Efficiency of use of metabolisable energy by non-lactating, pregnant dairy cows fed autumn pasture

Abstract

Two studies were conducted on pregnant, non-lactating dairy cows to investigate the metabolisable energy (ME) requirements for maintenance and the efficiency with which ME is used for body weight (BW) gain. In the first experiment, 53 non-lactating, pregnant Holstein-Friesian × Jersey cross dairy cows were fed one of three allowances [low: 7.5 kg,medium: 10.1 kg, or high: 12.4 kg of dry matter (DM)/day] of freshly cut autumn pasture for 38 ± 2 days (mean ± SD) in individual stalls to allow the measurement of estimated DM and ME intake daily. Animals were blocked by age (6 ± 2 years), day of gestation (208 ± 17 days), and BW (526 ± 55 kg). Cows on the low, medium, and high allowance treatments consumed 7.5, 9.4, and 10.6 kg of DM/day, and BW gain, including the conceptus, was 0.2, 0.6, and 0.9 kg/day. The ME content of pasture was estimated by wet chemistry from in vitro true digestibility and by near infrared spectroscopy. Total ME requirements for maintenance, pregnancy, and limited activity were 1.07 MJ ME/kg of measured metabolic BW/day. This is more than 45% higher than current recommendations. Differences may be due to an underestimation of ME requirements for maintenance or pregnancy, an overestimation of diet metabolisability, or a combination of these. Further research is necessary to determine the reasons for the greater ME requirements measured in the present study, but the results are important for on-farm decisions regarding feed allocation for nonlactating, pregnant dairy cows. In experiment 2, the amount of ME required for BW gain from pasture and commonly used supplementary feeds was investigated to estimate the efficiency with which ME is used for BW gain (kg). Control cows were offered autumn pasture to estimated maintenance requirements (-0.55 MJ ME/kg BW0.75), with an additional 20 MJ ME/d allocated for pregnancy and activity. All other cows received the same allowance of autumn pasture (Past) and an additional allowance (2.5 or 5.0 kg DM/day) of Past, spring pasture silage (Psil), maize silage (Msil), maize grain (Mgr), or palm kernel expeller (PKE). Regression analyses on BW allowed the determination of ME intake on BW gain for each feed; the kg was estimated from this, assuming each kg of BW contained 25 MJ of net energy. The kg of Past and Msil were 0.34 and 0.47, consistent with published literature and the kg of Psil (0.50) was in line with published values for spring pasture. The greatest kg (0.61) was for PKE, possibly due to its high fibre and fat content, and the fact that acetate and medium to long chained fatty acids are not substantially used for conceptus metabolism. The kg for Mgr was low (0.38), possibly because the end products of digestion of starch (i.e. propionate and the hepatic-derived glucose) are used preferentially for conceptus metabolism and less for BW gain, or because the composition of BW gain differed relative to the other treatments. Results confirm differences between feeds in kg when pasture is the base diet, and provide important information for feed budgeting and ration formulation in grazing systems.