Impact of parity differences on residual feed intake estimation in Holstein cows

Abstract

Residual feed intake (RFI) has been used as a measure of feed efficiency in farm animals. In lactating dairy cattle, RFI is typically obtained as the difference between dry matter intake observations and predictions from regression on known energy sinks, and effects of parity, days in milk, and cohort. The impact of parity (lactation number) on the estimation of RFI is not well understood, so the objectives of this study were to (1) evaluate alternative RFI models in which the energy sinks (metabolic body weight, body weight change, and secreted milk energy) were nested or not nested within parity, and (2) estimate variance components and genetic correlations for RFI across parities. Data consisted of 72,474 weekly RFI records of 5,813 lactating Holstein cows collected from 2007 to 2022 in 5 research stations across the United States. Estimates of heritability, repeatability, and genetic correlations between weekly RFI for parities 1, 2, and 3 were obtained using bivariate repeatability animal models. The nested RFI model showed better goodness of fit than the nonnested model, and some partial regression coefficients of dry matter intake on energy sinks were heterogeneous between parities. However, the Spearman's rank correlation between RFI values calculated from nested and nonnested models was equal to 0.99. Similarly, Spearman's rank correlation between the RFI breeding values from these 2 models was equal to 0.98. Heritability estimates for RFI were equal to 0.16 for parity 1, 0.19 for parity 2, and 0.22 for parity 3. Repeatability estimates for RFI across weeks within parities were high, ranging from 0.51 to 0.57. Spearman's rank correlations of sires' breeding values were 0.99 between parities 1 and 2, 0.91 between parities 1 and 3, and 0.92 between parities 2 and 3. We conclude that nesting energy sinks within parity when computing RFI improves model goodness of fit, but the impact on the estimated breading values appears to be minimal.

Summary: Residual feed intake (RFI) is typically obtained as the difference between dry matter intake (DMI) observations and predictions from regression on known energy sinks, and effects of parity, days in milk, and cohort. Understanding the impact of parity on RFI estimation is important because variation across parities might affect the estimation of RFI phenotypes and, hence, the prediction of RFI breeding values. To investigate this, we calculated RFI considering the energy sinks nested or not nested (constant) within parity using weekly records of 5,813 lactating Holstein cows collected from 2007 to 2022 in the United States. In addition, genetic correlations between weekly RFI for parities 1, 2, and 3 were estimated using bivariate repeatability animal models as the genetic breeding values for RFI in each parity. Our results showed that nesting energy sinks within parity when computing RFI improves model goodness of fit, but the impact on the RFI phenotypes and the estimated breeding values appears to be minimal.

Figure 1

Relationship between residual feed intake (RFI) phenotypes calculated using the constant model and nested model (A); and between sires' RFI breeding values in parities 1 and 2 (B), parities 1 and 3 (C), and parities 2 and 3 (D).