Efficient cooling systems based on water evaporation from cow's body surface have been developed in last four decades in dairy sectors of warm climate countries. The main effect of cooling is in reducing the drop in food consumption caused by heat stress, in addition to the normal drop in food consumption in the beginning of lactation, resulted by the mobilization of body reserves and with all its metabolic meanings.
Few years ago, we conducted a research in which we studied the possibility, by increasing (through dietary manipulations at the end of lactation and dry period) the body reserves (Body Condition Score, BCS ) of cows which are programmed to calve in early summer, to compensate with cow's low food intake in early lactation which take place in summer stressful conditions. This research was published in the Journal of the American Dairy Association (source appears at the end of the article)
Eighty adult cows, expected to calve in late spring and early summer were randomly assigned in the last trimester of the lactation into two groups. Body condition of the cows was manipulated through diet energy concentration in late lactation and dry period to reach high (3.50) and low (2.75) body condition at calving (increasing or reducing the amount of energy consumed, according to the body condition group the cow was assigned). After calving, cows of both groups were randomly assigned to one of two treatments. One treatment consisted of cooling the cows by combined cycles of wetting and forced ventilation (30 seconds of wetting every 5 minutes), given for a duration of 30 minutes at a time, for 7 times a day (every 2-3 hours). The other group was not cooled at all, and served as control group.
Due to technical limitations, we were able to measure only food intake of the cows (on a group basis), to compare cooled Vs non- cooled cows. Cooled cows consumed significantly more food during early lactation in the summer period (19.4 versus 17.8 kg / day), while water consumption of cooled cows was about 10 liters per day lower than that of control cows (93 vs 103 liters a day), respectively.
Average daily body temperature of cooled cows was lower than that of control cows (38.7 ° C and 39.2 ° C), respectively. Daily peak body temperature was 38.9 ° C, Vs 39.7 ° C in the cooled and control cows, respectively. Body temperature measurements tell us a lot about the effectiveness of the cooling treatment cows receive (maintain cows below 39.0 degrees Celsius) throughout the day. I will return to this important achievement, when discussing the fertility results presented in this article.
The milk yields and its composition in the 85, peak summer days (mid-July – late September), are shown in Table 1.
Table 1 - The average daily milk, milk fat, protein and total milk solids yield, in cows that have exercised in high and low Body Condition Score, with and without intensive cooling at the beginning of lactation occurring in the summer.
From the presented in table 1 we can see a clear positive effect of the summer cooling treatment on milk yield, as well as on the solids content. However, also higher body condition score in lactations starting in early summer has a positive effect on milk yield, and especially on its total solids content. Being with more body energy reserves in these cows is advantageous, especially in cows that do are not cooled. High body condition at calving moderates the decrease in milk and milk solids production in early lactation, following exposure to heat stress conditions. The results of this study show that, higher body condition score at calving, as recommended, in combination with intensive cooling the cows at the beginning of lactation, when this occurs in the summer, gives the best results. On the other hand, the worst occurs when cows calve in low body condition and are exposed to heat stress conditions in early lactation occurring in the summer. As can be seen in the table, these cows produced less milk and less milk solids and, as we will see later on in this article, will suffer also from poor fertility traits in subsequent lactation.
The relation between body condition at calving, occurring in early summer and different parameters related to cow's fertility in subsequent lactation of cooled Vs non-cooled cows are presented in Table 2.
Table 2 – The relationship between the body condition of cows calving in early summer and cooling treatment during summer, on time to renewal of ovarian activity, manifestation of estrus signs and conception.
From the presented in Table 2, it can be seen that calving in high body condition, as recommended, and cooling the cows in early lactation, occurring in the summer, are both positively related to earlier return of cows to normal sexual activity after calving. As seen before, with regard to milk yield, also in this case, the cows that calved in low body condition and were not cooled in early lactation in the summer, were the most inferior and had the longest time to return to normal reproductive activity after calving.
Due to limited number of inseminations given in the summer in this research, we were not able to analyze the conception rates, separately for each of the four sub-treatments in the experiment. In light of this, I decided to present in this article the conception rates of cooled Vs non-cooled cows, where the most significant result was obtained. The improvement in the conception rate obtained from cooling the cows in this study was the best known to me in the literature, and raises from the fact that cooled cows were in "thermal comfort" (below 39.0 C), during the entire day, for all the study period. These results differ from other publications, as well as the situation in commercial dairy herds in Israel, who tend to cool the cows properly.
What we found in these farms is that they are not capable to maintain cows in thermal comfort for the entire day, so cows experience few hours of heat stress (above 39.0 C), every day. This situation lead to a certain decrease in summer conception rate, even if it is quite above normal summer levels, but still 10 percentage units below winter levels.
It can be summed up that, in order to achieve winter conception levels in the summer, it is needed, in addition to cooling them and maintain them in thermal comfort all day and all summer, it is also needed that those cows expected to calve in early summer, will calve in proper body condition, as recommended. This is especially important under conditions where, for different reasons and limitations, farmers are not capable to cool their cows in a manner that totally prevent them from being heat stressed. These cows are found in part of the day time in body temperatures above normal, as is still the reality today, in most dairy farms in Israel and the world.
Practically, my recommendation is to evaluate the body condition of those cows entering to last trimester of pregnancy and in the beginning of the last trimester of pregnancy. I recommend to separate the cows with body condition lower than 3.0, and feed them with a diet with higher energy density in late lactation, and if needed, also in the dry period, to reach, as possible, the body condition closer to the range of 3.50 to 3.75.