A deep dive into the data on substitution rates based on real measurements reveals just how careful farmers need to be when it comes to budgeting on the returns from supplementary feeding. The Prime Minister’s Chief Science Advisor Dr John Roche shared his analysis at a Taranaki Pasture Summit field day in late 2025 and in this podcast episode he steps listeners through his findings, explaining substitution rates and the concept of marginal milk. Drawing on extensive research and practical examples, he outlines why more feed does not always translate to more profit, and how important it is to look at the real costs of the extra production. 

Host:

• Anne Lee, Senior Journalist, Dairy Country

Guest:

• Dr John Roche, Prime Minister’s chief science advisor

 

John outlines how concepts such as substitution rate and marginal cost underpin the true value of additional feed, and why focusing on average cost can mask the real cost of producing extra milk. He talks about how pasture utilisation, stocking rate and timing all influence the response to supplements, highlighting that higher input systems can increase costs faster than returns if not carefully managed. Alongside the technical insights, he emphasises the importance of system design, flexibility and clear decision-making, helping farmers weigh up when supplements are a tool for profit and when they risk eroding it.

 

Read the article featuring John here: https://dairycountry.co.nz/real-data-shows-real-costs/

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