Rose Jackson BVSc DBR MRCVS explains more about understanding the links between feeding and fertility in Dairy Cows.
It is widely accepted that maiden heifers have better fertility than milking cows, but why?
Genetically they are the same, so it must be related to management. It is easy to assume that it is something to do with milk production but, in actual fact, it is related to the dry matter intake (DMI).
Dry Matter Intake (DMI)
Increased DMI drives milk production but also increases the rate of blood flow through the organs, especially the liver. The liver is responsible for metabolising (breaking down) the sex hormones – oestrogen (E2) and progesterone (P4) – so dairy cows have lower levels of circulating hormones compared to heifers. Low levels of E2 cause shorter periods of heat expression, making it harder to spot cows to AI. Low levels of P4 leads to increased early embryo loss and longer inter-service intervals (ISIs).
We normally target for >40% of return services to be 18-24 days after a previous service. However, the farm shown here serves over 20% of cows at day 25-35; some of this might be normal variation (recent research shows the range of ISIs is 15-30 days) but some of this will be early embryo loss due to insufficient P4.
Body Condition Score (BCS)
Body Condition Score (BCS) is also closely related to fertility – absolute BCS at time of first AI and change in BCS from calving to 21 days in milk (DIM) are both significant factors. One American study with over 1100 cows showed that cows with BCS ≤2.5 had an average pregnancy rate of 40% whereas cows with BCS 2.75-3 had an average pregnancy rate of 48%. In addition, cows that lost ≥0.25 of a score had a reduced pregnancy of 25% compared to 38% for cows that maintained their BCS. Cows that lost condition also had a higher rate of
pregnancy loss – 9% compared to 6% in cows that maintained their condition.
The link between BCS and fertility is due to negative energy balance (NEB) – cows in NEB have smaller follicles leading to low E2 production which will present as cows ‘not seen bulling’. Smaller follicles lead to smaller CLs, reduced P4 and small embryos which are unable to signal to the cow that they are in calf.
Cows that are fat at calving (BCS≥3.50) are more likely to lose condition in early lactation compared to cows at the optimum condition of 2.75 – fat cows have poor DMI. These cows then take longer to get in calf, so are more likely to get fat in late lactation, and so the cycle continues! Transition management is key but management of BCS needs to start in late lactation.