Russ Daly

Weaning is the most stressful event in a calf's life. If calves can weather this stress unscathed, they have cleared a major hurdle to a productive future in the feedlot or as a replacement in the breeding herd, explained Russ Daly, Associate Professor and Extension Veterinarian at South Dakota State University.

"Prolonged stress in a calf's life results in elevated levels of the stress hormone, cortisol, in his bloodstream. High cortisol levels affect many parts of the immune system," Daly said. "In particular, lymphocyte function becomes weaker. Lymphocytes are the white blood cells that produce antibodies or kill infected body cells. Also, the activity of lung macrophages – the white blood cells that engulf germs in the depths of the lungs – is reduced. The immune response shifts away from actions that combat viral infections."
In other words, Daly said a calf undergoing prolonged stress is now more susceptible to bacteria and viruses that previously were not problems. Add in the effects of low energy intake and dehydration due to feed and water disruptions, and the physical conditions are just right for illnesses like shipping fever (Bovine Respiratory Disease Complex).
Preparing a calf for weaning can therefore be broken into two different goals: reducing stress and preparing the immune system for what stress might still occur.
Stress reduction: While stress reduction at weaning is usually thought of in terms of the actual weaning process itself, how the calf is treated pre-weaning can also help. Making sure stressful procedures are completed well before weaning is an example. "Three to four weeks prior to weaning is a good time to work cattle in preparation for weaning. This allows the calf to recover from stressful procedures before the next stress – weaning – occurs," Daly said.
If not yet done, bull calves should be castrated at pre-weaning processing.   
"The calf benefits the most when castration is performed as early as possible: shortly after birth or at branding time is better than waiting until pre-weaning," he said.
Likewise, Daly said if calves need to be dehorned, it should be done at pre-weaning processing time. This  allows enough time for healing prior to weaning. In addition, the stress of the procedures is diminished when the calf can reunite with his mother.
Castration and dehorning are less stressful at pre-weaning time compared to at weaning.
Daly explained that when calves are exposed to more than one stressful event at a time, cortisol levels are higher and take longer to recover. He added that an increasing number of veterinarians are addressing pain relief and prevention as additional measures to combat stress. Lidocaine nerve blocks and anti-inflammatory medications in conjunction with castration and dehorning are examples.
Bolster the immune system: "Even when we do everything in our power to eliminate stress on calves, these animals will still face some stress and immune suppression in the days to come," he said. For those reasons, it's necessary to bolster the immune system through vaccinations.
The three-to-four week interval prior to weaning will afford calves enough time to respond to vaccines such as those against respiratory viruses (e.g. IBR, BRSV, BVDV, and PI-3) and bacteria (e.g. Mannheimia hemolytica). This is especially true when calves have had previous vaccinations (at branding or turnout time, for example). When a calf sees a vaccine for the second time (compared to the first time), Daly said, the immune response is quicker and stronger. If pre-weaning processing is the time of first vaccination, it may be necessary to provide a booster dose around the time of weaning.
The choice of pre-weaning vaccines should be discussed with a veterinarian. Some modified-live vaccines will caution against vaccinating calves nursing pregnant mothers if the mothers have not been recently vaccinated. Every operation and situation is different, so veterinary input is important.
The timing of pre-weaning processing may also be good for other procedures such as implanting and de-worming, especially when calves will be weaned and backgrounded at home. Regardless of whether calves will be kept or marketed, reducing stress and preparing the immune system prior to weaning are things that can be controlled and will result in a calf's healthy transition through weaning.

SOURCE: | igrow