During the first few weeks of life, dairy calves are extremely susceptible to both enteric (intestinal) diseases as well as mortality. In the United States, the national mortality rate for heifers from 2 days old to weaning hovers between about 8% and 11%. And scours accounts for approximately 60% of all pre-weaned deaths. In weaned calves, respiratory disease is quite common, at close to 2%. Such high morbidity and mortality rates indicate that calf management in terms of colostrum and liquid feed, nutritional plane, and the possible use of prebiotics, probiotics and other forms of supplementation, need further examination. In his 2015 paper exploring various dietary strategies to improve dairy calf health, Dr. Michael Ballou from Texas Tech University sought to review where we are now, and what needs to be done in the near future to ensure better calf health.
What makes calves so susceptible to gastrointestinal disease, in particular?
While much of the gastrointestinal (GI) immune system begins to develop during gestation, it's only after birth that maturation of these barriers occurs. Indeed, Dr. Ballou points out that the GI tract of calves undergoes rapid maturation over the first few weeks of life. While there are several structures in place that work together to create a competent immune system in the gastrointestinal (GI) tract of calves including physical barriers, chemical and immunological barriers, and microbial barriers, an increased risk of infectious disease occurs if there is a defect in any one of these components.
"Gut closure," or rapid intestinal maturation, contributes to the physical barrier. But, many things can compromise this physical barrier, including changes in the mucus layer and reduced intestinal motility. During the early post-natal period, chemical and immunological barriers can also be compromised. While antibodies from colostrum offer some immediate protection from enteric pathogens, the fact is that most passively derived antibodies have relatively short half-lives. Therefore, until they develop their own active immunity, young calves will always have an increased infectious disease risk. The microbial barrier of the calves' GI tract is also compromised during early life as they switch from the relatively sterile environment in utero to being exposed to a greater diversity and volume of microorganisms. With many potential GI immune system defence issues during their neo-natal lives, calves are at an increased risk for enteric disease.
It should be noted that in Ballou's work, different studies were evaluated which were conducted at different facilities and the hygiene conditions and colostrum management for each facility were not described. Sanitation and colostrum administration have a strong correlation with morbidity and mortality, as well as an interaction with plane of nutrition, so some of the differences observed below might be explained by variations in hygiene. Indeed, keeping a clean calving facility and moving the calf to a clean area right after birth is truly the first step to a successful start.
While the bioactive compounds in colostrum and transition milk do indeed directly influence the GI immune system's maturation, Ballou points out that supplementation with prebiotics, probiotics and hyper-immunized egg protein have been shown to help improve enteric disease resistance. In his own 2010 study, he evaluated supplementing a blend of all three immediately after birth through to 21 days of age with a total of 90 Holstein calves in two groups, a prophylactic group and a control group. The prophylactic group had significantly lower incidences of scours (25.0% vs. 51.1%). Ballou believes that such supplementation could be a viable alternative to the use of metaphylactic antibiotics.
Does upping the plane of nutrition help?
Ballou on his own, and in collaboration with other researchers, has done numerous studies over the better part of the last decade to understand how the level of pre-weaning nutrition influences both resistance to infectious disease and leukocyte response during the pre- and immediate post-weaning periods. While his results indicate mixed responses during the pre-weaning period (with some improvement in enteric and respiratory health reported), and while some of the differences may also possibly be attributed to differences in colostrum supplementation, upping the plane of nutrition during the pre-weaned period does seem to influence both disease resistance and leukocyte responses after calves are weaned, despite potential differences in weaning strategy. One example from his body of research involves Jersey bull calves fed a higher plane of fluid nutrition pre-weaning going on to have improved neutrophil and whole blood E. coli killing capacities after weaning, compared to Jersey calves fed more conventional, low planes of nutrition. Ballou's group's recent body of research does indeed show that upping the level of pre-weaned liquid nutrition can and does influence the health of dairy calves within one month of weaning.
While Dr. Ballou calls for further study and more data to be collected to see if the results garnered in the first month post-weaning persist as long-term results, he does find that feeding a greater plane of nutrition pre-weaning improves disease resistance and leukocyte responses that extend beyond weaning.