Heat stress and cooling effects on feed efficiency, important factors in the economy of dairy farms in warm climates

A lot of information has been published in the last decades, dealing with the negative effect of summer heat stress on the productive and reproductive traits of the high yielding cow. Very limited information existed however, up until the last years, on the effect of heat stress on the feed efficiency of cows (as estimated by the feed to milk ratio). Knowing the full extent of the economic losses caused to the cows due to heat load can help to present to the dairy farmers their losses, and thereby, help to convince them to invest in the installation and proper operation of heat mitigate means in their farms.

A special NRC publication, from the early eighties showed that, as compared to cows in normal conditions, energy requirements for maintenance of milking cows are 20% higher, when exposed to ambient temperatures of 30 C, and 35% higher, for those exposed to temperatures of 40 C. Energy requirements of high yielding cows eating ×4 multiple maintenance diets and being exposed to heat stress conditions, will increase by 5 - 10% above cows maintained in normal thermal conditions.

Studies carried out in the USDA facilities in late sixties showed that feed to milk ratio was 10% higher in summer calving cows, as compared to those calving in the winter. Milk energy content was only 60% of the energy consumed when cows were in normal conditions, but only 35%, when cows were exposed for 2 weeks to heat stress conditions in climatic chambers (32 C). In a large-scale survey carried out in 13 commercial dairy farms in Alabama, cows produced 1.4 kg milk per each kg of DM they consumed, in winter months, as compared to only 1.3 Kg of milk in summer months (5% decrease in “feed efficiency”).

Researchers from the University of Arizona published a study carried out in their new climatic chambers located in Tucson. High yielding cows maintained in normal climatic conditions, and whose feed intake was restricted to that of heat stressed cows, dropped in production to only half the drop obtained in the heat stressed ones (30% and 15% in the heat stressed and feed restricted cows, respectively). In other words, the drop in feed consumption by heat stressed cows can explain only half the decline in milk production, assuming that the remaining half can be attributed to the possibility that part of energy consumed was used for activation of body mechanisms to dissipate heat, as well as to other metabolic changes in cow’s digestive system. In other words, heat stress causes “nutritional inefficiency”.

Making use of the same experimental protocol, we conducted few years ago in Israel, a research done in the facilities of the Israel ministry of agriculture experimental dairy farm. Two groups of 21 high yielding cows each, averaging 45 kg/d, were fed ad lib, a TMR ration (provided in individual feed boxes weighted daily), and milked 3 times a day. All cows were intensively cooled by a combination of sprinklers and forced ventilation, provided for 6 cumulative hours per day, in 8 “cooling sessions”. In the middle of the summer, cooling treatment was gradually stopped to one of the groups, while food supply to the cows in the other group (where cooling continued), was restricted (on pair basis) to that consumed by the non-cooled, heat stressed cows. As in the study in Arizona, also in our research, the 20% decline in feed consumption (from 24.4 to 19.4 kg/cow/day), in the heat stresses cows could explain only half of the decline in milk production. The drop in milk production in the heat stressed cows was almost double of that obtained in cooled and feed restricted cows (14 and 8 kg/cow/day), respectively. In other words, cooling the cows in the summer can improve cow’s feed efficiency by 5 - 10%, almost same rate obtained with cows in normal and heat stress conditions in Arizona.

An impairment in feed efficiency is also obtained from the direct effect of the drop in cow’s annual milk production, and it is additive to that obtained due to the direct effect of heat stress on feed efficiency, presented so far. Feed requirements for maintenance is constant and the same amount required to maintain cows that produce 10 or 40 kg of milk per day. Hence, maintenance expenses per unit of milk produced will be lower in high-yielding cows, since they “split” over more kg of milk per day. In a survey conducted in Israel with 40 dairy farms and over a period of 20 years, the amount of Dry Matter per liter of milk produced annually was recorded for farms whose cows annual production ranged from 9,000 to 14,000 kg of milk per cow. In this survey, it was found that 0.78 kg of dry matter is required to produce 1 kg of milk at an annual yield level of 10,000 KG, while at a yield of 12,000 Kg, only 0.70 kg of food is needed (10% less). Based on a well cited article, published almost 20 years ago by St. Pierre from Ohio University, a drop of 2,000 kg of milk per cow annually, can be expected in dairy farms in the south US, due to the summer heat and the lack of effective use of heat mitigation means. Today we know, based on our experience in Israel, that proper implementation of cooling means may prevent most of this decrease. An addition of 2,000 kg to cow’s annual production in these climates means a drop of 10 – 15%  in annual feed expenses for milk production, when, as mentioned, this amount is in addition to the economic improvement described before.

In order to illustrate to the reader the economic meaning of the drop in feed efficiency and the benefit arising from cooling the cows, I made a calculation, based on the results from the studies above presented. I describe a scenario where per cow daily feeding cost is of 8 USD, a dairy farm located in a region with 150 stressful summer days (Israel, south US and south Europe), and where annual feed efficiency drops in approximately 15%. In this case, properly cooling the cows have the potential to increase per cow annual profit by close to 400 USD, more than 3 times the required investment to cool the cows in the summer. I assume that we can expect half to one third of this benefit in dairy farms located in North America and North Europe.

In conclusion, the drop in feed efficiency under heat stress conditions have a significant impact on the economy of the dairy farm, especially for those farms located in warm climates, but also for those located in temperate regions. Presenting these numbers, as well as the expected benefit from intensively and properly cooling the cows, will encourage farmers all over the world to invest in the implementation and proper operation of heat mitigation means. This will be, first and above all, for farmer’s own benefit, but also for the environment, as milk can be produced with less cows, less food for maintenance and production, and in the same time also, less GHG emission to the atmosphere.