The following paper is a special collaboration from AFMA
(Animal Feed Manufacturers Association) www.afma.co.za
We thank their kind support.
Fertility in the UK dairy herd is falling at a rate of around 1% per annum, whilst
milk yield is increasing at around 2% per annum. Mostly, this infertility is due
to cows not ovulating, which may be because of a negative energy balance through
the transition period and up until pregnancy is achieved. This paper will review
two areas of transition cow feeding that may affect fertility: follicle development
and dry matter intake. Follicle development is largely controlled by IGF, FSH
and LH. When cows are in negative energy balance, IGF concentration may be reduced,
potentially lowering the number of follicles selected to grow. In turn, these
follicles may be smaller and less able to respond to the FSH in the middle stages
of development. The negative energy balance also reduces concentrations of LH,
decreasing the chance of ovulation. Since follicles take several months to develop,
formulating rations to contain feeds that promote IGF concentration pre calving,
such as propylene glycol, would increase the success of ovulation. Dry matter
intake immediately post calving is critical to reducing the energy deficit at
this time. Proper rumen function is required to enable full feed intake, therefore
reducing periparturient problems that affect rumen function is a must. By controlling
calcium homeostasis using a cation-anion approach, muscle function can be maintained.
At the same time, the occurence of milk fever and other associated metabolic disorders
(such as retained placenta, displaced abomasum) can be avoided. If follicles are
developing at the correct time and negative energy balance is minimised, milk
yield and fertility will be less likely to suffer.
- The aim of the dairy farmer is to maximise income from the sale of milk
and calves. Average recorded milk yields in the UK have risen from around
5,500 litres to 6,500 litres in the last 10 years (22). However, whilst there
is still much debate over the subject, it would seem that one calf per cow
per year would optimise returns, i.e., a 365 day calving pattern. The UK national
average calving interval is currently 397 days (7) with many herds well in
excess of this. This gives an average date of conception as being around 127days
post calving, whereas, to achieve a 365 day calving interval, conception should
occur around 95 days post calving. The average interval to first service is
83 days with a conception rate of 40 – 50%, with most producers taking 2 –
3 inseminations to achieve pregnancy. With a longer calving interval, dairy
farmers are losing money through less milk per year, less calves to sell,
more services per conception and increasing veterinarian costs.
- Fertility in the UK is on the decline at a rate of around 1% per year (25)
for the last 20 years. At the same time, milk yield has increased and there
has been a genetic shift from mainly Friesian cows to predominantly Holstein
animals. Royal (25) showed that this infertility was largely due to a failure
of the cows to ovulate.
- Many studies have shown the deleterious impact of a negative energy balance
(NEB) post calving on fertility (8,14, 23). The transition period generally
refers to the period of three weeks pre calving to three weeks post calving
when there is a decrease in dry matter intake whilst the energy requirements
of the cow are rising rapidly to meet the demand for milk production. The
Milk Development Council showed that for a cow giving 50kg of milk, there
may be up to 40MJ of metabolisable energy short in the daily ration (21).
- Our objective, therefore, must be to ensure conception around 80–100 days
post calving to optimise annual milk production but, not at the expense of
daily milk yield. There are many factors influencing this and this paper will
focus on just two: optimum follicle development and dry matter intakes in
the transition period.
2_ Follicle Development
- Negative energy balance may cause abnormal or irregular follicle growth
(23). The follicles take around 3–4 months to develop (28, 31). Therefore,
if we are aiming to achieve conception at 80–100 days post calving, the
follicles needed for fertilisation are beginning to develop 2-4 weeks pre-calving,
right at the start of the transition period. Furthermore, at the time of insemination,
the cow is often still in negative energy balance.
- There are three main factors that influence and are involved in follicle
growth and development.
- 2.2.1 Insulin-like Growth
Factor (IGF) is thought to stimulate and influence the number of follicles
selected at initiation of growth (30). IGF also has the potential to affect
the viability of follicles and therefore the developing egg (28). The
quality of the egg may well be compromised, leading to a poorer quality
embryo (23). IGF levels are closely related to insulin levels(23).
- 2.2.2 Follicle Stimulating
Hormone (FSH) as the name suggests, determines the growth of the follicles
in the middle stages of development (31) and is thought to be the primary
driver for follicular development during this phase (30). If the follicles
do not reach this stage, insufficient oestradiol is produced causing weak
signs of oestrus ("bulling") (17).
- 2.2.3 Luteinising Hormone
(LH) takes over from FSH in the final stages of follicle growth and is
essential for the follicle to ovulate (31). Pulse frequency of LH increases
markedly just prior to ovulation (23).
- Negative Energy Balance (NEB):
As the cow enters the transition phase she comes into NEB. As a result, IGF
concentrations may be reduced, potentially leading to fewer follicles starting
to grow that are smaller and less viable (16, 31). There is also evidence
that low IGF concentrations reduce the effectiveness of FSH on follicles (31),
even though circulating FSH levels remain unaffected by NEB (23). Fewer and
smaller follicles may continue to develop through this stage of growth as
a result. Finally, if the NEB continues up until insemination, the LH pulse
frequency can be decreased and so maturation of the follicles does not occur
and ovulation is delayed or fails and the follicle becomes cystic (29).
- Whilst the effect of NEB at the point of artificial insemination is essential
to ovulation, if the follicles are too small, they will not ovulate. The critical
period would appear to be in the dry period as the cow enters transition.
Since the follicles can take 3-4 months to develop, it would seem that stimulating
their initial selection and growth would increase the likelihood of their
responding to FSH later in development and LH at final maturation and ovulation.
Therefore, we must look to optimising IGF concentration in the early transition
period. Since IGF concentrations are closely related to that of insulin, an
insulin producing diet fed at this time may greatly benefit follicle development.
- Propylene glycol. This gluconeogenic precursor is known to increase insulin
levels (8,27). If insulin is increased, the IGF concentrations will similarly
rise, with the potential for greater stimulation of follicle growth and development
as well as responsiveness to FSH. Feeding propylene glycol at this time may,
therefore, be a method of increasing fertility in the dairy cow. Work is currently
underway to evaluate this hypothesis (9).
- Feeding of propylene glycol has also been shown to decrease the incidence
of ketosis (15,27). This is through insulin reducing the rate of fat mobilisation
from the adipose tissue (3). The method of feeding of the propylene glycol,
however, is important. It has been shown that feeding once daily as part of
a concentrate is more effective than small, regular quantities as part of
a total mixed ration (4). This is probably due to the amount of insulin produced
when fed as part of a TMR being insufficient to trigger the metabolic changes
required to reduce body fat mobilisation.
3_ Dry Matter Intake
- Dry matter intake decreases dramatically in the last 2-3 days pre calving
by as much as 20-30% and takes several days to recover post calving (11, 18).
Evidently, if dry matter intakes are not optimal, the cow is predisposed to
a NEB and thus reduced fertility and milk yield. Maximising dry matter intake
must, therefore, be a priority. Peri-parturient complications, such as milk
fever, retained placenta, displaced abomasum and ketosis are factors that
may reduce dry matter intake (11), with milk fever, even at sub-clinical levels,
being implicated in causing the other three (5).
- Calcium homeostasis. One of
the functions of calcium is to allow muscle to contract. Whilst milk fever
may not actually present itself until plasma calcium reaches 4mg%, it has
been shown that plasma calcium concentrations of 5mg% reduce abomasal motility
by 70% and the strength of the contraction by 50% (6). Clearly a reduction
in muscle contractility will lead to a decrease in dry matter intakes as rumen
function decreases, leading to a severe NEB. As a consequence, there is an
increase in fat mobilisation that may result in fatty liver syndrome and ketosis.
An excess of ketone bodies can further suppress appetite (14). (Low calcium
concentrations also prevent insulin production, further exacerbating this
situation (11)). Ultimately, milk yield will be reduced and, as previously
described, fertility will suffer. Muscle tone in the uterus will also be adversely
affected with cows experiencing prolonged calvings and retained placenta.
Uterine involution may also be impaired giving rise to fertility problems
- Hypocalcaemia occurs when the rate of calcium uptake into the mammary gland
for milk production is greater than that which is absorbed from the diet or
resorbed from bone. These mechanisms are under the control of the pituitary
hormone parathyroid hormone (PTH), which stimulates the bone resorption (12).
It also acts in the kidneys to produce 1,25 dihydroxyvitamin D (1,25 (OH)2
vitamin D), which causes the increased uptake of calcium from the gut (20).
High calcium diets cause this system to be quiescent, the cow receiving her
daily supply from passive gut absorbtion. At calving, the massive demand for
calcium may be too much and milk fever ensues, it taking 2-3 days for the
PTH cycle to become fully functional (19). "Traditional" low calcium
regimes used to prevent milk fever put the cow into a mild calcium deficiency
state, causing the homeostatic mechanisms to mobilise calcium from the bone
and absorb enough calcium from the diet (24). However, whilst sufficient to
stop clinical milk fever, the low calcium diet at calving can lead to those
other metabolic problems detailed above. Therefore, a system of supplying
high levels of calcium without causing milk fever is needed.
- Research into the dietary cation-anion balance (DCAB) of pre calving rations
has been extensive in recent years with the conclusion that acidifying the
diet can in fact allow such high calcium regimes without causing hypocalcaemia.
Calculation of the DCAB is based on levels of cations (K+ and Na+)
and anions (Cl- and S2-). High levels of potassium and
sodium in the ration cause the blood to be slightly alkaline that reduces
the effectiveness of PTH (12). In the UK, to prevent over-conditioning of
cows, green forage is usually restricted, with ad libitum straw and 1-2 kg
concentrates fed. It is generally supposed that the reduction in milk fever
seen as result of this feeding method is due to reduced calcium in the ration.
However, these green forages are generally high in potassium and this restriction
may have been beneficial in reducing K+ inputs. Even so, most UK
rations would have a DCAB of +200mEq/kgDM, whereas the target DCAB in pre
calving transition rations is –100 to –200mEq/kgDM. Few feeds are
naturally this low; therefore anions need to be added. There are many available
products and our own experience has been to use ammonium chloride and magnesium
sulphate. Others use calcium chloride but from a health and safety viewpoint,
this powerful chemical is not allowed through the mill! Whichever products
are used, no more than 3,000-3,500 mEq should be added from salts since they
may cause acute acidosis.
- Once the desired DCAB has been reached and the calcium homeostatic mechanisms
have been "switched on", other macro minerals need attention.
- 3.5.1 Calcium: Once the
PTH and 1,25 (OH)2 D are causing bone resorption and active
absorbtion from the gut, the system needs "fuel", therefore,
a high calcium diet is needed. The exact amount required is still under
debate, though practical experience has shown 120g or more of calcium
in the total diet is best to avoid problems.
- 3.5.2 Magnesium: This
should be supplied to provide 40-50g per day, especially with high K+
forages. A magnesium deficiency can adversely affect PTH secretion and
reduce the responsiveness of the kidneys to PTH, reducing the production
of 1,25 (OH)2 D (12).
- 3.5.3 Phosphorus: Target
supply of 35-40g per day. High levels of phosphorus inclusion at 80g per
day have been reported to reduce the effectiveness of PTH on 1,25 (OH)2
D production (11), causing milk fever. Experience under UK conditions
has found problems of milk fever at 60g per day, though this is not scientifically
- With variations in forage intakes, urine pH monitoring is an essential tool
to determine if the laboratory analyses and paper ration has the correct DCAB
in practice. Alkaline diets produce urine pH of 8.0-8.5, whereas acidic rations
cause this to fall to 6.0-6.5 (10). If the pH falls to 5.5 and below, there
is the danger of metabolic acidosis. This decline in urine pH is not linear,
though, so using the DCAB approach is an "all or nothing" system.
Timing of pH testing may also be critical. A recent study showed that, if
feeding the DCAB ration once per day, urine pH dropped rapidly after feeding
then rose again over the next 24 hours. Testing at 5-6 hours post feeding
gave the best indication if the diet was correct (13) and this has been borne
out in practice.
- Anionic rations, if fed and monitored correctly, may help reduce the extent
of NEB around calving by improving dry matter intakes as a consequence of
better calcium homeostasis and thus allowing a reduction of other associated
peri-parturient problems. Evidence (mainly anecdotal) suggests that feeding
a DCAB ration causes cows to have easier and quicker calvings, reduced incidence
of retained placenta and displaced abomasum. Interestingly, farmers have noted
that cows on such a ration are very hungry immediately after calving and want
to eat considerable amounts of forage. This obviously increases dry matter
intakes very quickly helping to reduce the NEB. Trials to substantiate this
are currently underway.
- Optimum fertility and performance can be greatly influenced by nutrition
of the transition cow. Feeding insulin-producing products, such as propylene
glycol, in the 3-4 weeks pre calving may help increase fertility by stimulating
IGF production and thus follicle growth and development. Propylene glycol
will also help to reduce fat mobilisation from adipose tissue, reducing the
incidence of ketosis and fatty liver, thereby lowering the negative energy
balance post calving, boosting energy available to the cow and increasing
LH pulse frequency and ovulation.
- Feeding an anionic ration pre calving allows for high calcium diets which
may help prevent metabolic problems such as milk fever, retained placenta,
displaced abomasum and ketosis, thus improving dry matter intakes and reducing
the NEB. In turn milk yield and fertility will be less likely to be impaired
and our goal of one calf per year at maximum milk output that much more achievable.
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