Between-individual variability on reticulorumen digesta passage rate in goats

Identifying factors that influence an individual's ability to utilize nutrients is a strategy for enhancing livestock sustainability. Digesta passage rate (k_p) is one of the most important determinants that influence nutrient utilization. However, there is limited knowledge regarding the individual's variability on k_p. The objective of this study was to investigate between-individual variability of reticulorumen (RR) k_p of solutes and particles in goats, and its relationship to total NDF digestibility (NDFd). A dataset consisting of 103 individual records from two studies of castrated male (n = 36), female (n = 34), and intact male (n = 33) growing Saanen goats fed ad libitum and slaughtered at around 16.62 ± 0.96 (n = 21), 23.09 ± 1.33 (n = 19), 30.67 ± 2.25 kg (n = 29), 38.02 ± 1.61 (n = 16), and 44.12 ± 1.95 (n = 18) kg BW was used. The RR k_p of solutes and particles were measured using Cr-EDTA and indigestible NDF (iNDF), respectively. Using mixed-effect models, considering sex and BW as fixed effects and study as random effect, we aimed to identify meaningful between-individual variance of RR k_p of solutes and particles. Given the characteristic of our dataset (i.e., one observation per individual), our approach focused on exploring the variance between individuals described by the residuals of our fitted models. The RR k_p of solutes and particles showed significant (P < 0.05) between-individual variation (CV ≈ 27%). Only 10% of RR k_p of solutes variance was explained by DM intake (DMI) (g/d and g/d and g/kg BW) and NDF intake (NDFI); g/d), while 51.8% of the variance in RR k_p of particles was explained by DMI and NDFI (g/d and g/kg BW), RR wet pool size, and iNDF:NDF intake ratio. The between-individual variation of RR k_p of particles explained 19.5% of the between-individual variation in NDFd (P < 0.001). However, the nutritional and individual traits cited above (i.e., DMI, NDFI, RR wet pool size, and iNDF:NDF intake ratio) explained only 51.8% of the between-individual variation of RR k_p of particles, while the remaining 48.2% was attributed to the individual's inherent characteristics, which in turn explained only 1.24% of the between-individual variation of NDFd (P = 0.146). The RR k_p of particles and solutes showed meaningful between-individuals variance, which was partially related to both individual inherent characteristics and nutritional and animal traits. However, the NDFd was found to be unrelated to the inherent characteristics of RR k_p of particles.