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• 1999 July

1999 July

How Many Replacement Heifers Do You Need?

It is easy to drift along each year with the number of replacement heifers reared being determined purely by how many arrive in the first four to six weeks of calving. In tight calving pattern herds which are using AI for two rounds, up to 80% of the herd may get in calf to AI.  By the time culls are taken out this figure will be closer to 70% in a herd of stable size.  If 45% of these result in a weaned heifer calf, then 31% of the herd size is being reared.

In most cases, less than 25% are reared, mainly because few farms AI for more than two rounds and submission and conception rates are not high enough to achieve a six week, 80% pregnancy rate.  It could be argued that if few culls are needed, then the required replacement rate of heifers is lower.  If herd fertility is excellent, mastitis is well controlled and production and temperament have been heavily selected against, then the number of lactations per cow may be closer to six.

In most herds the average number of lactations is about four years which means that to maintain the herd age profile, 25% of heifers need to be introduced each year.

Factors, which may change this figure, are:

•  Recent introduction of a high number of younger cattle.
•  Expanding herd.
•  Need to cull on mastitis, age or fertility.
  

To ensure that enough replacement heifers are available it is important to plan the likely requirements two to three years ahead since this is the lag time from either mating or rearing to herd entry.

If a poor run of heifers has resulted this year, it is not too late to buy some extras and reach the target required.  Alternatively, the AI program on cows could be extended for an extra round.  This provides more calves but two months behind the first calves.  To increase the number of cows mated in the first three weeks, use of induction and CIDR programs will assist.

It costs between $400 and $1,000 to rear a calf.  The big variation in costs depends on whether or not feed is available at no extra cost for these yearlings until they enter the herd.  If all rearing costs, feed costs and labour are included, then the price approaches $1,000.

To have an adequate number of well grown heifers entering the herd each year is like a release from prison.  All the problems encountered with high cell counts, low fertility, old age and low production can be addressed adequately.
Without these animals, cows will stay in the herd that either pose a risk to herd mating, high ICCC's or perform poorly because of short lactations (late calvers).

Abomasal Displacements (DA's)

Also known as twisted stomachs, left displaced abomasum (LDA's), right displaced abomasum (RDA's), or abomasal torsions (AT's).

So far this year we have seen an above average number of cows with these conditions.

What are they?
    The fourth stomach - the abomasum, normally sits on the bottom right hand side of
    the cow's abdomen beside the rumen. When the abomasum is displaced it can cross to
    the left hand side of the abdomen between the rumen and the ribs.  This is an LDA
    and is the most common displacement.

    The abomasum can also move up the right hand side of the abdomen, an RDA. RDA's
    can progress to the most serious form of displacement, a torsion.

Why do they occur?
    There seems to be a combination of factors that lead to the development of DA's.
   

    In most cases we see them in cows that have been calved, between 2 and 8 weeks
    and are high producers on a diet containing a lot of concentrates and low intakes of
    pasture and hay. After calving, organs that were pushed out of their normal places by
    the growing calf are able to move around freely, and there are several factors that can
    lead to digestive upsets. There is often a severe dietary change, high levels of
    concentrates and borderline blood calcium levels.

    This combination of factors leads to slower passage of material through the    
    abomasum, and the possibility of gas remaining in the abomasum. The abomasum
    then floats usually under the rumen and up the left side getting squashed between the
    rumen and the ribs, or sometimes up the right hand side.

Abomasal torsions result from the twisting of the abomasum as it rises on the right side. A lot of RDA's progress to torsions as the abomasum gets bigger and rises higher.

What do affected cows look like?
    Cows with LDA's drop in production and body condition, and are commonly seen by us
    after not responding adequately to treatment for acetonaemia or retained membranes.
    These cows don't look very sick, and are usually just not doing as well as they should.
    If left longer they can develop acetonaemia and go off their milk completely as they
    continue to lose weight. Cows with RDA's initially look the same as LDA's but they can
    quickly deteriorate if the abomasum twists. Then they go completely off their milk,
    look full on the right side, get sunken in the eyes, appear painful in the stomach, pass
    small amounts of dark to black faeces, and get weak quickly. Once twisted these cows
    can die quickly, sometimes in 24-48 hrs.

What can we do to fix them?
    We believe that the best treatment, in most cases, is surgical correction. This involves
    opening the cow in the right flank, removing excess gas or fluid, repositioning the
    displaced stomach to the correct location, and stitching it in place. Cows with LDA's
    respond well to surgical correction and usually recover quickly and come back on their
    milk fully. Cows that have had a LDA much longer than a week or that have lost a lot
    of weight may take longer to recover and may not regain full milk production, but will
    usually survive. We expect a better than 95% success rate with most LDA's. Cows
    with RDA's are usually more severely affected and consequently the success rate is
    lower. The major factor influencing a successful outcome is how long the cow has been
    affected. Cows with AT's often have a poor prognosis due to internal bleeding and
    shock.

What can you do to prevent them?
    Prevention of DA's is not always easy, perhaps especially this year due to the low
    growth rates and low average daily intakes of pasture.

    Some points to consider are:

• Minimise sudden dietary changes from a dry cow diet to a milking diet by allowing access to concentrates before calving and continue hay after calving.  If paddocks are too wet, offer hay in rings on track through July and August.
• Prevention of milkfever through the use of anionic salts before calving and limestone after calving.

Teat Disinfection and Water Quality

In the last newsletter Peter Younis wrote on the factors that influence teat disinfectant efficacy, especially the importance of good quality water for mixing the teat disinfectants.

As part of the recent mastitis seminars, and as part of recent mastitis investigations, we have tested numerous teat spray and water samples.  To date, only two samples of iodine teat disinfectant have had adequate levels of available iodine in them.  Some samples have had iodine levels less than one-third the recommended level despite having been properly mixed according to the label directions.  Most of these samples had been mixed with what appeared to be good quality water.  The people using rainwater tended to have higher levels of iodine in their mix than those using bore water, town water or water from dams.  However, the levels of available iodine were still below the recommended 0.5%.

This is a clear indication that the water being used to mix teat disinfectants must be tested and that it would be wise to have the mixed product tested at the same time.

For more information contact the clinic.