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Importance of optimum weight gains in pregnant heifers

In a bid to calve heifers before two years old, the focus has predominantly been on reaching target weights for service. Hannah Noble finds out why weight gain throughout pregnancy is just as important.


Hannah   Noble

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Hannah   Noble
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Nutrition at all stages while the heifer is growing is vital to meeting its potential
Nutrition at all stages while the heifer is growing is vital to meeting its potential
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Importance of optimum weight gains in pregnant heifers

Hefin Richards, Rumenation nutrition consultancy, explains why heifers need to keep growing once pregnant

When it comes to calving heifers to join the milking herd, young should not mean small, explains Hefin Richards of Rumenation nutrition consultancy.

 

Heifer weight rather than age usually determines the onset of oestrus, therefore growth rates need to be at an optimum throughout the whole rearing period to ensure heifers are big enough to cope with the rigours of calving and going on to start their first lactation.

 

The target weight at service should ideally be approaching 60 per cent of mature weight, aiming for service between 13-15 months of age to allow for calving by 24 months.

Mature weight

Mature weight can be calculated by weighing mature cows in the herd and working out an average, by collating cull cow weights or by using the standard breed average.

The benefits of calving heifers younger has been widely publicised including, overall higher lifetime output, increased productive life, better fertility and potentially faster genetic progress.

 

Mr Richards says: “It is all too easy to think once a heifer is in-calf ‘the job is done’. They are in-calf over a fixed period and they need to keep growing, it is easy for heifers to be too fat or too small at calving if you take your eye off the ball.”

Key stages

He says heifers should be at 85-90 per cent of their mature weight by calving. In order to achieve this it is important to regularly weigh them at key stages and at least once every three months.

 

“Aiming to grow heifers to the correct size before calving and not weighing them is like doing it with your eyes closed,” says Mr Richards.

 

Working on an average mature Holstein weight of 700kg, the weight at service, providing the heifer had reached her target 60 per cent of mature weight, would be 420kg.

 

Therefore, for her to reach 90 per cent by calving, the target daily liveweight gain (DLWG) during pregnancy should be at least 0.75kg/day. Over 270 days of pregnancy this amounts to a gain of 202kg. This gain includes the weight of the calf and calving fluids.

 

However, if a heifer is only gaining 0.5kg/day her overall weight gain could be nearly 70kg lower over her pregnancy.

Missing targets

Missing or exceeding DLWG targets can cause problems which significantly increase the risk of heifers being culled in their first lactation:

Underweight heifers:

  • Loss of growth potential – Resulting in a smaller mature animal
  • Growing while lactating – Heifers calving under the desired 85-90 per cent of mature body weight will still be actively growing once they have calved, therefore their demand for energy for growth and lactation means they may struggle to cope
  • Decreased production potential - Milk production will be lower in the first lactation when heifers are calved at a younger age. Milk secretory tissue, however, will continue to develop into the first lactation, causing a peak in yield slightly later than usual. This peak will be delayed further for heifers which are underweight at calving and may peak at a lower level
  • Calving difficulties – Underweight heifers are less able to cope with the stress of calving than heifers of an ideal body condition score (BCS) of 2.5 to 3.0, leading to problems when calving
  • Unable to meet required feed intakes – In order to keep up with the energy demand of milk production feed intake must be increased once a heifer calves. Heifers which are small when they calve do not have the physical capacity to reach required intakes and will be in negative energy balance for longer, therefore delaying rebreeding and limiting milk output

Overweight heifers:

  • Calving difficulties – Over-fat heifers tend to have a greater risk of calving difficulty due to fat deposits in the pelvis causing a constriction of the birth canal
  • Ketosis – Mobilisation of body fat is normal on the onset of lactation, however heifers with a BCS higher than 3.75 are at greater risk of developing ketosis, leading to a depression in appetite which, combined with increased demand for energy for milk production, causes rapid weight loss. This also impacts immune function, milk production and general metabolic health
  • Decreased fertility - Heifers above sub-clinical levels of ketosis in the first two weeks of lactation are 50 per cent less likely to hold to first service

Mr Richards says the ration provided for in-calf heifers is also important.

“You must know the quality of forage you are feeding, make sure silage is analysed and if it is not as good as it should be, the shortfall needs to be supplemented.”

 

“As a group, in-calf heifers are also often used to mop up late season grazing – this can be a feast or famine scenario. In a good season, they will over consume energy and lay down fat, and in a more challenging autumn, will fail to meet DLWG targets when grazing without additional buffer feeding.”

 

A pregnant heifer will require nutrients which cater for her maintenance, the requirement of the calf and, during her first two pregnancies, growth. Environmental conditions can also have an effect on the maintenance requirement.

 

Mr Richards says it is important crude protein requirement is met, as this is vital for maintaining the target DLWG without laying down excess condition. Protein is extremely important in the diet of growing heifers to ensure adequate frame size and growth and can be a limiting factor on many farms.

He says: “Use the protein and energy content of the forage to work out what the concentrate requirement is.

 

An in-calf heifer at 525kg, mid-way between conception and calving, will have a typical dry matter intake of 8.5-9kg/day and requires about 95-100 MJ of energy per day for maintenance and growth. The target for protein content of the diet should be 15-16 per cent.

 

“Macro minerals, particularly calcium and phosphorus, are important for bone development in growing animals, and micro minerals [trace elements] should also be balanced around the forage base,” says Mr Richards.

 

“The overall aim must be to calve a well-grown, healthy heifer with the potential to go and milk and rebreed within the target 22-24 month age range. Nutrition and management at all stages is key to this.”

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