US:Pre-puberty growth no
Research from the USA
sheds new light on heifer
rearing by suggesting that
growth rate to puberty need
not be restricted. But UK
scientists say treat the
results with caution.
Jessica Buss and
Sue Rider report
DAIRY heifers can be grown rapidly before puberty without a detrimental effect on milk production providing their diets contain enough protein.
So says US researcher Mike van Amburgh who used 270 heifers reared at three growth rates – 0.68kg, 0.83kg and 0.94kg a day – for a study at Cornell University, Ithaca, New York state.
Evdience from the trial suggests that growing heifers more quickly pre-puberty could increase profits by £65 a heifer by the end of the first lactation.
Dr van Amburgh believes that relying on udder development to show the effect of increased growth rates on milk yield has been misleading, because the animals requirements for protein have never been met.
"Heifers in previous studies have been fed more energy to accelerate growth, but lack of protein produced short, fat heifers with fat in the mammary gland."
For his trial, Dr van Amburgh said he improved the nutrition by supplying more protein, but without designing complicated diets. He then studied the effect on yield.
He estimates that growing heifers more rapidly at 0.94kg pre-puberty represents a saving of £80 a heifer – based on typical rearing costs of 90p a day – when heifers calve three months earlier at 21 months old.
"Typically heifers in New York state calve at 28 months old, reducing this to 21 months could save producers £190 a heifer reared."
He admits that in his study heifers grown quickly up to puberty had a lower average 305-day fat corrected yield at 8558kg, than those grown more slowly which averaged 9008kg. But the spread of results indicated that little of the 450kg difference in milk yield was due to growth rate pre-puberty.
"Actual bodyweight at calving has a bigger impact on milk yield than growth rate," he says. This was shown by ranking the heifers according to weight at calving.
He explains that heifers from the three treatment groups had to be mixed after service because space was limiting. Those that had been well fed in their treatment group were less competitive for food, despite an adequate ration being offered. Heifers that had previously received lower quality diets thrived on this diet and showed compensatory growth.
Because the heifers were all served, when they reached the required weight, before mixing, those grown fastest at 0.94kgm a day pre-puberty were three months younger and 30kg lighter at calving, than heifers grown at 0.68kg a day.
Heifers grown quickly to puberty weighed less and this proved to be the reason they gave less milk in their first lactation, he claims.
Even so, loss in milk at 420kg a heifer between the fastest and slowest grown up to puberty was small, and valued at £16 a heifer after feed costs. When this is compared with the saving in rearing costs of £80, these faster grown heifers produced an estimated £65 extra profit by the end of their first lactatio.
Dr van Amburgh now believes that growing animals at up to 1.1kg a day from 90 to 320kg bodyweight can increase profits, but protein supplied in the diets of animals grown quickly must be adequate. However, the type and digestibility of the protein fed is less significant. It does not have to be by-pass protein, he says. The diets fed in his trial included hay, maize silage, soyabean meal, a concentrate mix, urea, vitamins and minerals.
Heifer weight Average DM intake Diet crude protein
kg kg %
90-180 3.6 17-18
180-270 5.5 16-17
270-360 7.5 15-16
US-STYLE HEIFER REARING
• Growing faster saves costs.
• 1.1kg/day from 90 to 320kg.
• Weight at calving bigger impact on yield than growth rate.
• Must feed high protein diet.
Heifers in a study at Cornell University proved more profitable when grown at 0.94kg a day before puberty, says Mike van Amburgh (inset).