The relation between energy intake and chewing index of diets fed to nursing ewes

Abstract

The objective of this study was to determine whether a linear relation exists between the metabolizable energy intake (MEI) of nursing ewes in early lactation and the dietary chewing index (CI). This was investigated using five feeding trials with intake data from 108 nursing ewes in the first 4–6 weeks after lambing, giving a total of 528 observations. The ewes were fed grass silage ad libitum separately, supplemented with concentrates or fed a total mixed ration (TMR) ad libitum. The ewes were of different breeds, ranging from 2 to 7 years old, had a mean body weight (BW) in the first week after lambing of 91kg (SD=9.7), and nursed an average of 2.2 lambs (SD=0.37). The average daily MEI was 42.0MJ (SD=8.35). The NorFor CI (min/MJ ME) values of the diets was estimated from the content of neutral detergent fibre (NDF; g/kg DM), indigestible NDF (iNDF; g/kg NDF), and the theoretical chopping length (mm) of the forage. The CI values were adjusted for the BW of the ewes and for NDF intakes higher than 0.7% of BW. The mean corrected CI (CIcor) was 37.3min/MJ ME (SD=6.74). The relation between MEI and CIcor was analyzed using nonlinear mixed-effects modelling, using the equation MEI=ME0−k×ME02×CIcor, where MEI is the daily intake of metabolizable energy, ME0 is the theoretical maximum energy intake capacity of the animal in a situation with no physical constraints on intake, and k represents the decline in MEI with increasing CIcor. The model included random variation of week-within-experiment on the intercept and linear fixed effect of week after lambing on k. The maximum daily chewing time, CTmax, for the ewes was assumed to be 1/(4×k). The MEI declined linearly with increasing dietary CI (P<0.001), with difference within-experiment intercepts in relation to week after lambing (P<0.001), supporting the existence of a linear relation between MEI and CIcor for nursing ewes in early lactation. Despite an overestimated CTmax value, this model has the potential to predict the MEI of nursing ewes in early lactation.