It is estimated that in dairy herds silent heat varies from 10 to 40% in all cases, depending on milk yield and maintenance and feeding conditions.
The profitability of milk production is closely related to the proper reproduction, and any missed heat in a cow reduces its milk production in 305 days of lactation. Lack of visible heat until the 60th day after the birth is considered as a pathological condition and half of the cases are in silent heat. This is one of the most serious causes of the reproductive disorder, financial losses of farms caused by the extension of the inter-pregnancy period (usually about 2 months).
Silent heat consists of not showing characteristic heat symptoms, which should occur cyclically, despite hormonal changes that occur in the reproductive system during the cycle.
During a normal heat, cows show symptoms such as vocalizing, jumping on colleagues, sniffing, trickling in place, vaginal mucus, swollen and reddened vulva, bending of the back, daily decrease in milk production and decrease in appetite.
Normally the oestrus takes about 18 hours, in heifers for about 16 hours. In about 20% of individuals, the oestrus can be observed only for 6 hours, moreover, the symptoms are often shown only at night, which makes it very difficult to detect the heat properly and thus insemination in good time. In herds observed correctly, the detection rate of heat should be at least 80%.
Causes of silent heat
At first, the occurrence of silent heat was considered to be an organizational problem for staff or breeders, due to incorrect or too short observation, incorrect timing of insemination, or even incorrect performance.
However, the main cause is now considered to be a hormonal imbalance, since blood tests have shown that cows or heifers in silent heat had lower oestradiol levels and higher levels of progesterone than cows in the oestrus period with visible symptoms.
Disabled production of these hormones results from abnormal feeding of cows at the beginning of lactation and, above all, from energy shortage in the ration. This leads to a decrease in the production of gonadoliberine from the hypothalamus, which leads to a decrease in the production of gonadotropin from the pituitary gland and thus to lower growth of the ovarian follicles and finally to a lower level of estrogen production. The lower the level of hormone production, the less visible are the symptoms of heat.
In addition to an incorrect balance of the feed ration, a deficiency of vitamins, minerals or too many isoflavones (post-extraction soya meal) is also important. Most silent heat is found at the turn of winter and early spring, which may be related to worse maintenance conditions and changes in nutrition.
The first and the most effective solution is to balance the feed ration adapted to their needs, the second is the individual analysis of each cow’s reproductive cycle and the use of the herd calendar, and the third is the use of heat detection systems that help to detect even silent heat. All of them applied at the same time will ensure detection of all heat at the level of up to 98%. By simultaneously analyzing changes in animal activity, feed intake and daily milk yield, the e-stado shows the high efficiency of heat detection, which will improve reproduction parameters and thus the profitability of milk production.