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 (6).
- 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 substantiated.
- 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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