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Does heat stress affect breeding-age heifers?

Pedro Nogueira for Progressive Dairyman Published on 31 July 2017

Heifers are a fundamental part of the dairy operation. They are the future of the herd, and an important aspect of their development is to get them pregnant on time, calve in good condition and make a positive return.

Stress is not something we normally associate with breeding-age and bred heifers. These animals have already passed the phases of their development considered more critical in terms of susceptibility to stress, like weaning or commingling with other heifers.

Nevertheless, breeding heifers can also suffer from stress. In a recent Discovery Conference on Dairy Replacement Heifers, Jaymelynn Farney from Kansas State University indicated stress can have several origins: environmental, health and social.

Environmental stress

When talking about environmental stress, heat stress comes to mind since it can have such a negative effect on animals. Cold stress can also have a negative effect, but the focus here is on heat stress.

Heifers are considered more tolerant to heat stress than mature dairy cattle. Their greater body surface area relative to their body mass allows for more efficient heat dissipation. Nevertheless, heat stress in heifers also results in reduced performance in terms of growth rate and reproduction.

One of the reasons for reduced performance is the reduction in dry matter intake (DMI) associated with heat stress, and this is particularly true for growth rate. Nevertheless, studies done with pair-fed animals indicated in terms of reproduction, heat stress affects fertility more than what would be expected by just the reduction in DMI.

In these studies, two similar animals are fed in pairs. One of them is subjected to heat stress, while the other is kept in a thermoneutral environment. The DMI of heifers subjected to heat stress is recorded, and then that amount of the same diet is fed to heifers maintained in a thermoneutral situation.

If DMI was the only driver of reduced performance, we would expect the heifers kept in thermoneutral environment would have similar problems to the heifers subjected to heat stress. Of course, heat stress also has an effect in terms of expression of heat (estrus) with, for example, an increase in silent heats in heat-stressed animals.

Ronchi from the University of Tuscia, Italy, did a study to compare the effects of heat stress and feed restriction on hormonal secretion (progesterone, oestradiol-17b, luteinizing hormone, follicle-stimulating hormone, prolactin and cortisol) in Holstein heifers.

Relation between ambient temperature and predicted daily weight gain of heifers

The results of his study indicated heat stress conditions caused the development of ovarian cysts in two heifers (40 percent), an increase in plasma concentrations of prolactin, a decrease in concentrations of cortisol and progesterone, and a 23 percent reduction in dry matter intake.

When this same feed restriction was applied to the heifers kept under thermoneutral conditions, there was no change in any of the parameters considered, meaning the depression in cortisol and progesterone secretion observed under heat stress was due to heat stress and not to a decline in feed intake.

Other studies focused on the relationship between heat stress and the incidence of embryonic abnormalities. Researchers from the University of Florida used 16 Holstein heifers to determine whether heat stress prior to ovulation increases the incidence of embryonic abnormalities.

They super-ovulated the heifers and kept them at either thermoneutrality (24ºC) or under heat stress conditions (exposure to 42ºC for 10 hours) beginning at the start of estrus.

Following A.I. at 15 and 20 hours after the onset of estrus, heifers were continuously maintained under environmental conditions of thermoneutrality for seven days provided by shade structures.

On the seventh day after estrus, they recovered embryos and evaluated them morphologically for stage of development and quality. They verified the distribution of embryos classified as normal, retarded or abnormal, or as unfertilized eggs differed significantly (P less than 0.001) between heat stress and thermoneutral treatments.

Only 12 percent of 25 embryos recovered from heat-stressed heifers were normal, compared with 68.4 percent of 19 embryos from thermoneutral heifers.

Stressed heifers also had a higher incidence (P less than 0.001) of retarded or abnormal embryos with degenerated blastomeres. (These are cells that originate when the fertilized egg starts to divide.)

These data indicated thermal stress during the periovulatory period (follicular phase) increases the incidence of retarded or abnormal embryos in super-ovulated heifers.

For economic reasons, heifers on average are growing at a faster rate to be able to calve sooner. Nonaka from Japan noted this increases the internal heat production in response to the increased nutrient intake.

With increased heat production, regulation of the body temperature becomes difficult, which exacerbates the decline in the heifers’ intake. Nonaka did a series of studies to determine the effects of increasing environmental temperature and relative humidity on the performance of Holstein heifers.

Then he used the data obtained from these studies to evaluate the impact of global warming on the performance of dairy heifers.

He used a model assuming progressive global warming and did projections from now to 2060 for different regions in Japan. He concluded since high humidity under hot temperatures significantly affects heifer growth, global warming and high humidity in the Japanese summer will have a marked negative effect on future heifer performance.

In his studies, heifers were kept at 20ºC, 28ºC or 33ºC with two levels of humidity, 60 percent and 80 percent. The bodyweight was either around 200 kilograms or 400 kilograms.

The results indicated as the environmental temperature rose, the daily weight gain decreased, with a more marked decline in heifers older or heavier at the start of the experiments. It was also demonstrated high relative humidity has a strong impact.

The negative effect of high temperature on heifer performance at 28ºC and 80 percent relative humidity exceeded that at 33ºC and 60 percent relative humidity. High environmental humidity affected the latent heat dissipation from heifers, even at 28ºC. The increased body temperature contributed to decreased DMI.

Based on this evidence, and since Canadian summers can be hot and humid, it is recommended that cooling and barn environment as well as nutritional management and feeding strategies be kept in mind when dealing with breeding-age heifers.

References omitted but are available upon request. Click here to email an editor. 

Pedro Nogueira
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