Meeting Protein Needs of Growing Dairy Calves – Dr. Noah Litherland, Vita Plus
In the calf world, we often think of protein in terms of percentage of milk replacer powder, but a multitude of factors affect the growing calf’s protein needs.
Amino acids are the building blocks of protein and vital to maintenance, growth and structural development. Amino acids – and not protein per se – are thus the required nutrients for calves.
The three primary amino acids for growth are lysine, methionine, and threonine (Hill et al., 2008). Methionine is unique in that it is the first amino acid used when synthesizing a protein, which is a chain of amino acids. Additionally, methionine donates methyl groups, which play a role in gene expression. Cysteine, another amino acid, also is a methyl donor and serves as an antioxidant.
Other amino acids play important roles as well. For example, arginine aids in vasodilation (increases blood flow) in the intestines. Glutamate is used by intestinal cells as a source of energy, and, finally, threonine is needed for synthesis of mucin, which serves as a barrier to pathogens in the small intestine.
A better understanding of dietary amino acid supply is gained by splitting the calf’s first two months into three stages of development, which are associated with considerable changes in gastrointestinal tract function. These three phases include:
- Liquid-feeding phase: Essentially all the nutrient requirements are met by milk or milk replacer
- Transition phase: Both the liquid diet and starter grain contribute to meeting amino acid requirements
- Ruminant phase: The calf derives its nutrients from solid feeds, primarily through microbial fermentation (Figure 1).
Figure 1. Timeline of phase-feeding in nursery calf systems and relative contribution of amino acids (AA) from milk and/or starter grain.
Liquid feed phase
|Weeks 1-3||Weeks 4-7||Week 8|
|AA from milk or milk replacer
(some starter grain toward the end
of week 2)
|AA from milk or milk replacer
|AA from starter|
Amino acid balancing for the milk-fed calf is similar to that of monogastrics. However, over time, rumen development results in production of microbial protein. Microbial protein has an almost perfect amino acid profile for growth.
The majority of amino acid research for dairy calves has focused on supplemental amino acids in milk replacer. For example, Kanjanapruthipong et al. (1998) supplemented methionine, lysine, and threonine to milk replacer with or without soy protein and found milk replacers with plant proteins need amino acid supplementation to compete with milk-based milk replacers. When lysine and methionine were added to 20-percent crude protein milk replacer, average daily gain (ADG) was increased versus 20-percent crude protein milk replacer without added amino acids. Additionally, ADG was equal to calves fed 22-percent crude protein milk replacer without added amino acids (Hill et al., 2007).
The calf industry has steadily moved toward higher milk feeding rates. Let’s take a look at the supply of amino acids by whole milk.
Table 1 estimates amino acid supply by feeding increasing amounts of milk from 1 gallon (4 quarts) per calf per day up to 2.5 gallons (10 quarts) per calf per day. Not surprisingly, the amount of amino acids (grams per calf per day) increases with amount of milk fed.
Table 1. Estimated intake of selected amino acids (g/calf/d) by varying feeding amounts of whole milk containing 12.5% solids and 3.3% milk protein.
|Whole milk||Feeding amount (gal/d)|
What is not apparent in this table is the digestibility of each of these amino acids. What proportion of these amino acids are not absorbed by the animal and contribute to fecal nitrogen loss? It is likely that the amino acid digestibility is related to intake, with higher amounts of amino acids lost in feces when calves are fed a high versus a low plane of nutrition.
Other pieces of the equation are missing as well. What are the amino acid requirements for maintenance and growth? These values will depend on the size of the calf and the rate of growth. Environment likely impacts requirements as well. Calves mounting an immune response will have a greater maintenance requirement for amino acids to synthesize cells associated with battling pathogens and rebuilding damaged tissues.
Overfeeding protein comes with some challenges, including risk of overfeeding some amino acids if essential amino acid requirements are not met. If energy is limiting, then amino acids will be broken down to meet energy requirements. Excess amino acids must be metabolized through the urea cycle and disposed of via urine. Increased protein in the environment contributes nutrients for bacterial growth. A good goal should be to meet amino acid requirements for maintenance and growth without overfeeding crude protein.
We have a tremendous amount to learn in the area of amino acid nutrition in dairy calves. We have observed elegant relationships between the calf’s environment, rate of growth and amino acid supply. To optimize the calf’s protein intake, we need to find a balance in amino acid supply to the calf from milk, milk replacer and starter grain throughout all three feeding phases.
This article was originally written for the May 21, 2015 issue of Agri-View.
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