The protein requirement of dairy cows during pregnancy
The factorial method to estimate the nutritional needs of pregnancy requires understanding the rate of nutrient deposition in the gestational tissues (fetus, placenta, amniotic fluid and uterus) and the efficiency of feed nutrients for embryonic tissue growth. There is very limited information on this aspect of dairy cows.
This version of the NRC differs from the previous version (NRC, 1989) in estimating the protein requirement in the last two months of pregnancy. The current estimates come from the results of Bell et al. (1995). There are other estimates, but these estimates are either from beef cattle, from other breeds of cows other than Holstein, or from research data 25 years ago. However, the estimated value of Bell et al. (1995) is not much different from that of the predecessors, thus supporting the requirements issued by the NRC in 1989. Bell et al. (1995) carried out a series of slaughter experiments on Holstein cows of 190-270d gestation to determine the growth rate and chemical composition of embryos. A quadratic regression equation can be obtained to describe the deposition of protein in the uterus of pregnancy.
The estimated value is derived from a single pregnancy cow with an average weight of 714kg. The estimated protein requirement for pregnancy is only a function of the number of days of pregnancy and embryo weight. The MP requirement (MPPreg) required for pregnancy is derived from the equation of Bell et al. (1995), which includes variables such as embryo weight, calf birth weight, and gestation days. The efficiency of MP for pregnancy (Eff MPP reg) is assumed to be 0.33. Because the cows used in Bell et al.'s test were all gestational over 190 days, and the cows had a very low protein requirement before 190 days of gestation, only the pregnancy requirements of cows over 190d were calculated.
If the gestation period of a cow is between 190 and 279 days, the equation for calculating embryo weight (CW) is:
CW=(18＋((DaysPreg－190)×0.665))×(CBW/45) where DaysPreg= gestation days; CBW= calf birth weight.
The average daily gain during pregnancy (ADGPreg) is:
ADGPreg=665×(CBW/45) MPPreg=((0.69×DaysPreg)-69.2)×(CBW/45)/EffMPPreg model, the gestation period of 279 days or more cows requires the same amount as gestation 279d.
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