THE REPEAT BREEDER COW

Over the years a good deal of effort has been expended in investigating the repeat breeder cow, that is the cow which has been served 5, 6 or even 7 times and continues to return to service every 21 days.

• endometritis

• thickening of the fallopian tube or adhesions of the bursa to the ovary

• ovarian cysts, which occasionally are present in the normally cycling animal

• pregnancy: although this is very rare, I have examined occasional repeat breeders which were pregnant!

Adhesions

Adhesions of the bursa to the ovary could be due to stretching or tearing at calving, metritis or rough handling of the ovaries on rectal palpation. (Manual removal of the corpus luteum or manual cyst rupture can lead to haemorrhage and subsequent bursal adhesions.) Abursa which is fused to the ovary may not be able to function properly, so that any eggs released at ovulation drop into the abdomen, rather than pass down the fallopian tube. This would obviously lead to a repeat breeder.

Use of GnRH

Pregnancy rate (%)

Data summarised from reviews by Mee et al.

Table 8.11. The effect of GnRH at the time of insemination on the pregnancy rate of cows.

At 12 days after insemination (the timing is critical) GnRH works by prolonging the life of the corpus luteum and in so doing it increases the length of the cycle. This means that the embryo will be a few days older before the cow considers producing prostaglandin from her uterus to start the next cycle. By being slightly older, the embryo can produce a stronger signal and this may be sufficient to ‘alert’ the cow to her pregnancy and ‘cancel’ the next cycle. After the injection of GnRH at 12 days, the cow will have a slightly longer (1-2 days) cycle length. In fact, if repeated injections are given every 3 days, many cows will not come on heat at all!

The use of GnRH is particularly good if the cows are stressed, for example following inseminations car­ried out just after turnout to grazing. Results show a 9-12% improvement in conception rates after the first service (Table 8.10), and in one trial this increased to 13% for cows at their second service and almost 30% at their third service. The number of animals involved at the second and third service was quite small, how­ever, so the data needs to be interpreted with caution. Others have shown that the use of GnRH or LH at both service and at 12 days post service produces a better result in problem herds, viz where conception rates are inherently low.

Use of Embryos

An alternative approach is to implant an embryo into a repeat breeder cow. This would work even if the fallopian tubes were blocked or there were bursal adhesions present. The technique has become a practi­cal possibility using embryos frozen in ethylene glycol, because these can be thawed in a single step, in much the same way as frozen semen. (Originally embryos were frozen in glycerol and had to be thawed in several stages.) Sometimes two embryos are placed in each cow, in the hope that twins will give dou­ble the amount of embryo ‘signal’ (bTb) and thus improve chances of conception.

Dosing Individual Cows

A wide range of mineral, multivitamin and ‘nutrient boost’ products have been tried, some with apparent success. By all means try using them, especially if they are inexpensive, but do your own trial to monitor their performance. They may or may not work in your particular situation.

ABORTION

One of the final hurdles in our components of the calving interval (see page 233) is the maintenance of pregnancy to full term to allow the birth of a normal live calf. If early foetal death occurs, it is most likely that the foetus will be reabsorbed in the uterus and nothing is seen. If the calf is expelled from the uterus at any stage of pregnancy before full term, then this is called an abortion.

The age of the aborted calf in days can be estimated by the distance from the crown of its head to its rump (or anus), using the formula:

age = 2.5 x (crown to rump length in cm + 21)

Most abortions are expelled from the uterus soon after foetal death and appear to be quite fresh. However, sometimes all of the placental fluids are reabsorbed and the calf becomes dry and chocolate-brown in colour. This is known as a mummified foetus and an example is shown in Plate 8.21. Mummified calves will often remain in situ, with some being expelled several months later - for example a 3 month old calf at 7 months of gestation - while others remain in the uterus and the cow simply fails to give birth.

Even with brucellosis eradicated, the average abortion rate for cattle in Britain in 1998 remains at approxi­mately 4%. This is based on the num­ber of abortions reported and checked for brucellosis by the Ministry of Agriculture, however, and so the actual abortion rate might be some­what higher.

Plate 8.21. A mummified foetus may die in early pregnancy, although it may not be expelled from the uterus for several months.

In some countries (including the UK) the law obliges you to report all cases of abortion to the Divisional Veterinary Officer, so that samples can be taken to eliminate the possibility of brucellosis.

Most of the diseases causing abortion are dealt with in detail elsewhere in the book. They include:

• brucellosis, Chapter 11

• IBR (infectious bovine rhinotracheitis), Chapter 4

• BVD (bovine viral diarrhoea), Chapter 4

• leptospirosis, caused by Leptospira hardjo, Chapter 13

• salmonellosis, especially £ typhimurium and £ dublin, Chapter 11

• any peracute fever, such as summer mastitis, Chapter 7

• Bacillus Iicheniformis

Diseases discussed in this chapter are:

• mycotic abortion (Aspergillosis)

• Q fever

• listeriosis

• Neospora caninum

• chlamydia

Aspergillosis is the most common fungal cause of abortion. It is a mould with a green/grey colouring which is often seen growing on silage. A typical example was shown in Plate 1.1. Suspect food should therefore not be fed to late pregnant animals.

Neospora is thought to account for some 20-40% of all bovine abortions. A protozoan parasite, Neospora caninum originates in dogs, and one Canadian study showed a strong association between the presence of dogs on the farm and the number of cows which blood tested positive to Neospora. Little is known about its method of spread. The organism probably multiplies in the gut, spreads throughout the cow’s body and passes to the uterus, where it then invades the developing foetus. Infection may cause abortion, mummified calves or calves born alive with brain incoordination (cerebellar hypoplasia, see Plates 1.8 and 1.9).

Chlamydia and Q fever are both members of the Rickettsia family, that is organisms which share properties of both bacteria and viruses. They are mainly associated with tick infested areas (see Figure 13.7), and both have been known to cause abortion.

Listeria monocytogenes more commonly produces abortion in sheep than cattle but can cause a brain infection in both species. The infection may originate from big bale silage.

Less is known about the causes of mummified calves. Possible factors include:

• BVD

• Neospora

• genetics (very occasionally particular sires will produce a high incidence in their offspring)

• stress in early pregnancy (proven in pigs but not in cattle)

Farmers tend to be more careful when handling heavily pregnant cows and this is probably a good thing in order to avoid teat and leg damage. I suspect that fairly severe mishandling is necessary to cause abortion.