Declining Fertility - A Problem Of High- Yielding Cattle?

A rise in milk yield may point to signs of declining fertility in dairy cattle, says Prof. Heinrich Bollwein, Clinic for Cattle at the University of Veterinary Medicine, Hanover, Germany.
calendar icon 3 October 2010
clock icon 7 minute read

Animals with high milk yield are also more susceptible to diseases, which in turn is a cause of early foetal death in cattle.

To prevent this, care must be taken starting from the insemination stages, with attention paid to the energy balance of animals particularly in the first weeks after calving. Having the right prophylactic and therapeutic measures in place can help minimise the incidence of early foetal death while effectively countering problematic conditions such as inflammatory illnesses during this period.

A series of studies shows that a rise in the milk yield of cows goes hand in hand with a decline in their fertility. There are admittedly indications that this negative connection between milk yield and fertility is also due to genetic factors, but the heritability of the fertility attribute is relatively low.

Moreover, in many cattle herds the milk yield has been boosted in recent decades without a decline in fertility. That is why the question arises as to whether the reduction in fertility observed is in fact an inevitable problem of high-yielding cattle.

The cause of the drop in the pregnancy rate of about 0.5 to one per cent per annum observed in the USA and UK in recent decades is attributable not to a decline in the conception rate, but instead to greater occurrence of early foetal death during the first three weeks after insemination. Studies show that the conception rate with properly performed insemination using good quality sperm has remained almost constant at about 80 to 90% for decades.

By contrast, in many herds early foetal death occurs in about 30 to 40 per cent of the inseminated cows within the first three weeks after insemination. As positive pregnancy testing in cows is only possible as of day 25 after insemination with the aid of ultrasonics, early foetal death can only be proven by means of experimental studies and not in practice.

Negative energy balance

The negative energy balance occurring in high-yielding cattle appears to play a major role with regard to the early foetal death. Thus there is undisputedly a negative connection between the energy balance of the cows and the time required up to resumption of the oestrus cycle after giving birth.

The first ovulation takes place later in cows showing a high energy deficit during the first weeks after calving than in the case of animals with a more regulated energy balance. One important cause of delayed first ovulation after birth lies in the fact that concentration of the metabolic hormone Insulin-like Growth Factor I (IGF I) is reduced in these animals.

This hormone is produced largely in the liver and is partly significant for development of the egg sac. As cows with a negative energy balance suffer from liver insufficiency in the first weeks after giving birth, these animals do not synthesise sufficient IGF I for ripening of the egg sacs with subsequent ovulation.

However, a negative energy balance has a harmful effect on the quality of the egg cells too. In the case of cows that lose a large amount of body fat in the first weeks after calving, harmful fatty acids are released that are found not only in the blood, but also in the fluid of the egg sacs.

Delay insemination until after the first oestrus

In the case of high-yielding cattle with delayed resumption of the oestrus cycle after giving birth, the first oestrus is frequently used for insemination. However, cows that are inseminated in the first oestrus after the birth frequently show significantly lower pregnancy rates, as in these animals the uterine mucosa are not yet sufficiently formed to accept a foetus.

Changes in the uterine mucosa that promote the development of the foetus after insemination are only induced by a cycle with high levels of the hormone progesterone produced in the corpus luteum. However, a high progesterone level prior to insemination also appears to be responsible for ripening of the egg sac and hence for good quality of the egg cell located inside it. Admittedly egg cells of poor quality can also be fertilised, but the foetus developing from these frequently dies off.

Using hormone levels as indicator of pregnancy rate

In the case of cattle with a high milk yield, blood supply to the liver is increased in order to transport the nutrients ingested there and to metabolise them. However, not only nutrients but also oestrogens and progesterone are metabolised in the liver. That is why cows with a high milk yield often display a lower blood concentration of these hormones.

As oestrogens are of significant importance for the extent of oestrus, high-yielding cows often show only indistinct oestrus symptoms of short duration. Yet as oestrogens also induce the maturation of the egg sac, cows with low oestrogen levels display not only weak oestrus symptoms, but also low pregnancy rates.

The pregnancy hormone progesterone promotes the growth of the embryo in the early phase of pregnancy. That is why in an experimental study significantly smaller embryos were found after slaughtering in cows with low progesterone levels.

Lower pregnancy rates in cows with inflammatory diseases

A further cause of the high incidence of early foetal death in cattle lies in the fact that animals with a high milk yield are more susceptible to illnesses. In the case of inflammatory diseases such as for example udder and uterine inflammations, more prosta-glandines are released that can lead to dissolution of the corpus luteum and hence to a decline in progesterone with subsequent foetal death.

Furthermore, in bacterial inflammations toxins are formed that disturb the hormone balance. That is why cows with uterine inflammations develop ovarian cysts more frequently than healthy animals. As the toxins and mediators formed in inflammations also accumulate in the egg sac fluid, the egg cells too are harmed.

Optimal feed is important

The above causes of reduced fertility in high-yield cows lead to a series of prophylactic and therapeutic measures for avoiding this situation. It is crucially important around the time of calving that the animals be fed in line with energy needs as far as possible.

Moreover, for a few years now feeds with an elevated ration of Omega-3 fatty acids have been used to prevent an early foetal death through inflammation-related release of prostaglandins. However, it should be noted here that the corresponding fats have to be administered in protected form, as otherwise they are converted already in the rumen of ruminants.

Treat diseases in the first weeks after calving

During the first weeks after calving, special attention should also be paid in particular to the occurrence and consistent treatment of inflammatory illnesses. In view of the importance attached to knowing the time of resumption of oestrus cycles after giving birth, with both weaker and shorter oestrus symptoms in high-yield cows, in addition to conventional surveillance methods technical auxiliary means should also be employed to identify oestrus. Various systems are available on the market for this purpose. Equipment with which movement activity of cattle can be measured has proved particularly effective.

Hormone programmes not recommended

If no cycle is observed in cows over a relatively long period after calving despite these aids, a seven-day progesterone treatment and insemination in the subsequent oestrus can increase the pregnancy rate by comparison with animals inseminated in the first oestrus but without prior treatment.

Intravaginal administration of progesterone as of day 4 after insemination for a period of several days can also increase the probability of pregnancy by about 10 per cent, whereby such administration should only be carried out in animals with suspected progesterone deficits, in other words animals with high milk yield and negative energy balance.

The general deployment of hormone programmes for insemination of cows independently of oestrus symptoms is not to be recommended, as the pregnancy rates achieved here are generally relatively low.

Early pregnancy testing

Finally, the animals should be examined for pregnancy as early as possible, so that in the event of negative results renewed insemination carried out as soon as possible can avoid extending the inter-calving period unnecessarily. A reliable pregnancy diagnosis is possible by means of transrectal ultrasonic examination as of day 25, and via evidence of pregnancy-associated proteins in the blood as of day 30 after insemination.

To summarise, it can be ascertained that fertility in our high-yield cattle is reduced for a wide variety of reasons. By optimising management, especially feeding, and taking prophylactic and therapeutic measures in time, it is possible to prevent rising milk yield necessarily going hand in hand with a decline in fertility.

September 2010
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