Live weight and growth of dairy heifers are important for subsequent milk production and reproductive performance : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science, Massey University, Turitea, Palmerston North, New Zealand
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Date
2019
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Massey University
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Abstract
In the pasture-based farming systems that occur in New Zealand, dairy heifers tend to
follow a seasonal pattern of growth in response to pasture quality and availability. The
industry recommended liveweight-for-age targets for dairy heifers are 30% of mature
liveweight (LWT) at 6 months of age, 60% at 15 months of age and 90% at first calving.
Target growth rates are calculated by linear interpolation, thereby creating a mostly
linear planned trajectory of growth from three to 22 months of age. The general aim of
this thesis was to investigate the effects of LWT and growth on dairy heifer performance
in the New Zealand pasture-based system.
In this thesis, LWT records from 189,936 spring-born dairy heifers were provided by
Livestock Improvement Corporation to model growth curves from three to 22 months
of age. Holstein-Friesian (F) heifers were heavier than Jersey (J) heifers from three to
22 months of age, and FxJ crossbred heifers were heavier than the mean of the
purebreds due to positive heterosis effects. Additional data of calving dates and milk
production records were provided by Livestock Improvement Corporation. Live weight
between three and 21 months of age had significant impacts on milk production and
reproductive performance. As LWT of heifers increased, milk production and
probability of calving and calving early increased up to a maximum. Further increases
in LWT past the maximum point did not result in increases in milk production, and for
reproduction (stayability and calving rate) resulted in a decline in probability. For
heifers that were below average in LWT, significant improvements to milk production
and reproductive performance would be expected by increasing LWT. For example, the
mean LWT of 15-month-old Holstein-Friesian-Jersey crossbred (FJ) heifers was 301.5
kg. These “average” heifers were estimated to produce 436 and 1,477 kg more energycorrected
milk (ECM) than “below average” 250 kg heifers in first-lactation and threeparity
accumulated yields, respectively. Additionally, stayability to first, second and
third calvings were superior for “average” heifers (93.6 vs 89.3% for first, 78.2 vs 70.6%
for second and 64.7 vs 57.5% for third) and first calving 21-day calving rate (C21_2yo;
81.9 vs 78.0%) compared with “below average” heifers. For heifers that were above
average in LWT, significant improvements to milk production would be expected by
increasing LWT, however, at the heaviest LWTs a reduction in reproductive
performance would be expected. For example, FJ heifers that were 375 kg at 15 months
of age were estimated to produce 554 and 1,434 kg more ECM than “average” 300 kg heifers in first-lactation and three-parity accumulated yields, respectively. However,
stayability and C21_2yo were similar for 375 kg (93.4%, 78.9%, 63.8% and 79.8% for
stayability to first, second, third calving and C21_2yo, respectively) and 300 kg FJ
heifers. Heifers that were at the heaviest LWTs, for example, 425 kg at 15 months of age
were estimated to have the greatest ECM yields, but had a lower stayability and C21_2yo
compared with “average” heifers (88.4%, 72.5%, 55.3% and 76.8% for stayability to
first, second, third calving and C21_2yo, respectively). The greatest benefits to both
reproduction and milk production would be expected by increasing LWT of the lightest
heifers at each age studied.
In a prospective study, milk production did not differ between heifers that grew in a
seasonal manner (slow then fast) compared with the target growth trajectory (linear)
between six and 15 months of age. There was a difference in the age at which puberty
was attained, such that heifers that grew to the target growth trajectory were younger
at puberty compared with those grown in a seasonal manner, however, there was no
difference between treatments in the date of first calving or first lactation milk
production. These results indicate that there were limited disadvantages to growing
heifers slower over their first winter, provided they caught up to target LWT by first
mating.
Overall the results of this thesis indicate that having heifers heavier through the
precalving rearing phase (three to 21 months of age) has the potential to improve
reproductive success and milk production, with the greatest advantage seen by
increasing LWT of the lightest heifers. This information can be used to develop
guidelines which may improve productivity and survival of dairy cattle in New Zealand.
Description
Listed in 2019 Dean's List of Exceptional Theses
Keywords
Dairy cattle, New Zealand, Growth, Reproduction, Heifers, Physiology, Milk yield, Dean's List of Exceptional Theses