End of Project ReportIt has been suggested that negative energy balance (NEB) in the immediate
post-partum period is potentially an important factor in the association
between increasing milk output and declining reproductive performance. The
objective of this project was to design an experimental model that could be
used to impose different degrees of NEB immediately after calving and to
examine the effect of this model on dry matter intake (DMI), milk production,
energy balance (EB), metabolic and reproductive hormonal profiles, the onset
and pattern of post-partum ovarian cyclicity and reproductive physiology
around AI.
Two experiments were carried out to evaluate the effects of milking frequency
and diet on DMI, production, energy balance and blood metabolites and
hormones in the first 4 weeks after calving and subsequent reproduction.
Reducing milking frequency from either thrice or twice daily to once daily
reduced DMI but also reduced milk production. This resulted in a better EB in
once daily milked cows in both experiments, the reduction being significant in
the first. Milk production during the 4-week treatment period was reduced by
23 and 20 percent by reducing milking frequency from thrice to once daily in
experiments 1 and 2, respectively. There was a reduction of approximately 10
percent in the cumulative yield up to week 20 of lactation in experiment 1 and
of approximately 9 percent in total lactation yield in experiment 2. Reducing
milking frequency resulted in increased plasma glucose, insulin and IGF-1
concentrations and reduced non-esterified fatty acid (NEFA) and beta
hydroxybutyrate (BHB) concentrations. Conception rates to first service or
overall pregnancy rates were not different between milking frequency
treatments but once daily milking resulted in a shorter interval to first ovulation
than thrice daily milking, due to a higher proportion of cows on this treatment
ovulating the first post-partum dominant follicle. Increasing the energy density
2
of the diet increased DMI and milk production with no consequent effect on
energy balance.
Logistic regression on the combined data from the two experiments showed
that lower energy intake, greater NEB and lower milk protein content and
were significantly associated with poorer conception to first service. Lower
plasma IGF-1 concentrations in experiment 2 were also associated with a
lower conception rate to first service.
A third experiment which investigated protein concentration in the concentrate
combined with concentrate feeding level post calving (for two groups of cows
in different body condition score at calving) showed no effect of post calving
diet on BCS change.
Overall the results suggest that reducing milking frequency to once per day
during the first 4 weeks of lactation reduces NEB and appears to be a suitable
strategy for altering energy balance at this time. However, the short-term
reduction in milking frequency immediately post partum reduces total lactation
yields. Blood metabolite and hormonal concentrations indicate better energy
balance for cows milked once daily. Increasing dietary energy density or
reducing the protein content of the diet does not appear to be effective in
changing energy balance in early lactation. Decreased NEB in the first 4
weeks post-partum is associated with an improved conception rate to first
service
