DAIRYFARMERS across mainland Australia use shade and evaporative cooling to keep their milking herds cool over the hot months.
However, dry cows have received relatively little attention when it comes to managing heat stress.
Should autumn and year-round calving farms, that dry cows off over the hot months of the year, be doing more to ensure their dry cows stay cool? Cows generate less metabolic heat when dry than when lactating and have a higher upper critical temperature.
So, farmers could presume that dry cows are far less of a concern than the milking herd when it comes to heat stress.
However, research studies consistently show that if cows experience heat stress during late pregnancy (i.e.during their dry period), they produce less milk in the next lactation. (See Figure 1).
Why does this carry-over effect on milk yield occur? Researchers believe it is due to impairment of blood flow through the dry cow’s udder during the last two months of pregnancy, when the udder is growing and developing rapidly in preparation for the onset of the next lactation.
Other metabolic/hormonal mechanisms may also contribute to the carryover effect on milk production.
However, further research is required.
Not only does heat stress during late pregnancy affect development of the cow’s udder, it also affects development of her placenta, on which the unborn calf is totally dependent for its supply of oxygen and nutrients.
Cows that suffer heat stress during their dry period have smaller placentas, with reduced blood flow through the uterus and umbilical cord.
As a result, calves of these cows tend to be born several days earlier and several kilograms lighter than calves of cows that kept cool during their dry period.
When cows experience heat stress during late pregnancy it suppresses their immune system for many weeks.
Studies have shown that the neutrophils (the white blood cells which are the first line of defence against pathogens) of heat-stressed cows become less aggressive against bacteria.
The level of circulating antibodies is lower than 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).
Their calves go on to be less healthy, fertile and productive in first lactation.
Studies in many species of animals (including humans) show that 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 this negatively affects the calf’s metabolism and gene expression, pre-programming it for suboptimal health and performance.
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, so have lower blood antibody levels than calves from cool dry cows.
So if many cows in a herd experience heat stress during their dry period, it may impact significantly on productivity and profits, not just for a few weeks, but for many years.
Autumn and year-round calving farms with a high heat stress risk level should therefore ensure that their early dry cows and transition cows have access to adequate shade and cool drinking water at all times.
As per the milking herd, the aim should be to protect dry cows from direct sunlight, particularly during the hottest part of the day.
If existing natural shade from trees in paddocks on the home farm or support block used for dry cows is inadequate to provide four square metres of shade per cow at midday, then alternative paddocks should be sought.
In the longer term, the farm plan should be reviewed with the aim of establishing more tree belts along dry cow paddocks and springer paddocks.
Of course, the farm’s calving system could also be adjusted to reduce the number of cows dried off over the hot months of the year.
For further information on keeping cows cool, visit www.coolcows.com.au.
Article courtesy of WestVic Dairy Newsletter
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