Heat stress influences production and fertility of high producing dairy cows. In Israel, milk production declines in summer to almost 90% of winter level. Conception rate in summer reaches levels of 20%, compared to more than 40% in winter months. The summer decline in production and fertility creates a significant seasonality in milk supply to the market and an additional cost to the consumers, caused by the need for drying milk in winter and using it in summer.
A “summer to winter performance ratio” index was developed to evaluate the efficiency in which each farm deals with summer heat stress by implementing management tools which are mainly based on the use of cooling methods. The “summer to winter performance ratio” compares
average herd summer results to average herd winter results regarding milk, Economical Corrected Milk (ECM), milk fat and protein percentage, somatic cell count (SCC), and conception rate. Calculation of this index is based on data from the Israeli Dairy Herdbook Database.
Recently, a large scale survey was carried out to study effects of production level and heat stress relief on the performance of dairy cows in Israel. The survey was based on data for the year 2005 and included 22 dairy herds, averaging 300 cows each and a total of 6,600 cows. All the dairy
herds were located in the coastal part of Israel. Cows in all the herds were held under similar housing systems, milked 3 times per day and fed for ad libitum TMR intake, distributed twice daily. Twelve of the herds were of high production level and ten were of low production level (previous year winter ECM yields averaged 41 and 35 kg/d, respectively).
In eleven herds of each production-level group, cows were intensively cooled (IC) during the summer, using a combination of wetting and forced ventilation for 10 cooling periods for a total of 7 cumulative hours/d. In the other eleven herds of each production-level group, cows were moderately cooled (MC) by a combination of wetting and forced ventilation in the holding pen, only before milking. Winter production averages and the summer to winter production ratio, which were used for allocating herds to different groups, are presented in Table 1 and averages of milk production for the different seasons and groups are presented in Table 2.
Ranges of winter production averages (Kg/d) and of summer to winter production ratios for the different groups in the year prior to the survey
Average milk production (Kg/cow/d) for the different seasons and groups
Lactation curves in the first 10 months in lactation for the different groups of high and low producing herds are presented in Graphs 1 and 2, respectively.
Summer and winter milk production curves (kg/d), for intensively cooled (IC) and moderately cooled (MC) cows in High and Low producing herds.
Summer and winter ECM production curves (kg/d), for intensively cooled (IC) and moderately cooled (MC) cows in High and Low producing herds.
Averages of conception rates (CR) for the different seasons and groups are presented in Table 3.
Summer and winter averages of conception rates (%), for intensively cooled and moderately cooled cows of High and Low producing herds
The results of this survey indicate that intensive cooling almost eliminated the summer decline in milk production regardless of the level of production and reduced about half of the summer decline in conception rate. Intensive cooling had greater impact on improving conception rate in low producing herds, than in high producing herds.