Do we need to cool heifers too?

Adult lactating cows receive more attention when it comes to the negative effects of heat stress, as compared to heifers. This is because lactating cows need to dissipate more heat, running out of milk production and feed consumption and digestion to support it. The losses, caused by exposing adult cows to heat stress conditions include drop in milk production and composition, fertility, health and cow's performance in the subsequent lactation, when exposed to heat stress conditions in the late pregnancy. Heifers do not produce milk, and so, their heat production is lower, making them less sensitive to heat exposure. It is known therefore that the heat load threshold beyond which losses heifers performance begins is higher (THI 72 – 74), as compared to 68 in lactating cows.

Heifers exposed to heat stress conditions may be impaired in their growth rate throughout the growing period, their fertility may be impaired upon exposure to heat stress conditions around the time of insemination, and also exposure of heifers to such conditions in late pregnancy, may impair their productivity after calving. In this article, I will bring relevant information from studies carried out in last years, which deal with effect of the heat stress on the heifers in these stages. The possibility of reducing these losses by cooling, and its economic viability will also be discussed.

Impact of heat stress on growth - A study conducted in the US, evaluated the economical losses caused to farm animals following their exposure to heat stress conditions in different parts of the country. (St. Pierre et. al. 2003). With regard to dairy heifers, the study presents data showing that exposure of heifers to heat stress conditions slow their growth rate, with the extent of losses being directly proportional to the length of the growing period with THI threshold exceeding 72. Weight gain of heifers in the first year is reduced by 0.5 to 7.9 kg per year, in the northern and southern US states, respectively, while in the second year of heifers life, it is reduced by 1.0 to 17.4 kg per year, respectively, in same regions. The authors of this study found that summer negative impact on heifers "costs" US dairy industry almost $ 50 million each year. Total annual losses to US dairy sector is around $ 1.5 billion per year, so the losses caused to heifers is relatively negligible, and do not exceed 3% of the total losses to U.S. dairy industry.

The effect of heat stress on conception rate - naturally, the negative impact of heat stress on the fertility of heifers is less than that of adult lactating cows. However, recent data published by "SION" (Israel's largest AI cooperative), shows a drop of 10 to 15 percentage units, in conception rate of heifers inseminated in last summers, as compared to inseminated given in the winter. I faced same results in some of my projects done in last 10 years in extremely warm countries like Vietnam, South China and Central America. Can cooling improve heifers fertility, as it does with adult cows?. An answer to this question was given in a recent study (Maghaddam et. al. 2009), which studied the possibility of reducing summer decline in fertility of heifers by cooling them for a short period around the time of insemination. Ninety heifers around time of insemination in the hot summer of Iran were allocated into three groups of 30 heifers each. First group received only shade, while the other two groups received shade plus short wetting sessions, provided in the feed line, or, shade plus short sessions of wetting, combined with forced ventilation in the feed line. The heifers were cooled for only few hours before, during and after insemination, and were later on, put together in the same shed, with shade only. Body temperature, immediately after insemination was 39.7, 39.3 and 38.9 C, in the three treatments, and conception rate was 23%, 40% and 56%, for heifers in same treatment groups, respectively. Despite the limited number of heifers participating in this experiment, some insights can be learned from it. First, heifers can too, be in heat stress in the summer. Second, cooling treatment, combining wetting and forced ventilation allow heifers maintain normal body temperature under heat stress conditions and finally, that short-term cooling of heifers, around insemination time given in extremely hot summer conditions can significantly improve conception rate of heifers. The conditions and number of animals under which this experiment was carried out are limited in order to come into clear conclusions. Therefore, it is advisable to expand the knowledge on this issue in studies with high numbers of animals. In case these results will be repeated, then it would also be worth examining the economic aspect of implementing such technology. This will be done by comparing the ratio between the cost of investing in the installation of the cooling system and its operation, to the reduction in the cost of maintenance of these heifers, due to shortening the number of days from calving until they get pregnant.

Effect of heat stress in late pregnancy – There is a lot of information available regarding the effects of heat stress and cooling on cows being in the last trimester of pregnancy in the summer. But, there is very limited knowledge about heifers in same conditions. A study published recently (Davidson et. al. 2020) studied the effect of cooling late pregnant heifers in the summer, on their performance in their first lactations. Two groups of heifers in late pregnancy occurring in summer were compared. Heifers from one group received a cooling treatment combining wetting and forced ventilation in feed line, operating alternately, when ambient temperature exceeded 21 C. Heifers from the other group were provided only shade, and served as control. Respiration rate and body temperature were monitored throughout the experimental period. Respiratory rate was significantly lower in the cooled heifers, as compared to those in the control group (daily averages of 40 and 70 breaths per minute), respectively. Vaginal temperature of heifers in the control group was higher than that of the cooled ones by 0.3 - 0.4 C, throughout the day. Control heifers were above normal temperature during most of daytime, while cooled heifers maintained normal body temperature for most of the daytime. Average daily milk production of heifers from the cooled group was 4 kg higher than that of the control group (35.8 and 31.9 kg/d), respectively, and this difference was maintained along all the tested period.

With regard to the effects of heat stress on growth of heifers, it does not appear that the economic losses caused to this branch (3% of the total losses to the entire industry) can justify an investment in the installation and operation of cooling systems. This conclusion can be changed in case we will have new information from extremely warm regions showing larger losses in weight gain. When it comes to the effect of heat stress on fertility, it is clear that the summer decline in fertility occurs mainly in extremely ward regions. Before making any decision regarding the need to install cooling systems, more knowledge should be reached regarding the possibility of improving conception rate of heifers, through short-term cooling around the time of insemination. Also, simulations of the possibility of reaching these results through longer cooling period around insemination should be required. Anyhow, investing in cooling heifers to prevent summer decline in fertility will be justified only if it is economically worthwhile. Regarding the viability of investing in cooling heifers at the end of pregnancy, it is obvious today that cooling pregnant heifers is no less important than that of cooling late pregnant adult cows. Investing in cooling late pregnant adult cows has been examined and found to be highly economically justified. (An article of mine on this topic was published here not long ago). Hence, the viability of investing in cooling late pregnant heifers in summer is not worthwhile either, and this viability is not in doubt.