Dairy sectors all over the world have been using the cattle heat load index (THI) as a mean of characterizing the climatic conditions to which cows are exposed. This is true when analyzing the results of experiments and surveys, as well as the routine by which milk producers operate their farms.
The heat load index is based on a formula that includes the ambient temperature and relative humidity at a given time. Until recently, the value of THI 72 had been defined as a threshold value beyond which a cow may suffer from heat stress and its yield and reproductive performance may be impaired. Recently, Arizona researchers examined the issue on cows at a higher level of production than cows used to determine the 72 value in the 1950. These researchers found that the threshold value beyond which a cow was supposed to enter into heat stress, is when the minimum THI during the day exceeds the value 65, or when the average daily THI exceeds the value 68 (between 4 to 7 THI units, lower than the threshold value used until recently). This probably means that there is a need to start cooling the cows already in “more favorable” conditions than in the past, and probably, proper cooling needs more hours per day (sometimes the entire day), as well as more cooling months per year, in each of the world regions.
A recent survey, which included all US states, examined the effects of summer heat load on cow’s performance, as well as the improvement that can be achieved by applying the best knowledge available to cool the cows at that time. In order to characterize the conditions in different US states, the researchers calculated the number of hours per day in any given month, during which THI prevailed above the critical threshold (in this case the value was THI 70). As can be seen in Table 1, there is considerable variation between regions, in terms of the percentage number of hours per year in which cows are exposed to conditions above the threshold value, and accordingly, the reduction in food consumption and milk production, the increase in days open (above optimum), and consequently, the expected financial loses.
As can be seen in Table 1, the percentage of hours per year in which cows are exposed to heat stress conditions ranges between 6% of total year hours in northern Washington State, to 49% in southern Florida State. The reductions in feed intake and milk yield, as well as annual income per cow, are proportional to these percentages.
In recent years, Israel ministry of agriculture experts in cooperation with Israel Cattle Breeders Association (ICBA) developed an index called “summer to winter ratio index”. This index allows us to assess the degree of efficiency in different farms are coping with summer heat load. Summer to winter ratio index compares the performance of cows in summer (July-September) to those in winter (January-March), assuming that the performance of cows in winter represents the productive potential of the cows. Summer to winter ratio index is included in an annual report elaborated at the end of each year, for each dairy farm in Israel. The index analyzes “corrected averages” of milk yield, peak lactation, fat and protein content, Somatic Cell Count (SCC), as well as winter and summer conception rates.
The report above mentioned is elaborated every year for each individual dairy farm as well as by region, type of farming and level of milk production. Based on this report, ministry advisory department sets priorities and concentrate efforts, focusing on herds with poor results.
In collaboration with the agricultural meteorology department of the Israel Ministry of Agriculture and the Israeli Cattle Breeders Association computerized system, we examined the situation in the Israeli dairy sector. From the climatic aspect, we calculated the average number of hours per day, during the summer months where THI was above 68. Having this information, we examined the relationship of these numbers to performance conditions. For preliminary testing, we used data from two meteorological stations, one in the coastal part of Israel and the other in the inland. As first step, we calculated the percentage of hours per year and the average number of hours per day above THI 68 during the summer months (July – September), over 12 consecutive years (2008 - 2019). The findings per each year, in each of the two regions are shown in Figures 1 and 2.
As shown in Figures 1 and 2, it can be clearly seen that, in both regions, cows experience heat stress for more than 40% of year time (similar to the conditions in southern states in the US, with all the negative productive and financial impacts, described in table 1), as well as most of the day time. The number of hours during which cows in the inland region were exposed to heat stress conditions was more pronounced than in the coastal region. Cows in the inland stayed for more than 22.5 hours a day (between 22.5 hours in 2009 and 23.6 hours in 2010). In the coastal region, the number of hours per day ranged between 1.0 and 2.0 hours per day in comparison to that recorded in the inland region. In both regions, there is a worrying trend of rising and stabilizing relatively high heat stress values over the past five years (23 hours per day and more in the inland region and 21 hours per day and more, in the coastal region).
To examine the relationship between annual heat stress intensity and cow performance, we used the annual report of “Summer-Winter Ratio”, elaborated by the ICBA computerized system. Since the trends were not different between the two regions, and in order not to over-extend in this article, I have decided to present the relationships between hours of heat (in this case the threshold of THI 72) and cow performance for the large-scale cooperative dairy farms in the inland region. These are 27 dairy farms with about 10,000 cows. In each of the following figures, one of the parameters included in the summer to winter report is tested in relation to the number of hours above threshold THI 72.
As shown in Figures 3 - 5, the relationship between the percentage of hours per year and the average hours per day, beyond the threshold of 72, and the productive and reproductive parameters can be clearly seen. In those summers, where more than 18 hours per day above the threshold were observed, the summer to winter milk production ratio dropped from 0.97 to 0.93 and the summer monthly conception rate decreased from 26% to 16%. A decrease from 26 to 14 percentage units was observed in the gap between winter and summer conception rate. In same time, we observed a drop from 0.97 to 0.92, in the summer to winter peak lactation ratio (not presented graphically).
The results of this study are especially interesting, as these differences between years in almost all the parameters examined occur when the difference between summers ranges between 18 and 23 hours per day above THI 72. This indicates the great sensitivity of this parameter, when it comes to the summer performance of the cows.