Measuring Feed Conversion Efficiency (FCE) in dairy cows can seem straightforward – litres of milk produced/feed intake (dry matter) gives a figure somewhere between 1.0 and 2.0. This can be used to monitor herd performance, as well as compare performance between herds and individual cows. At first glance, it would seem that the higher the number, the better, ie more milk from a given amount of feed seems better. However, it is not quite that straightforward. As a rule of thumb, target whole herd FCE in the UK is 1.5-1.6 l/kg DMI. Figures that are too high generally reflect excessive weight loss in early lactation; too low often caused by rumen health/SARA problems.
In addition, FCE figures should be calculated in way that is consistent and allows realistic comparisons to be made. Firstly, feed intake should on a dry matter basis, and not ‘as fed’. The intake figure should be what is put out for the cows minus refusals, ie what they actually eat. Secondly, the milk output should be standardised, to what is called either fat or energy-corrected milk. Often in the US the standard is 3.5% fat, whereas in the UK 4.0% is more normal.
However, the ratio of litres milk:feed DMI does not allow for any weight gain or loss, and, of course, cows go through a cycle of losing weight in early lactation and then gaining weight in later lactation. Mike VandeHaar from Michigan State University in a recent talk ‘Advances in Production Efficiency in Adult Dairy Cattle’ said that while measuring FE based on milk litres:DMI was the norm, a more accurate metric is total milk energy:feed intake energy. This can be readily calculated – most rations will have an M/D value and DairyCo Feeding+ has a table of ME required at different milk fat and protein levels. This takes into account variations in milk solids (as above), as well as variations in feed component energy densities eg fat vs fibre. Including energy accounted for by weight loss or gain would be even more accurate, but more applicable to research institutions for now.
In the same way that an average BCS for the whole herd is not very helpful (because cows have different target BCS values at different stages in the production cycle), so an average FCE value for the whole herd is made up of values for groups with different targets. Theses are shown in the table below and reflect the facts that:
These values are from USA, using 3.5% fat corrected milk, and should be used in light of the considerations above. However, the differences in FCE between different groups still hold. Many farms will only be able to measure FCE for the whole herd. But bear in mind that there could be problems occurring to some degree in different groups, yet the whole herd FCE figure is still in the target range.
Optimum FCE will occur when energy and protein/amino acids supplied to the rumen microbes are in balance and when the amount of rumen-degradable starch in the ration doesn’t exceed the cow’s capacity to deal with the volatile fatty acids produced, which is achieved by the cow’s innate buffering, including from physically effective fibre, which is supplemented by adding Glycal Forte® to the feed.
FCE has approximately doubled over the last 60 years as yields have increased, allowing the relatively ‘fixed cost’ of maintenance to be divided up between many more litres of milk. However, figures from DEFRA suggest that FCE has been declining over the last 30 years (‘Ratio of compound and blend to milk’ slowly increasing – dashed line in graph below).
Why is this? One explanation may be that increased yields have been driven by increased feeding of starch. Increased starch levels in the ration increase DMI and hence milk yield, but also reduce forage digestibility which is likely the result of lowering rumen pH and its effect on fibre-digesting rumen microbes (de Souza et al, 2018). Increasing starch levels in the feed from 23% to 29% have been shown to reduce FCE (LaPierre et al, 2020).