The effect of dry period length and antibiotic treatment at drying off on somatic cell counts across the dry period

Management measures to reduce the risk of new intramammary infections (IMI) during the precalving period include use of dry cow antibiotics. Blanket dry cow therapy is not allowed in several European countries, among which the Netherlands. Moreover, shorter dry periods are of interest because of beneficial effects on the energy balance and metabolic status in the subsequent lactation. The aim was to study the effect of dry period (DP) length on SCC in the subsequent lactation and occurrence of IMI, based on SCC, across the dry period. This aim was approached in 2 separate experiments: experiment 1 was conducted with use of dry cow antibiotics and experiment 2 without use of dry cow antibiotics. In experiment 1 Holstein-Friesian cows (n=167) were randomly assigned to three DP lengths (0, 30, 60 days). Cows with a 30-d or 60-d DP were treated with dry cow antibiotics (Supermastidol®, Virbac Animal Health, Barneveld, the Netherlands) at drying off. In experiment 2, Holstein-Friesian cows (n=127) were randomly assigned to two DP lengths (0 or 30 days) and were not treated with dry cow antibiotics. Data were analysed using a logistic regression model (SAS Institute Inc., 2011), including DP length as fixed effect. Somatic cell count was log transformed before statistical analysis (LnSCC). Data are expressed as LMEANS ± SE. In experiment 1, cows with a 0-d DP had a greater average SCC in the subsequent lactation (LnSCC 5.01 ± 0.06) than cows with a 30-d (LnSCC 4.68 ± 0.06) or 60-d DP (LnSCC 4.52 ± 0.06) (P<0.01). The proportion of cows with a chronic IMI (SCC ≥ 200,000 both pre- and postpartum) was greater in cows with a 0-d DP (5/10), than in cows with a 30-d DP (1/13) or a 60-d DP (1/12) (P=0.04). In experiment 2, cows with a 0-d DP had a greater postpartum average SCC (LnSCC 4.51 ± 0.04), than cows with a 30-d DP (LnSCC 4.24 ± 0.04) (P<0.01). The proportion of cows with a chronic IMI during the precalving period was not different between cows with a 0-d DP (6/11) or a 30-d DP (2/6) (P=0.47). Postpartum average SCC for lactation is greater in cows with a 0-d DP, than in cows with a 30-d DP, regardless of use of dry cow antibiotics. Studies are ongoing to evaluate whether the greater SCC in early lactation in cows with a 0-d DP is actually correlated with intramammary bacterial infections.