tA controlled trial investigating the effect on conception of administration of 250 g ofgonadotropin-releasing hormone (GnRH) at artificial insemination (AI) in dairy cows inseasonal or split calving herds was conducted. Time of detection of estrus, body condition,extent of estrous expression, treatment, breed, age and milk production from the mostrecent herd test of the current lactation was recorded. Cows were tested for pregnancywith fetal aging between 35 and 135 days after AI. Sixteen herds provided 2344 spring-calved cows and 3007 inseminations. Logistic regression adjusting for clustering at herdlevel was used to examine the effect of treatment for first (2344) and second (579) insem-inations separately. For first AI, treatment significantly improved conception rate in cowswith milk protein concentrations of 3.75% or greater and for cows with milk protein con-centrations between 3.00% and 3.50% and less than 40 days calved; increased conceptionrate from 41.2% to 53.4%. Treatment reduced conception rates in cows with milk proteinconcentrations of 2.75% or less. Treating only cows identified as responding positively totreatment (11% of all study cows) was estimated to increase first service conception ratein herds from 48.1% to 49.4%. There was no significant effect of treatment on conception tosecond AI, nor any significant interactions. These findings indicate that GnRH at AI shouldbe limited to the sub-group cows most likely to respond. The positive effect of GnRH at AImay be mediated through improved oocyte maturation and/or improved luteal function,rather than by reducing AI-to-ovulation intervals.©