Quantification of changes in body weight and body condition scores during lactation by modelling individual energy balance and total net energy intake

Individual energy balances (EB) and total net energy intake (NEI) were mode lled to quantify the energy content of the explanatory variables body-weigh t change corrected for gut fill (BWC) and body condition-score change (BCSC ). A total of 6147 records from 213 lactating dairy cows, 71 Holstein-Fries ians, 71 Jerseys and 71 Holstein-Jersey F1-crosses was analysed (1st, 2nd a nd >2nd lactation). Data were collected during 210 days of lactation, from calving to week 30. Individual EB were calculated as total energy for lacta tion (MJ NEL) minus estimated energy necessary for maintenance and producti on. Body-weight changes were calculated as differences between weekly means of body weight corrected for gut fill (BWCw) as well as from polynomial re gression (BWCp). BCSC were differences between assessed body condition scor es. The study included analyses of 1- and of 4-week periods for BWCw and BW Cp, of differences between measuring periods for BCSC as well as separate a nalyses of data from cows in either negative or positive energy balance. Analysis of repeated measurements were performed applying mixed theory. Mod els for EB contained type of dairy cow, lactation and type of roughage as f ixed effects, week of lactation and either BWCw, BWCp or BCSC. Models for N EI contained type of dairy cow, lactation and quality of roughage as fixed effects, week of lactation as well as energy corrected milk, metabolic body weight and either BWCw, BWCp or BCSC as covariates. Most plausible energy contents per unit body tissue change were estimated f or BWCp from weekly data with either EB (with energy partitioned into milk production and maintenance fixed) or NEI (no restrictions concerning partit ioning) as the response variable. The estimated energy contents for BWCp we re 15.40 and 10.47 MJ NEL, respectively.