• Association between somatic cell count during the first lactation and the cumulative milk yield of cows in Irish dairy herds

      McCoy, Finola; Archer, Simon; Wapenaar, Wendela; Green, Martin J.; Teagasc Walsh Fellowship Programme (Elsevier, 2014-01-30)
      Reduced potential milk yield is an important component of mastitis costs in dairy cows. The first aim of this study was to assess associations between somatic cell count (SCC) during the first lactation, and cumulative milk yield over the first lactation and subsequent lifetime of cows in Irish dairy herds. The second aim was to assess the association between SCC at 5 to 30 d in milk during parity 1 (SCC1), and SCC over the entire first lactation for cows in Irish dairy herds. The data set studied included records from 51,483 cows in 5,900 herds. Somatic cell count throughout the first lactation was summarized using the geometric mean and variance of SCC. Data were analyzed using linear models that included random effects to account for the lack of independence between observations, and herd-level variation in coefficients. Models were developed in a Bayesian framework and parameters were estimated from 10,000 Markov chain Monte Carlo simulations. The final models were a good fit to the data. A 1-unit increase in mean natural logarithm SCC over the first lactation was associated with a median decrease in first lactation and lifetime milk yield of 135 and 1,663 kg, respectively. A 1-unit increase in the variance of natural logarithm SCC over the first lactation was associated with a median decrease in lifetime milk yield of 719 kg. To demonstrate the context of lifetime milk yield results, microsimulation was used to model the trajectory of individual cows and evaluate the expected outcomes for particular changes in herd-level geometric mean SCC over the first lactation. A 75% certainty of savings of at least €199/heifer in the herd was detected if herd-level geometric mean SCC over the first lactation was reduced from ≥120,000 to ≤72,000 cells/mL. The association between SCC1 and SCC over the remainder of the first lactation was highly herd dependent, indicating that control measures for heifer mastitis should be preferentially targeted on an individual-herd basis toward either the pre- and peripartum period, or the lactating period, to optimize the lifetime milk yield of dairy cows.
    • Association between somatic cell count early in the first lactation and the lifetime milk yield of cows in Irish dairy herds

      Archer, Simon; McCoy, Finola; Wapenaar, Wendela; Green, Martin J.; Teagasc Walsh Fellowship Programme (Elsevier, 2013-03-14)
      Change in lifetime milk yield is an important component of the cost of diseases in dairy cows. Knowledge of the likelihood and scale of potential savings through disease prevention measures is important to evaluate how much expenditure on control measures is rational. The aim of this study was to assess the association between somatic cell count (SCC) at 5 to 30 d in milk during parity 1 (SCC1), and lifetime milk yield for cows in Irish dairy herds. The data set studied included records from 53,652 cows in 5,922 Irish herds. This was split into 2 samples of 2,500 and 3,422 herds at random. Linear models with lifetime milk yield and first-lactation milk yield as the outcomes and random effects to account for variation between herds were fitted to the data for the first sample of herds; data for the second sample were used for cross-validation. The models were developed in a Bayesian framework to include all uncertainty in posterior predictions and parameters were estimated from 10,000 Markov chain Monte Carlo simulations. The final model was a good fit to the data and appeared generalizable to other Irish herds. A unit increase in the natural logarithm of SCC1 was associated with a median decrease in lifetime milk yield of 864 kg, and a median decrease in first-lactation milk yield of 105 kg. To clarify the meaning of the results in context, microsimulation was used to model the trajectory of individual cows, and evaluate the expected outcomes for particular changes in the herd-level prevalence of cows with SCC1 ≥400,000 cells/mL. Differences in mean lifetime milk yield associated with these changes were multiplied by an estimated gross margin for each cow to give the potential difference in milk revenue. Results were presented as probabilities of savings; for example, a 75% probability of savings of at least €97 or €115/heifer calved into the herd existed if the prevalence of cows with SCC1 ≥400,000 cells/mL was reduced from ≥20 to <10 or <5%, respectively, and at least €71/heifer calved into the herd if the prevalence of cows with SCC1 ≥400,000 cells/mL was reduced from ≥10 to <5%. The results indicate large differences in lifetime milk yield, depending on SCC early in the first lactation and the findings can be used to assess where specific interventions to control heifer mastitis prepartum are likely to be cost effective.
    • Association between somatic cell count early in the first lactation and the longevity of Irish dairy cows

      Archer, Simon; McCoy, Finola; Wapenaar, Wendela; Green, Martin J.; Teagasc Walsh Fellowship Programme (Elsevier, 2013-03-21)
      Reduced longevity of cows is an important component of mastitis costs, and increased somatic cell count (SCC) early in the first lactation has been reported to increase culling risk throughout the first lactation. Generally, cows must survive beyond the first lactation to break even on their rearing costs. The aim of this research was to assess the association between SCC of primiparous cows at 5 to 30 days in milk (SCC1), and survival over a 5-y period for cows in Irish dairy herds. The data set used for model development was based on 147,458 test day records from 7,537 cows in 812 herds. Cows were censored at their last recording if identified at a later date in other herds or if recorded at the last available test date for their herd, otherwise, date of disposal was taken to be at the last test date for each cow. Survival time was calculated as the number of days between the dates of first calving and the last recording, which was split into 50-d intervals. Data were analyzed in discrete time logistic survival models that accounted for clustering of 50-d intervals within cows, and cows within herds. Models were fitted in a Bayesian framework using Markov chain Monte Carlo simulations. Model fit was assessed by comparison of posterior predictions to the observed disposal risk for cows aggregated by parameters in the model. Model usefulness was assessed by cross validation in a separate data set, which contained 144,113 records from 7,353 cows in 808 herds, and posterior predictions were compared with the observed disposal risk for cows aggregated by parameters of biological importance. Disposal odds increased by a factor of 5% per unit increase in ln SCC1. Despite this, posterior predictive distributions revealed that the probability of reducing replacement costs by >€10 per heifer calved, through decreasing the herd level prevalence of cows with SCC1 ≥400,000 cells/mL (from an initial prevalence of ≥20 to <10%) only exceeded 50% for less than 1 in 5 Irish herds. These results indicate that the effect of a reduction in the prevalence of cows with SCC1 ≥400,000 cells/mL on replacement costs alone for most Irish dairy herds is small, and future research should investigate other potential losses, such as the effect of SCC1 on lifetime milk yield.
    • Association of season and herd size with somatic cell count for cows in Irish, English, and Welsh dairy herds

      Archer, Simon; McCoy, Finola; Wapenaar, Wendela; Green, Martin J.; Teagasc Walsh Fellowship Programme (Elsevier, 2013-01-12)
      The aims of this study were to describe associations of time of year, and herd size with cow somatic cell count (SCC) for Irish, English, and Welsh dairy herds. Random samples of 497 and 493 Irish herds, and two samples of 200 English and Welsh (UK) herds were selected. Random effects models for the natural logarithm of individual cow test day SCC were developed using data from herds in one sub-dataset from each country. Data from the second sub-datasets were used for cross validation. Baseline model results showed that geometric mean cow SCC (GSCC) in Irish herds was highest from February to August, and ranged from 111,000 cells/mL in May to 61,000 cells/mL in October. For cows in UK herds, GSCC ranged from 84,000 cells/mL in February and June, to 66,000 cells/mL in October. The results highlight the importance of monitoring cow SCC during spring and summer despite low bulk milk SCC at this time for Irish herds. GSCC was lowest in Irish herds of up to 130 cows (63,000 cells/mL), and increased for larger herds, reaching 68,000 cells/mL in herds of up to 300 cows. GSCC in UK herds was lowest for herds of 130–180 cows (60,000 cells/mL) and increased to 63,000 cells/mL in herds of 30 cows, and 68,000 cells/mL in herds of 300 cows. Importantly, these results suggest expansion may be associated with increased cow SCC, highlighting the importance of appropriate management, to benefit from potential economies of scale, in terms of udder health.