THE FASTING HEAT-PRODUCTION AND EFFECT OF LACTATION ON ENERGY-UTILIZATION BY DAIRY-COWS OFFERED FORAGE-BASED DIETS

Three experiments were carried out, each using eight Holstein-Friesian nonpregnant and late lactation cows, to determine the fasting heat pr oduction (FHP) of cows receiving forage diets at near ad libitum and t o examine the effect of lactation on the utilisation of metabolisable energy (ME). In experiments 1 and 2, energy metabolism data were recor ded in open-circuit calorimeter chambers during full lactation, half l actation (by drying off two of the four teats), and dry period (by dry ing off the remaining two teats), and afterwards the animals were fast ed for FHP measurement. In experiment 3 similar data were recorded dur ing full lactation and dry period (by drying off all four teats). The animals were offered dried grass nuts only in experiment 1, dried gras s nuts and concentrate (gross energy (GE) ratio of 0.80:0.20) in exper iment 2, and grass silage and concentrate (GE ratio of 0.80:0.20) in e xperiment 3. In each experiment, the feeding level was adjusted, on an individual animal basis, to provide an ME intake approximating to tha t required for both maintenance and milk production at the commencemen t of the experiments. This feeding level remained fixed throughout eac h experiment. The mean FHP data obtained in experiments 1 and 2 were s imilar, with an overall mean of 0.453 (s.d. = 0.0354) MJ/kg(0.75). Tot al intakes of GE and ME each were similar between full lactation, (hal f lactation) and dry period within each of the three experiments. Heat production during the dry period was significantly higher than during full lactation in experiment 3 (p < 0.05), but in experiment 1 or 2 t he differences were not significant, although the trend was similar. T his effect on heat production resulted in a decrease in productive net energy (retained energy) during the dry period in comparison with tha t (milk energy and retained energy) during lactation. Consequently, in experiment 3 the efficiency of ME utilisation for tissue retention du ring the dry period was proportionately 0.18 lower than that for lacta tion (p < 0.05), Similar trends were also obtained in experiments 1 an d 2, although none of the differences were significant. The results of the present study indicate that FHP (0.453 MJ/kg(0.75)) is proportion ately 0.41 higher than fasting metabolism (0.321 MJ/kg(0.75)) predicte d from Agricultural Research Council (1980), On average over the three experiments, the efficiency of ME utilisation for tissue retention du ring the dry period was proportionately 0.07 lower than that for milk secretion during lactation. (C) 1997 Elsevier Science B.V.