A randomised controlled study to assess the effects of an extension programme on the 6 week in-calf rate of seasonal calving, pasture-based dairy herds in New Zealand

•The effect of farmer participation in an extension programme (InCalf) on herd reproductive performance was quantified.•A multi-year, multi-region randomised controlled study design was used.•InCalf participating farms had a 6 week in-calf rate of 68% compared with 66% on farms that did not participate (P = 0.05).•The effect was not modified by region, study year or other biophysical and socio-demographic variables assessed.

The aim of this study was to quantify the effect of participation by New Zealand dairy farmers in a year-long extension programme designed to improve herd reproductive performance. This was estimated by comparing, over two successive years, the proportions of cows becoming pregnant during the first 6 weeks of the seasonal breeding programme (6 week in-calf rate) in herds involved in a full participation group (treatment), with herds in an actively monitored control group or a passively monitored control group. Possible interactions between treatment and various biophysical and socio-demographic factors were also assessed. Multivariable modelling was used to determine the effect of treatment on 6 week in-calf rate, adjusting for design factors (study year and region). It was estimated that the 6 week in-calf rate was 68% (95% confidence interval 65-67%) in the treatment group of farms that participated in the extension programme compared with 66% (95% confidence interval 67-69%) in the actively monitored control group of farms that did not participate in the extension programme (P = 0.05); thus the risk difference was 2.0% (95% confidence interval 0.0-3.9%). No significant interactions were found between treatment and region, study year or any of the biophysical and socio-demographic variables on the 6 week in-calf rate (P > 0.05). There was no significant difference in the 6 week in-calf rate between the actively and passively monitored control groups (P = 0.56). It was concluded that enrolment in the extension programme improved the 6 week in-calf rate, and that the treatment effect was not modified substantially by region, study year or any of the biophysical and socio-demographic variables assessed.