It is just under a month now since COP26 finished but it many respects it feels like it was longer ago now that the news has 'moved on' and returned to worries about Covid.

In some ways this is good as there seemed to be a lot of heat and not much light in a lot of the news coverage – scapegoats were found (preferably ones without the backing of rich lobbying groups) and not much actual progress in letting people know what life would be like in a world with lower greenhouse gas (GHG) emissions.

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So, I’m not wanting to rehash any of the arguments from last month. Rather I thought I’d look at how different management decisions might affect the farm’s CO2 emissions and move us towards 'net zero'.

First off though I think I should put my hand up and admit I’m not entirely sure about the phrase 'net zero'. Now maybe my understanding of it is wrong but based on a Carbon Audit we did 18 months ago, it strikes me as being a bit of an accounting trick.

The audit only looked at the activities we were doing (day to day expenditure or profit/loss if you like) but excluded any capital items. Which means that, that big new tractor (my brother can but dream!) just gets ignored, despite the fact that it undoubtedly took a lot of energy to manufacture.

Setting that slight issue aside, let’s move onto how different management can affect the emissions. What might surprise some people is that, based on research by SRUC at Dumfries, all-year housed cattle actually have a lower carbon footprint per kg of milk than those that graze through the summer. The main driver for this is that housed cattle had around 40% higher yields than their grazing counterparts. Therefore, although housing will inevitably lead to higher energy usage per cow, as we need to bring the forage to the cow and take the slurry away, this is diluted by the extra litres of milk they produce.

Just to be clear I don’t wish to denigrate grazing herds, as they have an important role in the agricultural tapestry, but I think we should be careful not to lionise them and be honest with the public about the drawbacks.

Another important way to lower the carbon footprint of milk production is to get the age at first calving down to at most 24 months. Youngstock are continually 'burning' energy so the sooner they become 'productive' members of the herd the better. Apparently a two-year- old stirk needs around 70MJ of energy for maintenance each day (roughly six times what you or I need) which converts to almost 20 kWh.

Having got them into the herd, it is then important for us to keep them there as long as we can so the energy we’ve used to grow and maintain them as calves is spread over as many litres of milk as possible. So really the important KPI (Key Performance Indicator) here is lifetime-daily-yield.

This also ties in well with a recent study from the University of Florida that showed that to gain maximum profitability from a cow we should aim to keep her for around 5-5.5 lactations, although their figures were quite heavily skewed by 'genetic opportunity costs' especially at higher lactations so they possibly underestimate the optimum lactation number.

Now, a few weeks ago I was at a meeting where one of the supermarkets was talking about the KPIs they used with their direct supply farmers. They believed there was a strong corelation between calving interval (CI) and the carbon footprint of the milk production such that they were asking farmers to commit to aiming for a CI of 380 days.

I think it’s maybe worth exploring how likely such a corelation is and also the potential unintended consequences for committing to such a KPI.

First off, and apologies that the next section is a bit dry, a CI of 380 days combined with an average gestation length for cows of 283 means that the average cow in the herd has to be back in-calf by 97 days into her lactation. So, if you have a Voluntary Waiting Period (VWP) of 40 days before you start inseminations then you’ll have to get half the cows in calf in 2.5 cycles which gives a required pregnancy rate of c25%.

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Take the VWP up to a more sensible (for high yielding cows at any rate) 60 days and you’re down to 1.5 cycles and suddenly you need pregnancy rate of 33%. For context I generally achieve a conception rate of 35% so I’d have to pick up every heat to get the required pregnancy rate. Alternatively, I could look to hormonal intervention by for example fixed time AI-ing, but I’m pretty sure the general public would disapprove of us using such techniques routinely.

A 380-day CI also means an average lactation length of around 340 days if the cow is to get a six-week dry period. This is going to limit the potential production of high yielding herds as even with an average daily yield of 40 litres they become limited to an annual yield of 13,500litres, and as I alluded to earlier, the higher the yield per cow generally the lower the carbon footprint of each litre will be.

Another way of constraining CI is of course to make culling decisions purely on the basis of fertility and get just rid of otherwise healthy stale old cows bringing in younger more fertile heifers. However as I’ve already said that doesn’t really help the carbon footprint…..