A lot of information has been published in last three decades on 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 few years, on the effect of heat stress on the feed efficiency of cows (as estimated by the feed to milk ratio).
A special NRC publication (1), showed that maintenance energy requirements of milking cows were 25% higher for cows exposed to ambient temperatures of 35 C (95 F), as compared to cows kept at 20 C ( 68 F). If we translate this data to high yielding cows eating ×4 maintenance diets, their expected feed requirements when being heat stressed is more than 5% above those cows maintained in normal conditions.
Studies carried out in the USDA facilities in the sixties (2, 3) showed that feed to milk ratio was 10% higher in summer calving cows, as compared to those calving in the winter. Milk energy used to be near 60% of that 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, 90F).
In a survey carried out in 13 commercial dairy farms in Alabama during winter months (4), the amount of 1.4 kg of milk was produced for every kg of DM consumed, as compared to 1.32 Kg of milk produced per kg DM consumed, in summer months (5% decrease in feed efficiency).
Researchers from the University of Arizona recently published a study carried out in the new climatic chambers located in Tucson (5). In high yielding cows maintained in normal climatic conditions, whose feed intake was restricted to that of heat stressed cows, milk yield decline was only half of that obtained in heat stressed cows (30% and 15% in the heat stressed and feed restricted cows, respectively). The reduction in feed consumption could explain only half of the decline in milk production, assuming that the remaining was energy used for activation of body mechanisms for heat dissipation, and changes in cow’s metabolism. In other words, heat stress causes “nutritional inefficiency”.
Based on the same protocol, I took part of a researchers team from the Israel ministry of agriculture research and extension services, in a research carried out in the facilities of the ministry’s research dairy farm (6).
Two groups of high yielding cows, averaging 45 kg/d (100 lib.) were installed in the same side of a barn, 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. The research started on July 1 and lasted until the end of the month. All cows were cooled, and feed consumption, milk production and other parameters were recorded. During the last week of July, cooling was gradually stopped in one of the groups, while food supply to the other one was restricted (on pair basis) to that consumed by the non cooled, heat stressed ones.
Results from the experiment carried out in Israel are presented in the table
* - Cows cooled in July and August.
- Consumption ad lib in July and restricted to group 2 level in August.
** - Cows cooled in July and without cooling in August.
- Consumption ad lib in July and August.
*** - Relation Milk : DM – (kg milk produced by 1 kg of DM consumed).
As obtained in the “Arizona study”, also in the “Israeli study” the 20% decline in feed consumption (from 24.4 to 19.4 kg, 54 to 43 lib. /cow/day), explained only half of the decline in milk production, and milk drop in heat stressed cows was almost double of that obtained in cooled cows (14 and 8 kg, 31 and 18 lib. /cow/day), although feed consumption was the same. It can be said that cooling cows in the summer improve feed efficiency by reducing 5 - 10% the feed required for certain amount of milk produced by heat stressed cows.
Based on the results obtained in our study, the following calculation can be made:
If we take into account that average per cow daily feeding cost is 6 USD and there are 150 stressful summer days in a year, then, cooling the cows in summer has the potential to reduce feeding cost and increase per cow annual profit by 50-100 USD. This amount of money is in many parts of the world, above the expected cost needed to cool the cows in the summer.
1. National Research Council. 1981. Nutrient requirements of dairy cattle. 5th ed. Nat’l. Acad. Press. Wash., DC.
2. McDowell et al. (1968), Interbreed mating of dairy cattle, 1. Yield traits, feed efficiency type and rate of milking. J. Dairy Sci. 51:767.
3. McDowell et al. (1969), Effect of heat stress on energy and water utilization of lactating cows. J. Dairy Sci. 52:188.
4. Britt et al (2003), Efficiency of converting nutrient dry matter to milk in Holstein herds. J. Dairy Sci. 86:3796-3801.
5. Roads et al. (2009). Effects of heat stress and plane of nutrition on lactating Holstein cows: 1. Production, metabolism and aspects of circulating somatotropin. J. Dairy. Sci. 92:1986.
6. Flamenbaum et al. (2010). Cooling cows improve feed efficiency of high yielding cows in the summer . from: “Meshek Habakar Veahalav” Vol 349, 12- 2010 P’ 65 (In Hebrew).