Influence of somatic cell count in heifers on lifetime milk yield and disease management

The appropriate management of mastitis is of particular importance to the Irish dairy industry where there is considerable scope for herds to expand in anticipation of the abolition of European Union milk quotas in 2015. Subsequent price volatility is expected and producing quality milk with a low somatic cell count (SCC) will become even more crucial. Expansion of dairy herds requires a surplus of heifers with low milk SCC to maximise first lactation productivity and survival. However the likely impact of SCC in heifers on cumulative milk yield beyond the first lactation is unknown. The aim was to assess the impact of SCC during the first lactation on the lifetime milk production of cows in Irish dairy herds and to estimate potential savings through the control of mastitis in heifers.

Milk recording data were available for approximately 6,000 Irish dairy herds from 2005 to 2009. Cow level somatic cell count (SCC) over the first lactation was summarised as SCC between 5 and 30 days in milk (SCC1), and the geometric mean of first lactation SCC (gmSCC). The impact of SCC1 and gmSCC on lifetime milk yield over follow-up times of 5 to 8 years was assessed. An increase in SCC throughout the first lactation was associated with large reductions in the lifetime milk yield of cows. A one-unit increase in SCC1 (on a logarithmic scale), for example from 50,000 cells/mL to 150,000 cells/mL, or 150,000 cells/mL to 400,000 cells/mL was associated with a median reduction in lifetime milk yield of 864 kg of milk. A similar increase in gmSCC was associated with a median reduction in lifetime milk yield of 1,663 kg. The relationship between SCC1 and gmSCC was found to vary between herds, emphasising the importance of a herd specific approach to the control of heifer mastitis that may need to be preferentially targeted towards the pre- and peri-partum period, or the lactating period. Both scenarios were investigated using micro-simulation, assuming that SCC during the first lactation could be improved.

Intuitively, most savings were possible for the worst 20% of herds that had ≥ 20% prevalence of cows with SCC1 ≥ 400,000 cells/mL. For these herds there would be 75% certainty of saving at least €115 per heifer calved into the herd if the prevalence reduced to < 5%. For the worst quartile of herds with first lactation geometric mean SCC ≥ 120,000 cells/mL (at herd level) there would be a 75% certainty of saving at least €199 per heifer calved in the herd, if this reduced to ≤ 72,000 cells/mL as achieved by the best quartile of herds. Although risk factors for heifer mastitis have been identified; the efficacy of management interventions in terms of impact on SCC through the first lactation are largely unknown. This means that although there appear to be potential cost savings through controlling heifer mastitis, at the present time it is not possible to provide robust advice on how this can be achieved in a cost effective manner on particular farms. Therefore, micro-simulation was used to investigate scenarios where specific management interventions with a known likely positive impact on SCC1 were introduced.

Budgets for the interventions were estimated to guide decisions as to what changes may be achievable. These budgets were found to be highly dependent on the characteristics of decision makers. Therefore, better understanding of the expectation of farmers when investing in disease control is required before achievable interventions to control heifer mastitis can be identified for different farms. Potential savings indicate that the control of mastitis in heifers should not be overlooked.

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Archer,_SimonSimon Archer qualified as a Veterinary Surgeon from University of Glasgow in 2000. He has an MSc in Veterinary Epidemiology and Public Health from University of London, and an Royal College of Veterinary Surgeons Certificate in Cattle Health and Production. After working in mainly dairy practices across the South of England, he joined the School of Veterinary Medicine and Science at University of Nottingham in 2007, initially as a clinical resident before embarking on a PhD and further research.