MILK REPLACER FORMULATION IMPACTS MORE THAN GROWTH
There is a wide range of milk replacer (MR) macronutrient profiles available on the market. As a producer or nutritionist, the focus is on selecting the right MR to achieve optimal growth rates aligned with the dairy's goals. Most people will usually focus on the fat and protein content within the milk replacer. Another macronutrient that might not be as commonly discussed in this conversation is lactose (energy source).
Growth performance alone may not fully capture how MR composition differences may affect calf health and metabolism. The question remains: How do the macronutrient profiles of these products affect calves from a health perspective? Wilms et al. (2023) evaluated the chemical and physical changes that the calf goes through after consuming a meal, and gut permeability in calves fed MR with different macronutrient profiles compared with a whole milk powder.
The MR treatments included a high-fat MR (25.0% fat, 22.5% protein, 38.6% lactose), a high-lactose MR (18% fat, 22.5% protein,44.6% lactose), a high-protein MR (18.0% fat, 26.0% protein, 41.5% lactose), and a whole milk powder (26.0% fat, 24.5% protein, 38.0% lactose). Holstein bull calves were randomly assigned to one of the four liquid diets over a 28-day period. Calves were offered water ad libitum, but no solid feed was offered during the 4-week treatment period. Milk was reconstituted to 13.5% solids and fed at a volume of 3 L per feeding, three times daily.
The researchers evaluated feed intake, body weight (BW), average daily gain (ADG), fecal scores, total tract digestibility (TTD), markers of gut permeability, and abomasal emptying rate. Urine samples were collected to measure markers administered to assess intestinal permeability. Blood samples were collected to measure non-esterified fatty acids (NEFA), alkaline phosphatase (ALP), total triglycerides, insulin, glucose, adiponectin, and leptin.
There were no differences in BW or ADG across treatment groups over the entire study period. This finding is important as it shows how different macronutrient profiles can lead to similar outcomes of growth performance, while producing different physiological responses. Therefore, evaluating a MR program solely based on BW or ADG goals may overlook differences in how calves digest, absorb, and metabolize nutrients.
Despite the similarities in growth, differences were observed in gastrointestinal function and metabolism. Gut permeability was the highest in the calves fed the whole milk powder, then the high-protein MR, and the lowest in the high-lactose MR treatment group. Calves fed whole milk powder and high-protein MR had the slowest abomasal emptying rates, likely due to a higher protein and casein content. A slower abomasal emptying rate would influence the rate at which nutrients are delivered to the small intestine and affect nutrient absorption and metabolism.
The authors evaluated gastrointestinal health through the percentage of days with diarrhea. One examiner used a three-level scoring system to determine diarrhea, 0 = normal feces, 1 = wet feces, 2 = watery feces. This was lower in the first week for calves fed the high-protein MR and higher in the third week for calves fed the whole milk powder compared with the other treatment groups. The urinary recovery of permeability markers was greater for the calves on whole milk powder and high-protein MR, indicating a possible decrease in intestinal barrier function. There were no differences across the treatment groups for the intestinal permeability index, indicating that there was no damage to the intestinal mucosa. Instead, these differences may be explained by protein digestion. The whole milk powder treatment group had a lower apparent TTD of dry matter (DM), which could be due to a greater amount of undigested protein reaching the large intestine, consequently leading to higher gut permeability.
Practically, on farm it’s important to consider intestinal permeability because of the susceptibility to disease. Calves with higher permeability may be more likely to be challenged with enteric pathogens. However, it is important to consider that looser feces doesn’t necessarily mean a disease state. The authors stated that the number of therapeutic interventions was higher in the high-fat MR treatment for diarrhea and respiratory disease. They attributed this due to the high-fat MR treatment having a lower blood IgG concentration than the other treatment groups (15.6 g/L, versus 19-20 g/L).
More liquid in manure doesn’t always mean the need for treatment. This has a lot to do with the increase in intestinal permeability, hypersecretion of water, and malabsorption of water and nutrients due to epithelial damage in the intestines. The differences observed here may predispose calves to certain situations, but it’s not a guarantee. In this study there were no differences in growth, so the macronutrient composition probably wasn’t influencing growth and the differences in permeability did not impair growth of the calves. Depending on the environment and stress situations on other farms, the outcomes could be different and influence some health outcomes.
Therefore, it’s necessary to consider all aspects of the dairy, including the environment that the calf is exposed to, feeding and health related protocols when selecting the appropriate MR formulation to meet the growth and health related goals of the dairy.
— Victoria Blaney
