Ruminants are a major producer of methane, which is both a potent greenhouse gas and a loss of energy, therefore mitigating its production is important. One strategy that has been shown to reduce methane production is to use feed additives to alter the rumen microbiome composition. However, once treatment with the additive is stopped the microbiome profile returns to its pre-treatment composition. But, shortly after birth there is a period of plasticity that provides a window of opportunity to manipulate the microbiome with a potentially long-lasting impact. To investigate this we fed linseed oil to dams (mother ewes) during gestation to see if this led to persistent changes in the lamb microbiome and methane emissions. This involved dams receiving one of three treatments: either they were fed a linseed oil supplement during gestation and lactation; only received the supplement during lactation; or received no supplement. The lambs of these ewes were then either directly fed the linseed oil or did not receive any direct supplement. We used 16S sequencing and shotgun metagenomics to track changes in dam and lamb rumen microbiomes at different time points up until lamb slaughter. Our study demonstrates whether dietary intervention in the ewes can lead to long-term, persistent changes in the lambs, and how these changes compare to being directly fed the additive. We also show the optimal timing for feeding the additive for maximal effect. Furthermore, we provide insight into the role of the dam in colonisation and establishment of the rumen microbiome.