Heat Stress & Dry Cows:
The Negative Effects on Herd Performance
The effects of heat stress on dry cows are profound. The production and physiological effects on cows and the effects on the calf from heat stress during the dry period are significant contributors to lower overall milk potential, productivity and profitability.
In this article we concentrate on the effects that heat stress during the dry period has on the subsequent performance of heifers born from cows that experienced heat stress during the dry period.
Some of the effects of heat stress on dry cows noted previously include the following:
- Decreased udder and placental development. Heat stress in late gestation reduces both mammary gland and placental blood flow, resulting in lower numbers of functional secretory cells in the mammary gland and a lower numbers of cells in the placenta. This limited development affects the functioning of both of these vital organs. There appear to be an association between mammary gland and placental development in late gestation.
- Lower calf birth weights. One aspect of the decreased placental development is a reduction in calf birth weight. The decreased blood flow, reduced placental development and placental hormones affect fetal development with calf birth weights up to 5kg lower than in cooled dry cows
- Shorter gestation period. Heat stressed dry cows decrease gestation by as much as 7 days which accounts for some of the reduction in birth weight. The 10 to 15% decrease in feed intake may also contribute to the lower birth weight.
- Decreased passive transfer of immunity. Calves from cooled (CL) dry cows had a higher total plasma protein (6.3 vs 5.9 g/dL; P<0.01) compared to the calves from the heat stressed (HT) animals. In addition, there were differences in the levels of total serum IgG (1577.3 vs 1057.8 mg/dL). There was a difference in the apparent efficiency of absorption (33.6 vs 19.2%) in the calves from the CL dry cows compared to the HT dry cows respectively.
- Higher weaning weight. In the same experiment, differences were evident at weaning (78.5 vs 65.9 kg) for calves from CL and HT cows respectively. The average weight for the remaining 3 to 7 months of age was 154.6 vs 146.4 kg and the wither height was 104.8 vs 103.4 cm. These marked effects on aspects related to placenta blood flow, passive transfer and lower birth weight raises important questions. Is there a carry-over effect on the heifers entering lactation two years later?
How did milk production of heifers from heat stressed dry cows (HT) compare with heifers from cooled dry cows (CL)? In a recent study reported by Monteiro et al. (2016) all the calves and heifers, regardless of in utero heat stress status, were fed and managed the same and they subsequently followed the animals through 35 weeks in 1st lactation.
The observations and results from this study are profound. There was no difference in age at first calving or body weight during lactation. The two most significant observations were the percent calves that completed first lactation (85.4 vs. 65.9% for CL vs HT heifers respectively) (Table 1) and the significant difference in milk and component production through the 35 weeks lactation (Table 3).
Table 1. Calf and heifer mortality for animals born to cows that were exposed to heat stress (HT) or cooled (CL) during an average 46 days dry.
Interestingly, the heifers from HT dry cows were lighter at 12 months of age but there was no difference by the time they calved (CL = 563 vs HT = 565 kg; P = 0.89). Age at first calving was not different between the two groups of heifers (Table 2). There was also no difference in body weight during lactation (CL = 567 vs HT = 568 kg; P = 0.92).
Table 2. Age at 1st AI, services per pregnancy (SPC) and age at first calving for animals born to cows that were exposed to heat stress (HT) or cooled (CL) during an average 46 days dry.
Considering the very small differences in the parameters reported above, the significant differences in milk and component production add substantially to the previous studies done on the effects of heat
stress on late gestation dry cows. Table 3 shows the differences in production between heifers from CL dry cows compared to heifers from HT dry cows.
Table 3. Milk and component production for heifers born to cows that were exposed to heat stress (HT) or cooled (CL) during an average 46 days dry.
The heifers from the CL dry cows produced about 19% more milk through 35 weeks of lactation compared to the heifers from dry cows that were heat stressed (HT) for the last 46 day pregnant. The graph opposite (Figure 1 – right) illustrates the differences in milk production.
Both kg of milk fat and milk protein were significantly lower for heifers from cows that were heat stressed during the last 6 weeks of gestation.
Not only do heat stressed dry cows produce about 23% less milk in the following lactation, their offspring are similarly compromised through their first lactation as evidenced by this research. This is thought to be mediated through the effect of heat stress in utero during the last 6 weeks of gestation that affects the phenotype negatively.
This leads to the next question – is this negative effect on gene expression permanent and will all subsequent lactations be similarly negatively affected? That will be the subject of further research.
However, from the research and information presented so far it is clear that every effort needs to be made to adequately cool dry cows during periods of heat stress!