Symposium review: Macronutrient metabolism in the growing calf

Abstract

Recent interest in increasing rates of body weight gain in heifer calves before weaning is based on the promise of an increase in milk production during first lactation. This increase is usually realized by increasing milk or milk replacer intake, delaying the onset of rumen development. Simultaneously feeding liquids and solid feeds brings about new challenges. Macronutrient metabolism in growing calves is reviewed, combining literature from heifer and veal calves with the objective to provide insights useful for developing novel feeding strategies. Growing calves are not efficiently retaining digested N when compared with other growing species. Energy and protein appear to be simultaneously limiting growth. With the possible exception of very young calves, low responses to incremental intakes of AA indicate that the limiting AA rarely explains the low efficiency of N utilization. Nonetheless, there are indications that disproportionate oxidation of AA as a result of AA imbalance may occur, notably in splanchnic tissues. Long-chain fatty acids, absorbed from the milk or calf milk replacer (CMR) are preferentially deposited as body fat, but this strongly depends on the need for ATP, fueled by the oxidation of carbohydrates. Hence, fatty acid oxidation typically decreases with an increased feeding level. Insulin sensitivity in calves is quite high at birth, but decreases independent of feeding strategy in early age to very low levels when compared with other species. Even though changes in insulin sensitivity may be provoked by early life nutrition, these effects are small and rather transient. In heavy calves, insulin sensitivity is invariably low. Large effects of dietary treatments on postprandial glucose and insulin responses, as often observed, are unlikely to be caused by differences in insulin sensitivity. Unlike in pigs, de novo fatty acid synthesis is not a significant route of disposal of glucose absorbed from the intestinal tract. Instead, high lactate fluxes in milk-fed calves suggest this may be an important route of disposal. When combining the feeding of milk or CMR with solid feeds, estimation of the contribution of the individual ration components is difficult, and interactions inside the gastrointestinal tract complicate the estimation of their feeding value. There are indications in veal calves that use of nutrients absorbed from a CMR is not dependent on the level of intake of solid feeds.

Effects of exchanging lactose for fat in milk replacer on ad libitum feed intake and growth performance in dairy calves

“…Protein intake was numerically lower (data not shown) for calves provided with an HF CMR compared with calves provided with an HL CMR, and a lower plasma urea was observed for HF at the end of periods 2 and 4. Unlike other species, both protein and energy are always limiting growth in calves (Gerrits, 2019). At a similar energy intake this reduced protein intake did not result in a lower ADG, yet animals might have responded to higher protein levels; in Bartlett et al (2006), higher CP levels were required for animals on high feed intake.…”

Invited Review: Nutritional regulation of gut function in dairy calves: From colostrum to weaning

“…There is concern that feeding more milk, especially when calves are only fed twice a day, may lead to abomasal ulcers and a decrease in insulin sensitivity in milk-fed calves (Berends et al, 2015); however, it was demonstrated that insulin sensitivity was not altered in calves fed elevated planes of milk at different meal sizes and frequencies (4 L/meal 2 times or 2 L/ meal 4 times of 150 g of MR powder per L; MacPherson et al, 2019). Previous studies have shown that veal calves fed large amounts of MR develop insulin sensitivity (Hugi et al, 1998;Blum and Hammon, 1999), but this decrease in sensitivity appears to be a natural progression as calves age (Gerrits, 2019). A study by Bach et al (2013) also investigated the effect of plane of nutrition (high vs. low) on insulin response to high plasma glucose and found that all calves were able to control glycemia.…”

Gradual weaning does not improve performance for calves with low starter intake at the beginning of the weaning process

“…Calves with low starter intake had lower plasma concentrations of glucose at the weaning day, probably due to the interruption of milk replacer supply and the fact that they consumed less solid feed. Lower starter intake suggests decreased rumen development, which consequently reduces production of glucose precursors for gluconeogenesis (Gerrits, 2019). When calves have an underdeveloped rumen, the glucose level depends on the liquid diet; therefore, weaning resulted in reduced circulating glucose.…”