by Dr Charlie Blackwood

Synchronising cows can have many advantages including:

1. Easier heat detection
Simultaneous display of heat by many cows enables easier detection of cows showing subtle or weak signs of oestrous. Heat detection aids such as scratchies, Kamars and tail paint tend to be more reliable when many cows are showing heat at the same time.
NB- some synchrony programs have a fixed time for insemination and do not require heat detection for the first insemination. However, good heat detection after the initial insemination can be a critical factor to achieve overall good herd fertility. Good observation and application of heat detection aids are important for the full duration of the AI period.

2. Earlier detection of non-cycling cows
Detection of heat earlier in the AI period allows those cows which are not cycling to be identified and therefore treated earlier. Pre-mating heat detection can also work well for some herds. Regardless, all cows that have not displayed a heat by day 21 into the mating period should be vet-checked. Delaying examination of these animals will be cost more money in the longterm.

3. Tighter calving pattern
Breeding more cows over a shorter time frame can help tighten the calving pattern. The earlier cows calve, the more likely they are to get back in calf at the subsequent mating period. An increased number of earlier calving cows also increases milk production early in the season.

4. More heifer calves born early
Early-born heifers have the advantage over later-born heifers in that they are more likely to achieve target weights for joining. Having a good replacement rate of heifers allows a greater ability to selectively cull cows with problems.

5. Efficient use of labour
On many farms, having a tight calving pattern allows more efficient use of labour and better observation of calving cows. Transition feeding is also easier to organise with groups of cows calving at a similar time.

6. Allows use of artificial breeding on heifers
Synchronising heifers can also be advantageous:

  • Early calving heifers (sometimes 2 weeks before the cows) can improve reproductive performance at the next mating period.
  • Decreased number of bulls required.
  • Increased genetic gain by using sexed semen.
  • Efficient use of time and labour. Synchronising allows you to efficiently plan when you will treat and AI heifers without the need for constant heat detection. This is useful when heifers are not close to the dairy.

• ODB (Oestradiol Benzoate) is not used in lactating cattle, but can still be used in heifers.
• CIDRs are commonly used to treat non-cycling cows.
• Farmers need to compare the extra cost of using CIDRs with the extra work and possible chance of a missed heat. 


Synchronising cows or heifers is NOT a magic cure for an extended calving pattern. Synchronising may be PART of the solution to the problem, but there are many other aspects to consider. You should discuss the benefits and costs of synchronising with one of our vets before deciding if it is the best option in your case.
Synchronising alone is unlikely to help if: 

  • Cows are only recently calved. 
  • Cows are in extremely poor condition and are not cycling. 
  • Heifers are poorly grown and are greatly below target weight. 
  • There are limited resources to handle a large number of cows and heifers calving over a short time next year.


PLANNING is the key issue to getting a good result from a synchrony program.
There are a number of programs available and in discussion with your vet, you need to decide which program fits best and how it will be done.

Synchrony programs for unmated Heifers

  1. AI for 5 days and inject all the unjoined heifers with PG on day 6. The remaining heifers should all come into oestrus within the next 5-7 days. This is a cost-effective program, since it involves injecting about three quarters of the group once.
  2. Inject the hiefers with PG 12-14 days before MSD, and again on MSD. All the heifers should be inseminated within a week.
  3. Programs using CIDRs, ODB, and PG injections. There are several variations of this program but they all allow for fixed timed insemination of all heifers on the same day. This is more expensive than the above programs, but only requires the heifers to be yarded 4 times (3 treatment yardings, and 1 AI yarding) to inseminate them all. 

Synchrony programs for Lactating Dairy Cows

  1. The Modified Why-Wait Program involves mating to detected heat for 7 days and injecting all the unmated cows with PG on day 7. All cycling cows should be mated by day 14, allowing identification of non-cycling cows by this time. 
  2. The Aggressive Prostaglandin Program or Double PG program involves injecting the whole herd with PG 14 days before MSD and again on MSD. All cycling cows should be mated within a week. NVO’s can be treated 11 days after the second PG, and will get two chances at AI in a 6 week AI period, compared with other programs they would probably only get one chance. Kamar heat detectors only last 10 days on cows, so if you are going to use Kamars and tail paint as your sole means of heat detection, then this may be a suitable program for your herd.
  3. Ovsynch involves 3 or 4 injections, and allows all cows to be inseminated on the same day. Giving 2 PG injections - one on day 7 and one on day 8 has been found to improve conception rates.

The secret to successful synchrony is choosing the right program for your needs, and careful planning. 

Talk to one of our vets for more information.
One of the aims of a synchrony program is to detect non-cycling cows as soon as possible and treat them.