The success of a calf rearing program is dependent on many management factors. The single most important factor in determining calf health and survival is to provide every calf with an early and adequate feeding of high-quality colostrum. Colostrum provides the newborn calf with the necessary antibodies for early immune protection along with providing an important source of nutrients. In addition, beneficial growth factors and hormones are found in colostrum.

Defining Colostrum
True colostrum is the very first secretion by the cow's mammary gland after parturition. Milk from subsequent milkings during the first three days after parturition is called transition milk. Table 1 illustrates the difference in the composition of colostrum, transition milk, and whole milk. Colostrum contains more total solids, fat, solids non-fat, total protein and, more importantly, a greater concentration of immunoglobulins (Ig). Furthermore, colostrum contains a higher concentration of vitamins and minerals, particularly zinc, vitamins A, E, and B₁₂. Without a doubt, true colostrum provides a much higher plane of nutrition to the newborn calf than either transition milk or whole milk.

The Colostrum–Immunity Link
A direct link between the nutritional importance of colostral consumption to improved calf health or growth has not been determined, but many positive relationships between serum Ig, growth, and health have been reported. The nutritional benefit from feeding colostrum is often overlooked. This is particularly true when considering the calf's requirement for energy. The newborn calf's body contains approximately 3% fat and 180 grams of glycogen. These energy sources will be utilized within 18 hours of life, if no feed is consumed. Colostrum provides high levels of fat and lactose, critical for maintenance of body temperature. This is crucial during periods of cold weather.

   The primary requirement for feeding colostrum is disease protection. The newborn calf is born without adequate immunity and is entirely dependent on the passive transfer of maternal antibodies from colostrum at birth. This process is called passive immunity. Passive immunity helps protect the calf until it can develop its own functional immune system.

   Prevention of disease(s) is the key to a successful calf rearing program. Providing the calf with adequate amounts of high-quality colostrum immediately after birth allows the calf the best opportunity to achieve passive immunity and, in turn, prevent disease(s). To establish successful passive transfer, a calf requires a minimum of 10 mg/ml or 1,000 mg/dl of IgG. Less than 1,000 mg/dl indicates failure of passive transfer (FPT). Calves that fail to achieve passive transfer are five times more likely to die. The National Animal Health Monitoring System (NAHMS) reported that during the first 21 days of life, 31% of all dairy heifer mortality could be prevented by improved colostrum management.

   Most of the immunoglobulins (Ig) in colostrum are in the IgG class, which are the main contributors to systemic immunity. Colostral IgM is important in early immunity and is most essential in the prevention of septicemia and intestinal immunity to enteric pathogens. Out of 2,177 calves tested, 41% failed to achieve passive transfer of immunity (IgG levels <10 mg/ml) and only 26% achieved an acceptable range of 10-20 mg/ml IgG. Approximately 33% of the calves had IgG levels greater than 20 mg/ml.
 
   The concentration of IgG in the blood of calves is clearly associated with calf survival and health. Figure 1 compares the survivability of calves with adequate and inadequate serum IgG levels. A higher percentage of the calves survived during the first 56 days of life when they had achieved an IgG concentration of 10 mg/ml or greater, compared to calves with less than 10 mg/ml. Low Ig levels do not necessarily lead to mortality or illness, just as high Ig concentrations do not guarantee survival. Other environmental and management factors can contribute significantly to calf health.

Colostrum Absorption
Colostrum timing, quality, and quantity are the three main factors affecting passive transfer of immunity. The period of time between birth and receiving colostrum is crucial to a good colostrum program. The calf's ability to absorb colostral Ig decreases rapidly following birth. Within three hours after birth, the calf's ability to absorb colostrum Ig falls below 70% (see Figure 2). The absorption capacity of the calf falls below 50% at 12 hours and for all practical purposes, non-existent at 24 hours. The sooner a calf is provided colostrum, the greater the absorption, the emphasis on quality and quantity is then not as crucial.

   The small intestine of the calf has the capability to absorb large proteins, such as IgG during the first 24-36 hours of life. Unfortunately, this also can include bacteria. Researchers have found that calves which did not receive colostrum, readily absorbed E. coli into the intestinal wall due to bacterial challenge. However, calves that received colostrum prior to or with the bacterial challenge of E. coli, did not absorb E. coli. Providing colostrum as soon as possible after birth ensures optimum IgG absorption and prevents the absorption of large bacterial proteins. Early consumption of colostrum is also important for prevention of septicemia.

Dystocia
   Severe dystocia decreases the ability of the calf to absorb colostral Ig. Calves that have experienced severe dystocia have significantly lower concentrations of Ig in the plasma compared to normal births when fed equal amounts of colostrum. In a study of 658 calves born with some degree of dystocia, 25% died within 48 hours. Prolonged births cause acidosis and reduced tissue oxygen level (hypoxia). Extra care and colostrum are needed to minimize death loss in these situations.

Colostrum Intake and Absorption
As stated earlier, a serum or plasma IgG concentration of 10 mg/ml or greater has been determined to indicate successful passive transfer. To achieve adequate serum levels of IgG, a general "thumb rule" is a calf needs to consume 100 grams (g) of IgG. How many quarts or liters would be required to meet the 100 g IgG level? The amount of colostrum required is dependent on the quality of colostrum being fed. Colostrum is classified poor if it contains <22 mg/ml of IgG, moderate if between 22 and 50 mg/ml, and excellent if the level is >50 mg/ml of IgG.

   Figure 3 illustrates the distribution of IgG₁ concentrations from cows on a single dairy farm. Most of the colostrum would be classified as moderate, with very few ranked as excellent. Calves weighing approximately 88 lb and fed colostrum containing 60 mg/ml of IgG₁ (excellent quality) would need to consume 1.83 quarts to supply the 100 g of IgG needed for successful passive transfer. However, the calf that is fed a lower quality colostrum, containing only 35 mg/ml, would need to consume 3.14 quarts for successful passive transfer. Calves that are larger and/or if absorptive efficiency is less, the amount of colostrum required would be even greater.
   Traditional recommendations indicate two quarts of colostrum within the first few hours are needed, followed by an additional two quarts within the next 12 hours. However, this may not provide adequate IgG to prevent disease or mortality if the colostrum is of low quality or not fed soon enough after birth. Figure 4 shows the percentage of colostrums estimated to provide 100 g of IgG when fed at three different volumes based on the variation of colostral IgG₁ levels shown in Figure 3. The mean IgG₁ content was 48.2 mg/ml with a standard deviation of 21.9 mg/ml. This data would suggest that calves fed only two or three quarts of colostrum at birth would have a high probability for failure of passive transfer.

   Recent studies from the University of Illinois have shown the benefit of calves consuming large amounts (four quarts) of high-quality colostrum. Calves receiving four quarts shortly after birth had higher serum IgG levels than calves fed two quarts, followed by two quarts at six and 12 hours of age. It was interesting, however, when low-quality colostrum was fed, calves fed two quarts at birth, six and 12 hours of age, recorded significantly higher IgG concentrations. This would suggest an increase in colostral absorption by maintaining a more constant flow of colostrum through the digestive tract the first day of life. This may not be practical for most producers, but it would definitely be beneficial, especially during stressful conditions.

Colostrum Quality
The sources of variation among cows within the same herd are not well understood. Age of the animal is one factor. Older cows generally have higher concentrations of IgG than do first calf heifers. It is also known that dry periods which are too short or too long cause variation in IgG concentrations. Exposure of cows to antigens and vaccinations result in production of specific IgG that are transferred into the colostrum from the maternal blood. Adopting an appropriate vaccination program will help increase the transfer of specific Ig to the colostrum and provide immunity to the calf. Beneficial vaccinations may include E. coli, rotavirus, coronavirus, and clostridial enterotoxemia.
   Dietary energy and protein concentrations late in gestation have little effect on the Ig concentration in colostrum, provided these nutrient needs are met. However, providing dry cows with appropriate amounts of macro and micro minerals which includes selenium, along with vitamins A, D, and E, will help ensure adequate levels in the colostrum. Supplementing dry cow diets with vitamin E and selenium are important to udder health at calving.

Measuring Colostrum Quality
Colostrum quality can be estimated by using a colostrometer. The colostrometer enables rapid on-farm assessment of colostrum quality. The colostrometer classifies colostrum quality (Ig level) as:

• Poor (red)–<22 mg/ml.

• Moderate (yellow)– 22 to 50 mg/ml.

• Excellent (green)– >50 mg/ml.

An accurate measure is highly dependent on temperature and should be read at room temperature (68-77°F). Readings below 68°F overestimate the Ig content, while measurements made above 68°F underestimate the colostral Ig concentration. For producers who feed calves in excess of four quarts of colostrum immediately after birth, the colostrometer adds little significance to the colostrum program. However, for the producer feeding only two quarts or less of colostrum, the colostrometer would be beneficial for selecting colostrums that contain 100 mg/ml Ig or greater to ensure passive transfer.
   High-quality colostrum can be stored in a refrigerator for approximately one week without loss of quality. Colostrum can be frozen for over one year without loss of immunity activity. Never thaw colostrum rapidly. Use either warm water or thaw slowly in a microwave. Overheating can denature the Ig and decrease quality.

Supplements
Remember, there are no substitutes for true colostrum. Several products are available which are advertised as replacements for colostrum. These products include spray-dried or freeze-dried colostrum, whey protein concentrate, mixture of whey and dried colostrum, colostrum ultrafiltrates, and purified immunoglobulins. Most of these products contain only 20 g of IgG/liter, so five liters would be required to meet the 100 g necessary to achieve passive transfer of immunity. The efficiency of absorption of these products vary, ranging from a low of 25% to a high of 65%, for <30% average efficiency. In studies that compared these substitutes to colostrum, the serum IgG levels were significantly higher for calves fed the true colostrum and the incidence of disease was dramatically lower. Colostrum substitutes are not recommended to replace colostrum; however, they could be used to supplement low-quality colostrum or when adequate amounts of colostrum are not available.

Summary
Adopting and implementing a colostrum program will not guarantee a healthy calf, but it definitely improves the chances for success. A colostrum program should include the following:

• Feed true colostrum, the very first milk, immediately after calving.

• Feed three to four quarts of high-quality colostrum.

• If only low-quality colostrum is available, feed four or more quarts.

• Feed colostrum frequently to calves that have experienced stress (such as dystocia).

• If possible, feed colostrum often within the first 24 hours after birth.

• Do not let calves nurse cows.

• Use colostrum substitutes only as a supplement to true colostrum.