Effect of converting weaned dairy calves from a component-fed diet to a total mixed ration on growth and nutrient digestibility

The objective was to determine the effects of converting calves from a component-fed ration to a total mixed ration (TMR) at 8, 10, or 12 wk of age on intake, growth, and nutrient digestibility. Holstein calves (n = 40) were randomly assigned to 1 of 4 groups (no TMR, TMR conversion at 8, 10 or 12 wk; T0, T8, T10, and T12, respectively). Calves were weaned at 6 wk of age, housed individually, and studied from 7 to 14 wk of age. Rations, consisting of a 20% crude protein texturized starter and grass hay, were offered ad libitum as separate components or as a TMR with 85% starter and 15% grass hay on a dry matter (DM) basis. Intakes and body weights (BW) were measured weekly. Component intake for TMR was calculated from the proportion of grass hay and starter contained in the TMR. Fecal grab samples were collected every 9 h over 3 d for a total of 8 samples that formed a composite at 9, 11, and 13 wk of age from the same 4 calves per group. Rumen fluid samples were collected via esophageal tube at -1, 0, 3, and 7 d relative to conversion from component to TMR. Feed and feces were evaluated for DM, neutral detergent fiber, acid detergent fiber, and acid detergent lignin (internal flow marker) to estimate digestibility. Average daily gain and final BW tended to be least for T8. Empty BW gain was significantly less for T8 than for T0 but not different from T10 or T12 (T0 = 1.07, T8 = 0.93, T10 = 1.02, T12 = 1.04 kg/d). Hip growth tended to be least for T8 and resulted in lower final hip width (T0 = 25.9, T8 = 25.2, T10 = 25.6, T12 = 25.8 cm). Intake tended to be reduced for calves converted to TMR earlier. Throughout the study, calves fed TMR ate more hay and less starter than component-fed calves. Metabolizable energy intake was less for T8 versus T0 but not different from T10 or T12 (T0 = 8.46, T8 = 7.55, T10 = 8.01, T12 = 8.23 Mcal/d). We observed no differences in feed efficiency for the duration of the study. Differences in DM digestibility were not observed, but fiber digestibility was greater for calves fed TMR at 9, 11, and 13 wk of age. Conversion to TMR increased rumen pH. These results indicate that TMR conversion increased hay consumption and subsequently decreased starter and total DM intake. This led to reduced weight and structural growth; however, calves that were converted to TMR as early as 8 wk still achieved adequate growth. The increase in rumen pH and subsequent increase in fiber digestibility allowed for calves to be converted to a 15% grass hay TMR as early as 8 wk and still achieve desirable growth goals.