Twelve lactating dairy cows (Holstein and crossbred Holstein x Zebu) wereblocked by days in milk and randomly assigned to three replicate 4 x 4 Latinsquares to compare two schedules of total fecal collection (two versus six days)and the reliability of two internal markers [indigestible neutral detergentfiber (INDF) or indigestible acid detergent fiber (IADF)] to estimate fecalDM excretion (FDME), apparent total tract digestibility of nutrients, and thetotal digestible nutrients (TDN) content of the diets. The summative equationsfrom the NRC (2001) model were also used to estimate the energy value of feeds.A diet containing 60% of corn silage (DM basis) plus one of the following proteinssources was fed to animals: soybean meal (SM), 38% CP cottonseed meal (CM38),28% CP cottonseed meal (CM28), or SM plus 5% of urea/ammonium sulfate in theconcentrate DM (SMU). Fecal collection schedule and markers had no effect onthe estimates of FDME, nutrient digestibility, and dietary TDN. The summativeequations from the NRC (2001) overestimated the intake of digestible CP andNDF and underestimated that of digestible NFC, EE, and TDN. It was concludedthat INDF was the most precise marker and two days of total fecal collectionwas enough to estimate FDME, nutrient digestibility and dietary TDN. The NRC(2001) summative equations did not reliably predict the energy value of tropicalfeeds.
