How to manage a tight block calving

Block calving can have financial benefits compared to all year round (AYR) calving herds when it is managed correctly. However, for block calving to be economical, 90% of your herd needs to calve in six weeks and your entire herd in 12 weeks. This means you need to achieve a 6-Week In-calf Rate of 75% and aim for a failure to conceive culling rate of below 8%.

Meeting these targets requires excellent fertility performance and management. This article from Zoetis looks at some key focus areas on managing a tight block.

Maintaining a tight calving period is the top priority for block-calving herds. It is not unusual in AYR calving herds to see cows having as many as eight oestrus cycles before they get back in calf. In a block calving herd this isn’t viable.

A 12-week (90 day) calving period will allow only four oestrus cycles and a 60-day just three cycles for successful pregnancy. During this time, you have to accurately detect heat and get the cows pregnant. Cows failing to get in calf during this time may need to be culled if a tight calving period is to be maintained.

There are two factors determining the fertility- heat detection (submission rate) and the pregnancy rate.

The Agriculture and Horticulture Development Board (AHDB) has devised a table which determines the Fertility Factor. This is calculated by multiplying the submission rate by the pregnancy rate (see table 1). The aim should be a Fertility Factor of 40%.

It’s important to remember the earlier you can get a cow in calf the more days in milk she will have and more time she will have to recover post calving before the next breeding cycle.

If you are using a 6-week calving block (42 days) then breeding must commence from 38 days post calving period. Ideally you want to know if cows are cycling before then, which means heat detection begins as animals calve down.


Working with your vet to achieve a tight calving block is important. They ideally should be involved in post-calving checks to identify any problems such as metritis or any other problems post-calving, which could have a knock-on effect on cows cycling. The use of a nutritionist will also ensure any dietary issues are resolved a couple of weeks before the start of the service period.

There are some essential things that are needed when block calving. They include:

  • Accurate heat detection. Sufficient time should be allowed to detect heat with ideally three-four 30-minute observations a day
  • Use of heat detection aids such as pedometers, tail paints, heat mount detectors
  • A vasectomised or penned bull can be useful to maximise signs of oestrus if you have the facilities to accommodate a bull on farm
  • Accurate records of heats are essential to ensure cows are inseminated at the correct time
  • Making sure cows at the correct body condition score of 2.5-3 at drying off and 2-2.5 at service
  • Making sure AI facilities are ready and semen is stored and handled correctly


Oestrus synchronisation programmes can be really valuable in block-calving herds. They have been found to greatly improve pregnancy rates and reduce the reliance on heat detection1.

Using a CIDR synchronisation programme involves the following steps:

  1. Injection of a Gonadatrophin-releasing hormone (GnRH) at the time of inserting the CIDR device. GnRH e.g. Acegon ensures any dominant follicle present at the time of CIDR insertion is ovulated so that a fresh follicle develops ready for service. Dairy cows will benefit from the increased fertility of a fresh follicle being ovulated, particularly if they are not cycling normally.
  2. Inserting the CIDR device. The CIDR contains the hormone progesterone which is important for a healthy dominant follicle. Progesterone priming improves subsequent luteal phase duration to help with pregnancy or the subsequent cycle. It also improves the growth and health of the developing embryo and improves the signs of heat.
  3. 24 hours before the CIDR is removed you need to inject a prostaglandin. Lutalyse and Prellim are both examples of prostaglandins that act to destroy a corpus luteum. Using a prostaglandin, the day before CIDR removal ensures that any corpus luteum present is removed so that once the CIDR is removed ALL progesterone is eliminated.
  4. Remove the CIDR 7-8 days after insertion, 24 hours after the prostaglandin injection
  5. 36 hours after removing the CIDR give another injection of GnRH to ensure that ovulation takes place and to control (synchronise) the timing of ovulation ready for fixed time AI. This injection is not needed if heat detection is performed.
  6. AI can take place at a fixed time of 16-20 hours after the GnRH injection

Table 1. Heat Detection, Pregnancy Rates and Fertility Factor

In conclusion, there are many benefits to block calving herds as we have covered previously. To maximise success in these herds, fertility of the heifers and cows should be carefully planned, monitored and treated where required. The quicker the animals are able to get in calf, the more milk they produce in their lactation and therefore the more profitable they are. Through working with your vet you can use synchronisation programmes such as CIDR-sync to reduce the reliance on heat detection by allowing fixed time AI and ensure that noncycling cows are quickly and effectively treated so that they have more opportunity to get in calf. Speak to your vet for more information about how they can help you manage cow and heifer fertility in your block calving herd.


ACEGON contains 50 μg/ml gonadorelin (as gonadorelin acetate): POM-V.

LUTALYSE contains dinoprost 5 μg/ml (present as 6.71 μg/ml of dinoprost tromethamine: POM-V.

CIDR contains 1.38 g progesterone: POM-V.

PRELLIM contains 0.075 μg/ml d-cloprostenol (asd-cloprostenol sodium): POM-V.

For further information, please contact your veterinary surgeon or Zoetis, Walton Oaks, Dorking Road, Walton-on-the-Hill, Tadworth, Surrey, KT20 7NS. Use medicines responsibly ( Always seek the advice of your medicines provider. AH507/18

1. Source: Diskin and Others (2001) BSAS Occasional Publication 26.

1st April 2022

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