Not necessary high milk production needs to be related to lower fertility under heat stress conditions

Achieving high conception rate is one of the conditions for achieving efficient production of milk at farm level. For many years the Israeli dairy industry suffered, like other farms in warm countries, a significant decline in the ability to pregnant the cows during summer months, causing reduction in the efficiency of milk production and seasonality in milk supply to the industry and market.

Unlike the effect of heat on cow's production which is largely depends on the cow's food consumption and less sensitive to short-term changes in the thermal comfort of the cows, the effect on cow's fertility in the summer is much more complex and spread out over a longer period around the time of insemination. Studies conducted recently by Israeli researchers from the University in Jerusalem, showed that cow's fertility is sensible to heat stress conditions for a wide period, starting in the beginning of the development of the ovulating follicle till the implantation of the fertilized egg in the uterus (a period of around 50 days). Hence for the purpose of obtaining normal fertility levels in the summer, inseminated cows must be in normal body temperature, at least along this period, not an easy task.

The main reason for writing this article is to change the belief in the public, that when obtaining high milk production, it is necessary to "sacrifice" something in fertility and, at the same time, to show that, with good management practices in general, and an intensive and effective cooling in the summer, in particular, high producing cows can also achieve good fertility levels, even higher than those obtained with low producing ones, being in the same conditions.

In order to show this, I am doing the use of the "summer to winter ratio" index report which was elaborated these days for the year 2019. This index was developed almost 15 years ago by experts from the Israeli ministry of agriculture and the Israel Cattle Breeders Association (ICBA) and was adopted recently by institutions in US and Italy. This index (described already in an article in Engormix), enable us to subjectively characterize every year the different Israeli dairy farms, in terms of their capability to cope with summer heat stress.

In the beginning of every year, I receive the data from all the dairy farms in Israel. In order to examine the relation between level of milk production and intensive cooling the cows in the summer, I selected from this data base the 50 dairy farms with the highest annual milk producing in 2019, and compared them to the 50 dairy farms with the lowest annual production. Average per cow daily milk production in the summer and the winter, the summer to winter production ratio, the conception rate in the two seasons and the summer to winter CR ratio are presented in table 1.

From the presented in table 1 it can be seen clearly that the summer to winter ratio is much higher in high producing farms than in those of low producing ones (0.97 and 0.92, respectively). Most probably, these results could be reached due to having better management practices in general, as well as the properly  installation and operation of intensive cooling means there, in particular. this can be confirmed, as milk production in the high producing herds dropped in the summer by 1.5 kg/day, half of the drop (3.0 kg/day), obtained in the low producing farms.   

Looking at the fertility numbers, we can see that, differently from what could be expected, summer drop in conception rate was smaller in the high producing farms, as compared to the low producing ones (15 and 22 percentage units, respectively), and summer to winter ratio for CR was 0.63 and 0.46, in high and low producing herds, respectively.

In conclusion, our study shows that, not necessarily high milk production needs to be associated with low fertility in the stressful months of summer. The recent data from Israel shows the opposite. My belief is that, obtaining high milk production is related mostly to better management practices in general and in proper cooling the cows in the summer, in particular. The better summer to winter ratios for milk (0.97 vs. 0.92), and CR ( (0.63 Vs 0.46), confirms very well the fact that these farms better cool their cows in the summer. This fact is translated, in addition to higher annual milk production, also to higher conception rates in the summer.