Pregnancy testing of beef cattle
The key to profitability for all beef breeding enterprises is high reproductive efficiency. This means achieving:
- 95% calves weaned to cows joined
- an average calving interval of 12 months
- a calving spread of 10 weeks or less.
These are all realistic objectives in Victoria.
Pregnancy testing is one method of monitoring reproductive efficiency and detecting any problems early in the breeding cycle, so that these objectives can be achieved.
Rectal palpation is the cheapest and most convenient method of pregnancy testing cattle. Using this method, veterinary practitioners can identify pregnant cows as early as six weeks after conception. They feel for the calf's head, a pulse in the artery supplying blood to the uterus, and the shape of the cow's uterus.
Pregnancy testing is normally carried out 8-10 weeks after the end of mating. Cows need to be restrained in a race; it is not necessary to headbail each one. In well designed yards and with labour provided to keep cattle moving into the race, up to 60 cows can be pregnancy-tested per hour.
Benefits from pregnancy testing
Early detection of non-pregnant cows is the main benefit from pregnancy testing, but there are others.
In many cases, the age of the calf and the likely calving date can be estimated during rectal palpation. Cows expected to calve early can then be separated from cows expected to calve late. This can provide a useful basis on which to cull cows if it is necessary to reduce herd size, perhaps in times of feed shortage. The calving spread can also be quickly reduced if late-calving cows are replaced with heifers that conceived early.
Various abnormalities responsible for infertility in cows can also be identified. The more common of these include cystic ovaries and uterine infection. The occasional freemartin heifer and other abnormalities of the reproductive organs may also be detected during rectal palpation. Diseases and management problems affecting the whole herd can also be identified much earlier if cattle are pregnancy tested. Low pregnancy rates in one particular mob, for example, might indicate problems with an individual bull. Poor fertility throughout the whole herd might be caused by an infectious disease, or perhaps inadequate nutrition prior to mating.
Using this information
Pregnancy testing is of little use as an aid to management unless the information gained is used to make management decisions.
Feed availability, current beef prices, and the management system used on the property must all be considered when deciding the fate of non-pregnant cattle. By pregnancy testing, the beef producer is in a position to make the best possible decision. In most situations non-pregnant cows are best culled as soon as possible. The cost of owning and maintaining a beef cow for a year is very high, so it is important that every cow on the property is fully productive. Even if they have calves at foot, non-pregnant cows are only partially productive. Mature cows sometimes fail to conceive after a late calving. Such cows wean the youngest, smallest calves, and are therefore best culled.
Under certain circumstances it might be better not to cull non-pregnant cows immediately. If feed is plentiful, and particularly if beef prices are depressed, the best strategy might be to draft off non-pregnant cows and fatten them for sale at a later date. Alternatively, the most profitable decision might be to sell them after their calves are weaned. It is a common practice to "bang" the tails (that is, remove about 100 mm from the switch) of cows to be culled at some time in the future, so that they can be identified for up to six months.
The other option is to re-mate non-pregnant cows, particularly young cows, for a later calving. This strategy may have application on properties that have more than one calving period each year. On properties with one calving period, small groups of late calvers create management difficulties.
Where non-pregnant cows are re-mated, good records are needed to ensure that barren cows are not being kept and moved between mobs calving at different times.
On many properties cows are not pregnancy tested but are simply culled when they reach some predetermined age. Quite often cows culled for age at 9, 10 or 11 are still rearing good calves each year. On the other hand, some of the younger cows retained in the herd fail to conceive.
Old, pregnant cows are obviously more productive than young "empty" ones. Culling cows on the basis of a pregnancy test is much more efficient than simply culling on age.
Pregnancy diagnosis also has implications for marketing. Cows sold by live weight will attract a higher price if they have been certified by a veterinarian to be non-pregnant.
The two main considerations of whether non-pregnant heifers are given a second chance to conceive are the breeding value of the heifers and the cost of carrying the heifers over. When a group of heifers have been reared and mated under similar conditions, those that fail to conceive are less fertile than the group. It is possible that these heifers will fail to conceive if kept for a second joining, or if the heifers conceive, the tendency displayed toward lower fertility may be passed on to heifer daughters.
For maximum profitability, heifers should usually be brought into production as early as possible. The cost of feeding and maintaining non-pregnant heifers for an extra year cannot be justified, except in exceptional seasons where feed is plentiful. Heifers that fail to conceive at first mating should therefore be culled. The exception to this is on properties with a split calving, where non-pregnant heifers could be re-mated with another mob in the same year.
It is desirable to mate heifers for a short period, (6-8 weeks) to ensure a compact calving. It is also a sound practice to mate more heifers than are needed for replacements. Heifers can then be pregnancy tested eight weeks after the end of mating, and only those pregnant need to be kept.
Other methods of pregnancy testing
Portable ultrasonic pregnancy detectors are an alternative to manual manipulation to detect pregnancy 6-8 weeks after conception. "Doppler" scanners have an external probe containing both transmitting and receiving elements, which project a beam of low-energy sound waves.
The beam is reflected by the uterine artery, umbilical vessel or foetal heart, and undergoes a change in frequency which is converted to sound or a light display which allows the operator to determine pregnancy status. A more accurate but more expensive alternative is the sector linear or "Real Time" scanner, which has a probe which is inserted into the rectum, as close to the uterus as possible. Reflected sound waves are transmitted to a light display, from which an experienced operator can interpret pregnancy status.
Ultrasound technology is ideal in a research situation, where high accuracy of determining pregnancy status and age of foetus are required. However, as this method is slow and expensive when compared to rectal manipulation, it is unlikely to be adopted on a large scale in a commercial situation.
Alternative to pregnancy testing
Producers may choose not to pregnancy test on grounds of cost. They may choose instead to detect pregnancy by monitoring oestrus or returns to heat. Once cows conceive, all but about 5% cease cycling for the duration of the pregnancy. Therefore, oestrus detection, after the end of the mating period, can be a useful alternative to pregnancy testing, with only slight inaccuracy. Any error could be reduced by pregnancy testing cows that display oestrus after the end of mating. Bulls or teaser steers fitted with a chinball harness or tail-painting methods are probably the most convenient aids to heat detection.
Due to the extra labour required to frequently check cattle for signs of heat, oestrus detection is unlikely to substantially reduce the cost of identifying non-pregnant cows over pregnancy testing. Other alternatives to pregnancy testing are measuring the levels of progesterone in the blood or milk, but involve extra labour and laboratory costs. In dairy cows, when the date of insemination is known, determination of the concentration of progesterone in milk taken 24 days later has been shown to be 84.5% and 97.0% accurate for positive and negative tests respectively. However, the milk progesterone test is not generally applicable to beef cows as service dates are frequently not known and milk samples are less readily obtained.
Non-pregnant cows are unproductive and should be removed from the herd. Pregnancy testing 8-10 weeks after the end of mating is a cheap and convenient method of identifying non-pregnant cows at an early stage. Pregnancy testing can also identify herd fertility problems, enabling earlier investigation and action than would otherwise be possible
Geoff Kroker, Bendigo and Lisa Clarke, Hamilton