Ruminant gastric anatomy and physiology undergo significant changes during early development. Calves are born with their abomasum as the only functional part of the four-stomach system, with the reticulorumen very undeveloped. The rumen needs to change both structurally and functionally as the calf grows. In addition, early weaning is desired in dairy calves because pre-weaning calf growth is expensive in both feed and management. For successful economic weaning, early rumen development is necessary and has been studied extensively.
Mechanism of rumen development
Scientists have explored various dietary and management regimens to accelerate early calf rumen development. Rumen structural development depends on muscular growth, resulting in an overall increase in size. It can be achieved by physical stimulation from feeds in the rumen adding weight and volume. Even inert materials, like sawdust and plastic, have been inserted in the rumen and found to improve muscular development, showing that it is the weight of material in the rumen that enhances muscle development (Flatt et al., 1958). However, functional development is characterized by papillary growth and microbial colonization. This depends on early consumption of nutrient-containing concentrates (calf starter) by neonatal ruminants. Rumen microbial fermentation of starch produces volatile fatty acids (VFA), primarily butyrate and propionate (Stobo et al., 1966), that stimulate rumen papillae development (Sander et al., 1959).
In the first 2 to 3 weeks of a calf’s life, milk remains the primary source of nutrients and solid feed intake lags. Milk bypasses the rumen via the esophageal groove and has little to no effect on rumen development other than increasing the overall size of the calf and therefore its rumen size, in general, can be impacted. Ultimately, it is the quantity of liquid (milk/milk replacer) feeding that dictates the calf’s solid feed intake (Terré et al., 2007). When feeding high levels of liquid feeds, solid feed intake is suppressed (Huuskonen and Khalili, 2008). This reduction in solid feed intake delays functional rumen development and weaning. For that reason, liquid feeding must be limited in early weaning programs. However, this is a compromised solution; reduced liquid feeding means a reduction in maximum possible growth and body weight gain in the pre-weaning phase. On the other hand, if ad libitum liquid feeding is practiced with early weaning, calves will undergo severe weaning stress (Jasper and Weary, 2002) and reduced growth rates as their rumens are not sufficiently developed to adapt to solid feed at weaning. This often leads to the loss of the benefits of increased liquid feeding in the pre-weaning phase.
To achieve optimum body growth and weight gain in the pre-weaning phase with increased liquid feeding and at the same time achieve significant rumen development, there needs to be a mechanism to provide sufficient stimulation for rumen development.
Butyrate can be a possible solution
Rumen VFA provide 70% of the total energy needs of a ruminant. The three most abundant VFA produced in the rumen are acetate, propionate, and butyrate. Acetate is primarily needed for peripheral energy, and in the adult cow part of it is incorporated into milk fat. Propionate is used to produce glucose for energy in the liver. However, the role of butyrate for the ruminant is quite different. More than 80% of butyrate produced in the rumen is metabolized to ketone bodies (BHB and acetoacetate) before being transported to the rest of the body for energy needs. It also has an alternative pathway to convert it directly to available energy for the rumen, and therefore it is of primary importance for the development of the rumen.
Since the 1950s, butyrate has been shown to be a key component in rumen papillae development (Sander et al., 1959). In the last fifteen years, different researchers have investigated the effects of butyrate on calf rumen and intestinal development and performance by adding it to milk or milk replacer. It has also been studied in calf starter.
The type of liquid feeding, calf age, and inclusion rate have been the key factors determining the effectiveness of adding butyrate to calf diets. The type of liquid feed being fed is an important determinant for the inclusion rate of dietary butyrate. This is because feeds may naturally contain some butyrate. Colostrum has the greatest butyrate concentration (approximately 2.1% of DM), followed by milk (approximately 1.2% of DM), whereas milk replacer has the least butyrate quantity depending upon the source and amount of fat in it (Górka et al., 2018). The addition of butyrate to milk replacer has compensated for its butyrate deficiency and has shown positive effects on rumen and intestinal development (Niwińska et al., 2017; Górka et al., 2018). A 0.3% inclusion rate of butyrate (DM basis) has been studied extensively in milk replacers. However, the fat sources and their quality in different milk replacers are highly variable, making it difficult to estimate the exact amount of naturally occurring butyrate in milk replacers.
Calf age is another important factor determining the inclusion of butyrate. Feeding butyrate to young calves has a beneficial effect on gastrointestinal development and performance (Niwińska et al., 2017). When fed within the first week of life, butyrate increased rumen papillae length, intestinal development, pancreatic secretions, and nutrient digestibility, thereby improving average daily gain. Addition of butyrate to the calf diet has minimal impact once starter intake is sufficient to produce butyrate via rumen fermentation (Górka et al., 2018; Niwińska et al., 2017).
In the pre-weaning phase, diet has the greatest influence on the amount of butyrate available to the gastrointestinal tract of calves. Its production within the calf rumen relies on the intake of calf starter, which is negatively affected by the quantity of liquid feed (Gelsinger et al., 2016). Apart from quantity, the source of liquid feed is another critical factor determining butyrate availability in young calves. It is a natural constituent of milk, while highly variable in milk replacers.
In pre-weaning calves where the rumen is still developing, butyrate can also accelerate small intestinal development, which is critical for nutrient absorption and overall development of calves. Guilloteau et al. (2010) evaluated the effect of butyrate on pancreatic secretions and digestibility of soybean protein in milk replacers. Butyrate increased pancreatic secretions, particularly of enzymes chymotrypsin and lipase, thereby improving dry matter and nitrogen digestibility of the milk replacer. Increased lipase secretion following butyrate supplementation can help calves digest high-fat milk replacers and protect them from scours. Further, Guilloteau et al. (2009) compared supplementation of butyrate against the antibiotic and growth promoter flavomycin in calf milk replacer. In this study, butyrate improved feed efficiency and body weight gains in calves by stimulating small intestine development, indicated by an improvement in villi length. Further, Górka et al. (2011a,b; 2014), evaluated the effect of butyrate addition in milk replacer and calf starter on rumen-reticular and small intestinal development for 3 weeks in the pre-weaning phase. Butyrate addition to both milk replacer and calf starter improved reticulorumen weights and stimulated rumen papillae development. Butyrate addition to milk replacer had more pronounced effects on small intestine development, improving cell growth, decreasing cell turnover, and stimulating enzyme secretions in the small intestine. In pre-weaning calves, butyrate addition to the starter improved intake and reduced scouring and treatment with electrolytes. Moreover, butyrate addition to milk replacer improved average daily gains in pre-weaning calves.
Therefore, we can add butyrate to the liquid feed in early weaning programs, especially when calves are on a high milk replacer feeding regimen and are consuming very little calf starter. This will accelerate rumen and intestinal development, thereby improving feed efficiency and protecting calves against stresses from weaning and diarrhea.
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