preventing the absorption of IgG from the small intestine.

Until recently, it was recommend that calves should ingest colostrum before gut closure ^[20][40], which was considered to be at 24 h after birth ^[21][41]. However, Fischer et al. ^[22][42] demonstrated that the time for optimal IgG absorption is shorter than was popularly believed and that colostrum should be provided immediately after birth and not beyond 6 h post-partum. In addition to timing, the volume of colostrum fed should be taken into account. According to Godden et al. ^[23][43], it is preferable to feed larger volumes of colostrum, because it allows for a greater mass of immunoglobulins to be delivered to the small intestine to be absorbed. In order to manage the timing and volume offered, the method of colostrum feeding has to be considered ^[19][39].

The method of colostrum feeding is an incremental factor in providing a sufficient concentration of IgG to the intestinal lumen within the critical time frame. Allowing calves to suckle has been discouraged. This is largely due to the fact that there is a greater chance of delayed suckling and colostrum consumption ^[10][31]. However, according to the Nation Animal Health Monitoring System (NAHMS) ^[24][13], only a minority, approximately 17%, of calves are left to suckle. Therefore, rates of FPI and digestives diseases cannot be solely attributed to calves suckling. Although the manual feeding of colostrum, by a nipple bottle, bucket, or esophageal tube, is considered to be more desirable, there are additional factors to take into account. For instance, the volume of colostrum provided should be considered. When feeding small volumes of colostrum, it is preferable to utilize a nipple bottle or bucket to ensure that all of the milk reached the abomasum and intestines. If larger volumes of milk are provided, then it may also be appropriate to use an esophageal feeder, where a portion of the milk will enter the reticulorumen and will be delayed in reaching the abomasums ^[23][43].

According to the authors Godden et al. ^[17][37], the quality of colostrum, including IgG concentration and bacterial count, is also important for the active absorption of maternal immunoglobulins. There are several available strategies for optimizing the quality of colostrum. In particular, it is suggested that, excluding the dam effect (age, parity, and breed), other factors, such as the length of the dry period, season of calving, vaccinal status of the dam, and nutrition during the periparturient period, can affect the colostrum IgG concentration. However, these factors are relevant to the management of the dam as opposed to the newborn calf. Therefore, more focus will be given to management practices that can be applied if there is a shortage of high-quality colostrum and the influence of the bacterial count on colostrum quality.

In case there is a shortage of colostrum with sufficient concentrations of IgG, there are various management strategies that can be utilized. According to Cardoso et al. ^[25][44], this the use of colostrum supplements and replacers. One strategy not mentioned in the aforementioned review, but often used at the farm-level to overcome the issue of poor-quality colostrum is the use of high-quality colostrum from other cows within the farm. This can be easily achieved by storing surplus high-quality colostrum produced by other cows at −20 °C, which can then be used after thawing to feed newborn calves.

Another factor that may influence SPI is the colostral bacterial count. It is postulated that the bacterial contamination of colostrum may interfere with IgG absorption, and this may occur because colostral antibodies bind to bacteria ^[26][45]. Unacceptably high levels of bacteria, more than 100,000 colony-forming-units (CFU)/mL total plate count or 100,000 CFU/mL total coliform count, may bind to IgG and hinder absorption ^[17][37]. On the farm, colostrum contamination can be prevented by practicing good hygiene during collection and feeding, prohibiting prolonged exposure to ambient temperatures, and in some cases by appropriate heat treatment of colostrum. Heat treatment should not damage the IgG molecules; therefore, it is advisable to use a lower temperature for a longer time (i.e., 60 °C for 60 min) ^[17][37].

Based on the most recent literature of colostrum management, it is advised that calves should be provided with colostrum consisting of no less than 50 g/L of IgG at a volume of approximately 8.5–10% of the birth weight within 2 h after birth ^[17][27][37,46]. If these recommendations are met, it is likely to reduce the risk of an FPI.

Although, FPI has been regarded to be the main reason for the high occurrence of digestive diseases in calves, there may be another factor that is equivocally import; this includes the bacterial colonization of the intestines ^[28][47]. The composition of the intestinal bacterial community has been shown to be correlated with the incidence of diarrhea. Calves that have a higher proportion of fecalibacterium in the feces during the first week of life have been shown to have a lower occurrence of diarrhea ^[29][48]. Therefore, colostrum-management strategies for promoting SPI should also be reviewed for the influence that they have on the bacterial colonization in the gut. It is possible that colostrum may provide two equivocally important protective mechanisms, SPI and microbial colonization.

Unsalable milk is often used to describe the term “waste milk”. It may consist of low-quality colostrum, transition milk, and milk from morbid cows consisting of high somatic cell counts or antibiotic residues ^[30][49]. Waste milk may be utilized as the liquid diet for pre-weaned calves, because it is deemed to be economically favorable. However, milk derived from infected cows may increase the risk of pathogen transmission, posing a direct threat to calf health ^[10][31]. In an attempt to minimize the pathogen load, it is preferable to pasteurize waste milk before feeding it to calves. In some cases, this has been successful in eliminating Mycobacterium paratuberculosis and Mycoplasma species ^[31][32][50,51]; however, others have reported that it is not completely effective in destroying pathogenic organisms ^[33][52]. Alternatively, ultraviolet light treatment may also be used to control the bacterial count in waste milk; however, it is not able to completely eliminate the presence of pathogens ^[34][53]. The antibiotic residues that may be present in waste milk can also have an impact on calf health and welfare. Trace amounts of antibiotics in the calves’ diet may disrupt the microbiota in the gastrointestinal tract (GIT), thereby negating the physiological and immunological development of the calf ^[30][49]. Due to the risk of pathogen transmission ^[10][31] and exposure to antibiotic residues ^[30][49], it is recommended that waste milk should not be fed to pre-weaned calves. However, it appears to remain a common practice in the dairy industry; in a nationwide study conducted in the United States, it was found that 40.1% of calves were fed whole or waste milk ^[35][54]. Whereas, a smaller study conducted in Chile found that 51.7% of the calves that were included were fed unpasteurized waste milk ^[36][55].

However, the term waste milk is often used ambiguously, without clarifying its composition, i.e., transition milk or mastitic milk. It may be helpful to refer to these two types of unsalable milk separately, since they are likely different in composition and the effect that they will have on calves. Transition milk is the milk produced by the cow on the second to sixth milking after producing colostrum. Transition milk is unique in that it differs from colostrum and mature milk, containing an intermediary amount of bioactive compounds ^[8][29], such as of sialylated oligosaccharides ^[37][56] and insulin-like growth factor 1 ^[38][57]. Transition milk may be beneficial in assisting with the early development and maturation of the GIT; however, there is currently no evidence to support this idea ^[8][29].

There is a lot of variation in calf nutritional management between different farms; differences are usually present in the feed composition and plane of nutrition ^[39][58]. Traditionally, calves have been fed according to a restrictive milk feeding regime, which allows only the daily provision of restricted amounts of whole milk or milk replacer solids (10% of birth weight) ^[40][59]. The reasoning is that if calves consume less milk, they will consume more grain and forage, thereby promoting earlier rumen development and greater post-weaning growth ^[41][60]. The development of the rumen is an important part of the GIT maturation in calves and is considered to be important for upholding good welfare ^[42][61]. However, it has been found that the consumption of solid feed in the first three weeks of life is negligible and that digestion may be impaired due to an underdeveloped rumen ^[43][14]. This may put calves in restrictive feeding regimes at risk of being underfed. Upon reviewing literature on calf feeding strategies, Khan, et al. ^[41][60] found that in order to improve calf performance and welfare, it may be more beneficial to provide milk volume at 20% of their body weight per day (dry solids at 2% of their body weight per day).

Underfeeding calves is likely to infringe directly on welfare by imposing distress due to hunger ^[44][62]. The presence of stress may enhance or suppress immunity. Enhanced immunity is likely to occur in the presence of a short-term acute stressor. Whereas prolonged chronic stress may induce a continuous glucocorticoid release, which may depress the activity of the immune system and increase disease susceptibility ^[45][63]. It is unclear whether or not restrictive feeding would illicit an acute or chronic stress response in calves. However, it has been found that calves receiving lower planes of milk during the pre-weaning phase have elevated neutrophil L-selectin protein concentrations, which may indicate that their immune system was more active ^[46][64]. It is speculated that the increased activity of the immune system was related to non-nutritive suckling and exposure to environmental microorganisms as opposed to the presence of stress ^[46][64].

Feeding higher planes of milk has been shown to positively improve growth during the pre-weaning phase ^[47][65]. However, some calves have been shown to have a loose fecal consistency ^[48][66]. In another study, calves that were provided with non-restricted quantities of milk also appeared to have a loose fecal consistency; however, no difference in fecal dry matter between the calves on a restrictive and non-restrictive diet was observed ^[49][67]. It is possible that calves receiving more milk only appeared to have loose feces, due to the greater provision of fluids ^[50][68], but did not in fact have any GI infection. The authors suggest that fecal scores cannot be used alone in determining the status of GI health. When implementing an intensive milk feeding protocol for the purpose of promoting early growth, dietary protein and energy should be taken into consideration. It may not be appropriate to simply increase the volume of a conventional milk replacer meant for a restrictive milk feeding protocol, because it might result in insufficient protein for lean tissue growth and the excess energy, which may be converted to fat ^[39][58].

The prevention of calf morbidity and mortality should start with the implementation of a nutritional management program that ensures the successful acquisition of passive immunity, prevents excessive exposure to pathogens, minimizes disturbances in the gastrointestinal (GI) microbiota, and promotes satiety by meeting the appropriate nutritional requirements. If these milestones are achieved, it is likely to promote calf welfare by improving health, providing adequate nutrition, encouraging natural feeding behavior, and reducing the occurrence of distress. There are additional management strategies that may be utilized to further assist in controlling or preventing morbidity and mortality. This may include a sound antibiotic treatment program for infectious diseases and the administration of DFMs to further aid in preventing pathogen overgrowth and microbial disturbances.