Optimizing grain processing in calf diets, along with ensuring adequate forage intake, is critical to promoting effective papilla development while maintaining stable fermentation and supporting stable rumen pH.
In dairy calves, the early differentiation of the rumen epithelium and the morphogenesis of the papillae are precisely regulated by changes in gene expression that prepare the rumen for the efficient digestion of solid feed. The rumen initially constitutes only 32% of the total stomach mass, but it rapidly increases in size and surpasses the abomasum, so that by 6 weeks of age it constitutes 63% of the total stomach weight. This growth is supported by an expanded vascular network under the tunica mucosa, which improves the supply of nutrients to the proliferating epithelial cells that are important for rumen growth and function. At the same time, gene-regulated muscular hypertrophy causes thickening of the tunica muscle, which improves rumen motility and the efficiency of food processing. When solid feed intake increases, the tunica mucosa undergoes distinct morphological adaptations with a significant increase in papillae area, length, and width, which increases the capacity for nutrient absorption. which are crucial for adaptation to a solid diet. The basal layer, which is rich in mitochondria, promotes proliferation and thickening of the epithelium and creates electrochemical gradients that are important for the transport of nutrients and electrolytes. The spinous layer actively contributes to rumen metabolism by converting butyrate to β-hydroxybutyrate for energy production.
Optimizing calf starter diets for optimal growth and rumen development requires a careful balance between grain composition, processing methods, and physical form. Grains such as corn, barley, and oats are essential for the calf starter diet because they have a high energy content that is essential for the diet. The starch content and digestibility of grains vary significantly due to differences in biochemical structure, including starch type and protein matrix, which affect the availability of starch for microbial fermentation. Processing methods such as milling, steam flaking, and rolling play an important role, as increasing the surface area through processing can improve digestibility by increasing enzyme availability. However, excessive milling, especially of grains such as barley, can reduce rumen pH and impair rumen function due to rapid fermentation. For example, steam-shelled corn has been shown to increase nutrient digestibility, improve ADG, and maintain rumen pH better than milled corn, a benefit consistently observed in several studies. In comparison, textured starters containing rolled or whole grains stimulate greater chewing and salivation, increase rumen pH, and improve rumen health, while finely pelleted starters without adequate particle size can reduce feed intake and negatively impact rumen fermentation and growth. In studies comparing physical forms, textured diets consistently promote rumen pH, better tissue integrity, better intake, and better weight gain than pelleted diets, especially when formulated with at least 45% whole grains or steam-shelled grains with a particle size greater than 1190 μm. Support adequate chewing and rumination for optimal growth. Maintaining pellet integrity with minimal fines is also critical, as fines can reduce palatability and absorption. Given the multiple interactions between grain type, processing, and form on rumen physiology and calf performance, these factors must be considered when formulating a balanced starter diet that promotes rumen health, energy intake, and growth to achieve optimal calf growth.
Inclusion of forage in the pre-weaning diet supports rumen development but must be balanced to avoid reduced energy intake and growth performance. Studies show that feeding green forage stimulates rumen muscles and minimizes keratinization of rumen papillae, enhances rumination, and increases rumen buffering capacity through saliva, thereby reducing the risk of low rumen pH. Adding forage also improves animal welfare by reducing non-nutritional oral behaviors. However, forage intake poses a challenge in terms of energy density and fermentation patterns, as many forages contain lower NFC and have lower total energy content than grain-based starters. Young calves with limited rumen fermentation capacity and lower NDF digestibility often struggle to utilize forage energy efficiently, which can hinder optimal growth during the challenging pre-weaning period. Forage intake also shifts the rumen fermentation pattern towards acetate production, which buffers the rumen. It reduces the risk of acidosis but stimulates rumen papilla growth less than butyrate or propionate. This shift may limit the amount of papilla growth required for effective nutrient absorption, potentially reducing growth benefits. Therefore, adjusting the forage to concentrate ratio in calf diets is critical to improve rumen fermentation, support rumen growth, and ensure adequate energy intake for healthy growth. In addition, the effects on gut filling should be considered when forage intake, as the volume of gut contents may increase, which if not distinguished from true tissue gain, could be misinterpreted as growth. Studies show that gut filling from forage increases when it constitutes more than 15% of the total feed intake, and alfalfa generally results in greater gut filling than grasses (NASEM, 2021). Failure to account for this difference may result in overestimation of calf growth performance. Furthermore, many growth studies lack accurate measurements of gut filling, often relying on the John and Chandler equation, which may not apply well to young calves. Future research should focus on developing accurate tools to measure gut filling and differentiate it from actual body tissue gain to provide a better understanding of the effects of forage on calf growth and rumen health.
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