Dry cows & Heat Stress
Dry cows have received little attention when it comes to managing heat stress.
Autumn and year-round calving farms that dry cows off over the hot months should be doing more to ensure their dry cows stay cool. Heat stress during pregnancy can have consequences on calf health and cause far-reaching health problems later in life.
A growing body of evidence demonstrates that dry herds in heat stress have reduced milk yields on their next lactation, reduced calf weight and a greater risk of health problems.
Reduced milk in next lactation
Cows generate less metabolic heat when dry than when lactating and have a higher upper critical temperature. This may contribute to the belief around heat stress being a lesser concern. However, research studies consistently show that when cows experience heat stress during late pregnancy they produce less milk in the next lactation. (See Figure 1).
Figure 1. Nine research studies examined the effect of heat stress and cooling during the entire dry period on milk production in the following lactation. (Source: Tao and Dahl, 2013)
Researchers believe milk yield reduction is due to impaired blood flow through the dry cow's udder over the last two months of pregnancy. The udder is rapidly growing over this period in preparation for the next lactation. The impaired blood flow stunts udder growth, which means they possess fewer functioning mammary cells with reduced secretory capacity. As a result, they will produce less milk and become disadvantaged while calving.
smaller, lighter calves
Studies in many species of animals show the conditions that offspring are subjected to while still in the uterus affect their lifetime health and performance. When the foetal calf's body temperature is increased, as it is when its mother experiences heat stress during late pregnancy, it appears that this negatively affects the calf's metabolism and gene expression, pre-programming it for sub-optimal health and performance.
When cows suffer heat stress during their dry period, they have smaller placentas with reduced blood flow through the uterus and umbilical cord. The calves tend to be born several days earlier and a few kilograms lighter than calves of cows that kept cool during their dry period.
greater health risks at calving
When cows experience heat stress during late pregnancy it suppresses their immune system for many weeks. Studies have shown the neutrophils of heat stressed cows become less aggressive against bacteria. Neutrophils are the white blood cells acting as the first line of defence against pathogens. The level of circulating antibodies is lower in cows that remain cool during late pregnancy e.g. cows that calve in spring.
Cows that experience heat stress during late pregnancy may therefore be at greater risk of health problems such as mastitis and retained foetal membranes around calving (when their immune function is already naturally suppressed).
calves less productive and fertile
Calves born go on to be less healthy, fertile and productive in first lactation. Calves born to cows heat stressed during the dry period have been shown to be less able to absorb maternal antibodies from the first colostrum consumed soon after birth. This results in lower blood maternal antibody levels than in calves from dry cows that don't experience heat stress.
These calves cell-mediated immune function may also be compromised. This results in them being more susceptible to infections that commonly occur pre-weaning. The ultimate outcome is poor growth rates, higher levels of illness and higher mortality rates.
Researchers have recently found that calves whose dams are heat stressed during pregnancy go on to become maiden heifers that are less fertile. This means a heifer is produced that has both lower production and more difficulty getting back in calf.
In the short term, protect dry cows from heat stress:
- if existing natural shade on the home farm or support block can't provide 4 square metres of shade per cow at midday, then find alternative paddocks
- portable paddock shade structures or a permanent shade structure are also options.
In the longer term, the farm plan should be reviewed for:
- more established tree belts along dry cow paddocks and springer paddocks
- adjust the farm's calving system could to reduce the number of cows dried off over the hot months of the year.