The negative impact of extreme heat wave on the performance of dairy cows in Israel

The summer of 2020 will be remembered as one of the hottest summers in Israel in decades. The summer heated many agricultural sectors, including the dairy industry. Beyond high heat load conditions throughout the summer, two unusual heat waves stood out in particular. The first heat wave was in May and the second in early September, as can be seen in fig 1.

In this article, I would like to present the heat wave events of September. I will describe first last year's September characteristics, compared to previous years and then present its negative impact on the breeding performance in Israeli dairy farms. Above all, it is of my interest to present in this article the breeding performance during this challenging period, in a group of dairy farms that managed, despite the intense heat wave, to maintain conception rates, which are close to the levels achieved in the winter months.

In order to characterize the climatic conditions prevailing in September 2020 and compare them with previous years, I used a heat load index (THI), and defined it for two representative and main regions of the dairy industry in Israel (Hot valley and Coast), the number of hours per month with extreme heat load conditions prevailed (above TUHI 80). This information is presented in figure 2.

From the presented in figure 2, it can be seen clearly that September 2020 was particularly hot, when more than 40% of all hours of the month were above THI 80, double than that of September in regular year. It was surprising to see that there was almost no difference in heat load intensity between the valley area and the coastal region. This is probably due to the higher relative humidity in the coast region, which probably "offsets" with the high temperatures that are usually recorded in the hot, but less humid valley. From the presented in figure 1 it can also be seen that some additional "Warm Septembers" occurred also in 2010 and 2015, but the heat load intensity in these years was lower than that of 2020.

The conception data in the different months of the year for the years 2016 - 2020, in the adult cows of the cooperative large- scale dairy farms is presented in Figure 3.

Conception rates to all inseminations, given in all 160 large-scale cooperative dairy farms of Israel (approximately 70,000 cows), is presented in table 1, comparing inseminations given in all summer months of 2020 (June - September), to those given only in September of this year.

From the described in table 1, we can see a significant decrease in the conception rate in inseminations given in September 2020, as compared to inseminations given along all the summer. Conception rate decreased from 23% to 17% in first inseminations and to 14% in all inseminations given in September 2020. It is interesting to see that the percentage of dairy farms, in which the conception rate was lower than 15% ranged between 40% and 60% of the total number of farms in September, as compared to no more than 10% in inseminations given during the entire summer. In contrast, almost no difference found in the percentage of "high fertility" dairy farms, where the percentage of farms remained around 10% of the farms, in the two tested periods.

Naturally, we are interested in finding those dairy farms that managed to achieve relatively high conception rates, even in extreme heat load conditions. To do this, we sorted all the 160 cooperative farms, according to the conception rate from all the inseminations given in September 2020, and selected the twenty farms with the highest conception rate and the twenty with the lowest conception rate in the analyzed period. The results of this evaluation presented in table 2.

From the presented in table 2, we can have the following insights:

Regarding the conception rate in heifers, no difference found between the two groups of farms, which was to be expected, since it is not customary for us to cool heifers in the summer. However, the conception rate of the heifers is relatively low than expected in this age group, and falls more than 10 percentage units, as compared to the conception rate obtained in the winter months. It is not inconceivable that if this trend continues, there will be room to discuss the possibility of cooling also the heifers in Israel in the inseminations given in the summer months as well, or at least, to be prepared for this during periods of exceptional heat load.

The conception rate in the first lactation and adult cows of the "high fertility" group was around 30%, in inseminations given in months with the extreme heat load. In contrast, the conception rate of the same cows from the "low fertility" group ranged between 14% in first lactation cows and 7% in adult cows, no doubt, very low conception rates. If we add to this the data in table 1, showing that there were more than 90 farms in the country in this situation, then, the negative impact to the decreased fertility at the national level may be particularly great.

What is interesting is that here too, as can be seen in table 2, there is a group of dairy farms, located in different parts of the country (hot valleys, coast and mountain area), reaching conception rates of 30% and more in adult cows. Most of the farms in this group known to us as those that, over the years, implement effective cooling means and protocol in the summer. In those farms we found higher summer to winter ratio index (appearing in the annual report issued at the end of each year by the Israel Cattle Breeders Association), as compared to the national average ratio. The results obtained in these farms confirms us that it is possible, by proper installation and operation of cooling means developed in Israel, and in accordance with the recommendations of professional institutions, to prevent the decline in summer fertility of the cows, even during periods characterized by extreme heat load, higher than usual.