<p> Background: Improving feed efficiency has become a common target for dairy farmers to<br>meet the requirement of producing more milk with fewer resources. To improve feed<br>efficiency, a prerequisite is to ensure that the cows identified as most or least efficient will<br>remain as such, independently of diet composition. Therefore, the current research analysed<br>the ability of lactating dairy cows to maintain their feed efficiency while changing the energy<br>density of the diet by changing its concentration in starch and fibre. A total of 60 lactating<br>Holstein cows, including 33 primiparous cows, were first fed a high starch diet-low fibre (diet<br>S+F-), then switched over to a low starch diet-high fibre (diet S-F+). To know if diet affect feed<br>efficiency, we compared the ability of feed efficiency to be maintained within a diet over<br>subsequent lactation stages, known as repeatability of feed efficiency, with its ability to be<br>maintained across diets, known as reproducibility of feed efficiency. To do so we used two<br>indicators: the estimation of the error of repeatability/reproducibility, which is commonly<br>used in metrology, and the coefficient of correlation of concordance (CCC), which is used in<br>biology. The effect of diet change could also lead to a change in cows sorting behaviour<br>which could potentially affect feed efficiency if for example the most efficient cows select<br>more concentrate than the least efficient. We therefore analysed the relationship between the<br>2<br>differences in individual feed refusals composition and the differences in feed efficiency. To<br>do so, the composition of each feed refusal was described with its near infra-red (NIR)<br>spectroscopy and was performed on each individual feed ingredient, diet and refusals and<br>used as composition variable. The variability of the NIR spectra of the refusals was described<br>with its principal components thanks to a principal component analysis (PCA). The Pearson<br>correlation was estimated to check the relationship between feed efficiency and refusals<br>composition, i.e. sorting behaviour.</p>
<p><br>Results: The error of reproducibility of feed efficiency across diets was 2.95 MJ/d. This error<br>was significantly larger than the errors of repeatability estimated within diet, which were 2.01<br>MJ/d within diet S-F+ and 2.40 MJ/d within diet S+F-. The CCC was 0.64 between feed<br>efficiency estimated within diet S+F- and feed efficiency estimated within diet S-F+. This CCC<br>was smaller than the one observed for feed efficiency estimated within diet between two<br>subsequent lactation stages (CCC = 0.72 within diet S+F- and 0.85 within diet S-F+). Feed<br>efficiency was poorly correlated to the first two principal components, which explained 90%<br>of the total variability of the NIR spectra of the individual refusals. This suggests that feed<br>sorting behaviour did not explain differences in feed efficiency.</p>
<p><br>Conclusions: Feed efficiency was significantly less reproducible across diets than repeatable<br>within the same diet over subsequent lactation stages, but cow’s ranking for feed efficiency<br>was not significantly affected by diet change. This loss in repeatability across diets could be<br>due to a more pronounced feed sorting subsequent to the change in diet composition.<br>However, the differences in sorting behaviour between cows were not associated to feed<br>efficiency differences in this trial neither with the S+F- diet nor with the S-F+ diet. Those<br>results have to be confirmed on diets having different forage to concentrate ratios to ensure<br>that the least and most efficient cows will not change </p>
residual feed intake, repeatability, diet, dairy cattle, sorting behaviour
Anonymous, Ioannis Kaimakamis, Giuseppe Conte, Angela Schwarm
