The use of the "summer to winter ratio" index, to assess the effect of summer heat stress on the performance of dairy cows in different regions of the USA

Summer heat stress causes significant economic losses to the U.S. livestock and especially the dairy industry. Different studies conducted in the United States indicate an annual loss of about $ 1.5 billion to the dairy US industry. This is due to unsuccessful confrontation of US dairy farmers with the heat stress problem. The losses differ of course, between different regions of the United States, with northern temperate climates, the annual milk loss per cow in Midwest farms being 200 lt. and the financial loss of about $ 50, compared to southern states where the annual milk loss per cow reaches 2000 liters and the financial losses are more than $ 600.

The "summer to winter ratio" index was developed in Israel nearly 20 years ago. It was done in cooperation between researchers from the Israel Ministry of Agriculture and Israel Cattle Breeders Association (ICBA). This index was developed as a tool to assess the impact of summer heat stress cow's performance in Israel's dairy farms, as well as to evaluate the degree of improvement achieved at farm level, when recommended measures are used to relieve heat from the cows in the summer. The summer to winter ratio index includes the daily average of milk, milk fat and protein output per cow for each given month, also measured the peak of lactation, milk somatic cell content (SCC), as well as summer and winter conception rate (CR). The closer the ratio is to 1, the less impact summer heat stress has on cows' performance, usually thanks to optimal use of heat mitigation measures (cooling).

Almost 15 years ago, I presented this topic at the American Dairy Science Association's annual meeting, held that year in Phoenix, Arizona. A young researcher from the University of Texas contacted me at the end of my lecture, showing interest in how we perform and use the summer to winter index in Israel. Following my explanations, he decided to come to Israel, meet us and learn from much closer this topic. Back to US, he gathered a group of researchers from several US universities, most of them from the southern US and together, they ran the survey appearing in the article published in the October 2019 issue of the Journal of Dairy Science. The aim of writing this article is to share with the readers of different languages, the findings of the survey, confront the findings from the US with those obtained in Israel and give some of my insights on the results.

The U.S. survey included more than 15,000 dairy farms, located in 40 states. For the comparison, these farms were divided into five different regions. On the one hand, the Northwest, Midwest, and Northern Mountains (regions characterized by temperate and relatively favorable climate for dairy cows in the summer) and, on the other, Southeastern regions and the Southern Mountains (regions characterized by tropical climates in the first, and desert in the second), which are more similar to the climate conditions existing in Israel. The US researchers decided to define the two seasons, based on how it is characterized by the meteorological authorities in the US, were winter is between December 21 to March 21 and summer in between June 21 to September 21.

Being the data in the article particularly large, I decided to concentrate on presenting data from two regions that represent different climatic conditions. Herds in the Midwest region (summer conditions close to Western European countries), with about 8000 herds in the survey, and the Southwest region (summers similar to those we face here in Israel), with close to 1,500 herds in the survey.

The climatic conditions in both regions expressed by Temperature Humidity Index (THI), presented in Table 1.

Table 1 - Heat load values (minimum, maximum and average), in the summer and winter months, in the Midwest and Southeastern United States.

From the presented in table 1, it can be seen that in both regions, the maximum THI values are of 80 (ten units above the critical value) were recorded, while summer minimum THI values were of 42 and 56, in Midwest and Southeast regions, ten units below the critical value. The article does not specify (and it is a pity), how many hours above and below the critical value were recorded in each of the regions, which might have allowed better understanding of the conditions for dairy cows in each region.

Data on the milk production and breeding performance in the summer and winter, in farms of both regions, shown in table 2.

Table 2 - Milk yield, milk composition, conception rate (CR and pregnancy rate (PR), obtained in the summer and the winter in the Midwest and Southeast of United States.

The milk yield of cows in the Midwest herds was higher in 2-3 liters per day, as compared with those of cows in the southern states. In any case, the gap between summer and winter was similar in both regions, ranging between 2 to 3 liters. The fat and protein content in milk was higher in the herds of the Midwest, compared to herds in the Southeast, for the two seasons under consideration. The fat and protein content in milk decreased significantly in the summer months, with no difference between the two regions.

The summer to winter ratio of different performance indexes for the farms in the two regions is presented in table 3.

Table 3 – Summer to winter (S:W) ratio (upper quarter, middle median, lower quarter and average), for milk yield and composition, conception and pregnancy rates in the Midwest and Southeast of United States farms.

From the presented in table 3, it can be clearly seen that summer's negative effect on milk yield and composition, as well as reproductive performance (especially the Pregnancy Rate, which incorporates effects on heat detection ability and conception rate), was greater in the farms of the southern United States than in the Midwest ones. It should be noted that in both regions, there is a significant gap between the top and bottom quarter results. The study does not explain whether the causes of the disparity are differences in t climatic conditions within each region, or better management practices, which may also include cooling treatment to the cows in the summer.

It is interesting that the summer to winter ratio related to milk production in the herds of the Midwest United States is very similar to that of the cooperative dairy farms in Israel (presented at one of the recent conferences by ICBA researcher), despite the climatic differences between this two regions. On the other hand, the average S:W ratio regarding milk production in Southeast herds (a region closer in its climatic characteristics to Israel), is very similar to the S:W ratio obtained in the lower quarter of the farms in Israel. This can probably be explained by the extent of the implementation of cooling means today in Israel, as compared to that in the southern US.

The use of the "summer to winter ratio" index, to assess the effect of summer heat stress on the performance of dairy cows in different regions of the USA

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