Feed efficiency in dairy cows: individual cow variability in component traits

Abstract: Feed efficiency (FE) varies between cows, and this variation is linked to the variation in energy metabolism variables. Respiration chambers are needed for measuring energy metabolism variables while individual cow dry matter intake (DMI) records are necessary for measuring FE, but these are difficult to obtain due to cost and logistic constraints. This thesis evaluated the between-cow coefficient of variation (CV) in the components of FE and their contribution to FE. Also, marker techniques of measuring DMI and the use of an upgraded GreenFeed system (GF) to measure energy balance (EB) in lactating dairy cows were evaluated. Marker-based estimates of DMI underestimated observed DMI. The use of external markers for faecal output estimates gave the best prediction of FE suggesting that faecal output measurements with external markers are enough to determine FE thereby removing the need for analysing feed samples. However, the direct measurement was more precise making it a method of choice unless otherwise not feasible due to facility limitations. The between-cow CV in gross energy (GE) intake was the highest among all component traits while that of digestibility (DE/GE) was small. Although the between-cow CV in methane (CH4) as a proportion of GE was important, it was positively correlated with DE/GE, suggesting that selecting for low CH4 emitters may result in unintended selection for low DE/GE which is an important trait for ruminants. The between-cow CV in residual energy corrected milk (RECM) was double that of residual feed intake (RFI) indicating that RECM is more amenable to genetic selection than RFI. Using respiration chamber data to predict DMI and ECM for RFI and RECM calculations, respectively, the partial regression coefficients were biologically meaningful. About 65% of the difference between low and high-FE (RFI or RECM) cows was due to improved utilisation of metabolisable energy. Residual CO2 could be the FE index of the future as it eliminates the need for measuring individual animal DMI. The replacement of cereal grain with by-product did not have negative effects on production and EB, suggesting that by-product can replace cereal grain in early lactation cow diets. The GF proved to be a promising tool for measuring EB. Milk mid-infrared (MIR) spectral data also gave a good prediction of EB which presents an opportunity to estimate individual cow EB without added investments as MIR is an on-farm routine analysis.