Energy balance calculations

The daily gross energy intake (GEI) was determined by multiplying the caloric content of the feed by the total feed intake. The DE intake (DEI) was calculated from the difference between the GEI and the amount of energy lost as feces (E_feces).

The ME intake (MEI) was obtained by subtracting the energy lost as urine (E_urine) and methane production from the DEI (E_CH4).

The urinary energy content was determined by the total combustion of samples using an adiabatic calorimeter, and this value was applied to the average daily urine production. Energy lost as methane was quantified by multiplying the daily production of methane by the caloric equivalent of 9.45 kcal/L of methane [11]

To obtain the net energy (NE) values, it was necessary to measure the energy lost as heat from basal metabolism (fasting heat production, FHP) and the heat from the heat increment of the diet (HI). The heat from fasted animals was previously determined during a period of 72h of fasting, which corresponds to the net energy for maintenance (NE_m) of the same cows prior to impregnation [15]. The NE_m were, respectively, 92.02 and 76.83 kcal/kg of metabolic BW (BW^0,75) for F1 HxG and Gyr animals. The efficiencies of 0.63 and 0.64 were used for F1 HxG and Gir cows, respectively [15], to determine the ME for maintenance (ME_m). With those values, we found 120.05 kcal/kg BW^0.75 for Gyr cows and 146.06 kcal/kg BW^0.75 for the crossbred cows.

To spare the pregnant F1 HxG and Gyr cows from stress promoted by fasting, the HI associated with digestive functions was determined using six adult, non-pregnant, nulliparous Zebu dairy cows with an average BW of 594 kg. These animals received the same experimental diets and the DMI level, set at 1.5% of BW, was equal to that used for pregnant F1 HxG and Gyr cows. Those cows also had their FHP measured using the respirometry system described above. The HI of the diet was obtained from the difference between the HP of fed animals and the FHP.

In pregnant females, heat production by gravid uterus should also be considered as a source of energy loss. This variable was obtained from difference between the pregnant fed HP from the FHP and HI.

The metabolizability (q) of the diet was calculated by divining MEI by GEI as proposed by the [16]:

The efficiency of utilization of ME for pregnancy (k_p) corresponded to the ratio between NE for pregnancy (NE_p) and ME for pregnancy (ME_p).