Improve your grades: Transfer of passive immunity is no longer pass or fail
Drew A. Vermeire
September 13, 2021
The value of adequate colostrum intake to the health and well-being of calves cannot be overstated. Unlike humans, in which babies are born with immune protection to diseases to which the mother has been exposed, calves are born without immune protection because the bovine placenta does not permit transfer from dam to calf through the blood. As a result, calves only receive protection against disease through ingestion of colostrum, which is rich in immunoglobulins.
For maximum benefit, colostrum must be fed to calves within hours of birth. In addition to immunoglobulins (IgG, IgA, IgM), colostrum contains many other bioactive compounds such as leukocytes, cytokines, hormones, growth factors (IGF-1, IGF-2), lactoferrin, lysozyme and an abundance of vitamins and minerals. Colostrum intake also increases blood volume, and it contains twice the solids content of whole milk, making it a rich source of protein and energy.
Measuring and monitoring
Management consultant, educator and author Peter F. Drucker is attributed to saying, “You can’t manage what you don’t measure.” Indeed, if we are to improve our colostrum management, we first must evaluate the current colostrum management. Monitoring how effectively a dairy manages their colostrum requires collecting a blood sample and measuring serum IgG levels or measuring some proxy such as zinc sulfate turbidity, sodium sulfate turbidity or, more recently, serum total protein using a refractometer.
Determining serum IgG levels usually requires laboratory testing such as radial immunodiffusion (RID) testing, which is considered the gold standard. The correlation between serum total protein and serum IgG levels is high in scientific terms (0.80), but the “real world” interpretation is that serum total protein level can predict IgG level about 80% of the time. For most people, the balance of a relatively high cost to test IgG versus inexpensively measuring serum total protein on the farm using a refractometer with either BRIX or specific gravity scale makes on-farm testing very popular. I only recommend digital refractometers, which contain their own light source and are sensitive, repeatable and remarkably inexpensive, costing only a few hundred dollars. They are more accurate and user friendly than handheld optical refractometers.
Pass-or-fail is out and A-B-C-D grades are in!
Recommendations to define whether or not a calf has had adequate colostrum have been revised periodically, but the result has been a pass-or-fail grade. The cutoff has been 10.0 g of IgG per liter of serum with the rationale being research that showed higher mortality in calves with serum IgG levels below 10.0 g per liter and calf survival is higher when serum IgG levels are higher than 10.0 grams per liter.
While the old guidelines served an important purpose, the pass-or-fail system is now being replaced with a newer system of excellent, good, fair and poor. The new system is more like a traditional A-B-C-D grading system with four tiers based on serum IgG levels: "excellent" (greater than 25.0 g/dL), "good" (18.0-24.9 g/dL), "fair" (10.0-17.9 g/dL) and "poor" (less than 10.0 g/dL). The new guidelines were published in 2020 by Lombard et al and are based on research that shows that higher colostrum levels reduce both mortality and treatment rates, increase growth rates, improve feed efficiency, decrease age at first calving and increase milk production in the first and second lactation.
Transfer of passive immunity ‘grade point average’
I believe that the greatest benefit of the new guidelines is the ability to assign grades to each category and calculate a grade point average for the herd in the same way that letter grades are assigned a number in school. Asking what percentage of calves passed and failed in the old system makes it difficult to measure improvement. With a letter grade point value, we can calculate an ongoing grade point average and easily determine whether or not we are making improvements.
What can we do to improve our grades?
The recommended management practice of vaccinating dry cows will improve colostrum quality. While cows exposed to a short photoperiod (eight hours light: 16 hours dark) produced substantially more milk (greater than 7 pounds per day) than cows exposed to a long photoperiod (16 hours light: eight hours dark), there does not appear to be an impact of photoperiod on IgG status in calves.
Several other factors can dramatically impact immunoglobulin uptake in young calves, including heat stress and nutritional factors. It has been shown that heat stress during the dry period not only negatively impacts milk production in the subsequent lactation but also negatively impacts birthweights, growth rates, survival rates and IgG uptake in calves born from cows exposed to heat stress. Many studies have shown that shade, fans and sprinklers are much more effective at keeping cows cool and keeping cows and calves healthy than by providing shade only.
Nutrition of the cow prepartum and of the calf immediately after birth also impacts IgG uptake. Vitamin E status impacts absorption of IgG by the calf. It was observed for many years that serum IgG was higher in calves born during summer than calves born during winter. By feeding supplemental vitamin E during the dry period, we no longer see this seasonal difference in calf IgG levels. Researchers at Oregon State University fed a supranutritional dose of selenium once per week for the last eight weeks of gestation to cows. They found that calves born from cows given selenium had 9% heavier birthweight and 43% higher serum IgG after colostrum feeding. Other studies have shown that selenium added to colostrum increased IgG absorption.
Finally, high levels of bacterial contamination in colostrum are known to reduce colostrum absorption. Pasteurization can also reduce colostrum absorption. The benefits of higher serum absorption of IgG and other colostrum constituents include higher survivability, increased milk production and lower treatment rates, which have profound economic impact on the dairy. Commercially available dried colostrum has consistent high quality and defined IgG levels. These products eliminate the problems of lower absorption due to bacteria or pasteurization. Feeding dried colostrum to calves could provide a mechanism to ensure maximum transfer of IgG. By monitoring serum IgG or total protein levels, dairy farms can take steps to improve their grades and their results!