Planning the integration of heifers into the herd is critical for good lifetime fertility performance. Heifers that have reached 65% of mature cow body weight should be cycling and ready to breed (Fig 1). For herds breeding their own replacements for 2 year-old calving this means a target body weight of 420kg at 15 months (assuming a mature cow weight of 650kg). This requires an average daily LWG of around 0.85kg from birth. In practice this means heifer calves destined as homebred replacements should be selected from the calves born in the first month of the subsequent calving to ensure these growth targets are possible.
Early maturing breeds or breed crosses will be easier to rear for 2 year-old calving. If using AI, then bulls can be selected with good maternal trait EBV's. Selection Indices are now available for many bulls to help in the selection of bulls for breeding replacement heifers. The Self Replacing Index figures should be used (Fig 2) if this is a priority in the herd whereas Terminal Sire Index is designed for herds solely breeding calves for slaughter.
As well as monitoring growth rates and nutrition to ensure target mating weights are met there are other factors that can affect heifer fertility.
Ideally maiden heifers should be bred for 6-7 weeks only to an easy calving bull or put through a double synchronisation programme (see synchronisation bulletin). This will ensure only the most fertile animals are kept as cows - it is not really sensible to keep a heifer that takes 3 or more serves to conceive as a breeding animal. In 6 weeks (two cycles) a fertile bull should settle at least 85% of the heifers so plan to mate a few more than needed.
A six week mating period will ensure maiden heifers calve down in the first half of the calving period and this is critical to avoid the risk of becoming barren as first calvers during the next breeding period. As discussed in bulletin 1, young cows are at risk of extended anoestrus due to the pressure of suckling a calf and still trying to grow to mature cow size.
If possible, especially if heifers are 18 months or older, the heifer mating should be planned to start 2-3 weeks ahead of the cows as this will give the 1st calvers significantly longer to overcome anoestrus before the mating period starts.
Managing heifers as a separate group through first calving and subsequent mating is ideal as will remove the competition stress from being mixed with mature cows however on many farms this is impractical due to limitations on housing and group sizes for mating.
High levels of calving problems (dystocia) will lead to reduced fertility in the subsequent mating period (Fig 3). Cows that have an assisted calving or caesarean will have delayed uterine involution, increased chance of uterine infection, retained placenta, uterine prolapse (Fig 4) and extended anoestrus - all of which can reduce the chance of conceiving at the next mating period.
If herds have >5% assisted calvings then it highly likely this could be having a significant effect on herd fertility. Clearly this will also affect the calf crop % as there is likely to be increased losses due to dead calves at birth or neonatal disease associated with calves not sucking colostrum promptly.
If a beef herd is going to achieve and maintain target fertility figures then infectious diseases that affect fertility cannot be tolerated. Losses can occur throughout the production cycle ranging from reduced conception rates, to embryonic deaths, abortions and poor calf viability (Fig 8).
As part of the farm's health plan a risk analysis should be carried out for the main infectious diseases that can affect fertility which would include:
All of these diseases can cause significant fertility losses in a beef herd and your vet can advise you which need to be controlled on your farm. Once herd status and risk factors are established for these diseases, control may take the form of strict biosecurity, vaccination, erradication or a combination of these options. Luckily we have effective vaccines against many of these diseases and rather than consider vaccination an unnecessary expense it should be looked upon as essential insurance for herds wishing to maximise fertility and productivity.
BVD can cause devastating losses if introduced into a susceptible group of breeding cows or heifers during the mating period as can venereal Campylobacteriosis. Johnes disease may manifest in a herd as a high % barren in 1st and 2nd calvers without the classical signs of scour, as sub-clinical disease leads to extended anoestrus due to poorer than average BCS . Herds planning to home breed replacement heifers must take Johnes control seriously if it is endemic on the farm. Herds purchasing heifers must take steps to try and source replacements from accredited or low risk herds if possible.
Many herds for years have ignored the potential of the bull stud to be the limiting factor in herd fertility (Fig 9). If poor results are found at the annual PD session the temptation is to look for cow infertility causes. In herds that are managing cows and heifers well and controlling infectious disease it is often the bulls that are limiting herd fertility performance especially when restricted 9-week mating periods are being used.
Having examined hundreds of bulls for pre-breeding soundness on farms over the past few years it is common to find bulls that were about to be used that are subfertile or infertile for various reasons. For a wider discussion of this topic and how your vet can help to identify problem bulls refer to the bulletin on this subject.
For beef herds to achieve and maintain key fertility performance targets that will maximise profitability the following areas should be considered:
NADIS hopes that you have found the information in the article useful. Now test your knowledge by enrolling and trying the quiz. You will receive an animal health certificate for this subject if you attain the required standard.