the average value of 13 percent efficiency results in the following equation to estimate the daily ME requirement for pregnancy in cattle:

Eq. 4–4

Some evidence is available to indicate that efficiencies of ME use for maintenance and pregnancy vary similarly (Robinson et al., 1980). Values for efficiency of utilization of ME for maintenance (k_m) may be calculated from the equation of Garrett (1980a) as follows:

Eq. 4–5

or,

where NE_m is net energy required for maintenance. The estimate of ME required for pregnancy may be converted to NE_m equivalent (kcal/day) by use of appropriate estimate of k_m as follows:

Eq. 4–6

If it is assumed, for example, that cows typically consume primarily forage diets containing 2.0 Mcal ME/kg, k_m is expected to be 0.576. With this assumption, the NE_m required for pregnancy may be estimated from the following equation (kcal/day):

Eq. 4–7

Estimates of the NE_m required for pregnancy, from this equation, are shown in Table 4–2. For comparison purposes, previous estimates from NRC (1984) and CSIRO (1990) are also shown.

Protein requirements for pregnancy may be estimated using the approach used with energy. Estimates of nitrogen (N) content of gravid uterine tissues at various stages of

gestation have been reported by Jakobsen (1956), Ferrell et al. (1976a), and Prior and Laster (1979). The equation derived by Ferrell et al. (1976a) to relate N (g) content of those tissues to day of gestation (t) was

Eq. 4–8

As with energy, this relationship may be scaled by predicted calf birth weight (38.5 kg) to derive the following equation

Eq. 4–9

Daily accretion of N in gravid uterine tissues may be calculated by differentiation of Eq. 3–9 with respect to t as follows:

Eq. 4–10

Supplementary net protein required for pregnancy is estimated from daily N accretion in gravid uterine tissues as

Eq. 4–11

Resulting values are shown in Table 4–3 for several stages of gestation. It should be noted that because of the high rate of metabolism of amino acids by uteroplacental and fetal tissues relative to accretion (Ferrell et al., 1983; Battaglia, 1992), as well as changes in extrareproductive tissue metabolism, these should be considered minimal estimates.

Milk production in the beef cow is difficult to assess. In contrast to the dairy cow, which is generally milked by machine two or more times daily, the beef cow is generally in a pasture or range environment and milk produced is consumed by the suckling calf. Numerous efforts have been made to assess milk production of beef cows with suckling calves with minimal disturbance of the normal routine of the cow and calf (Lampkin and Lampkin, 1960; Neville, 1962; Christian et al., 1965; Gleddie and Berg, 1968; Lamond et al., 1969; Deutscher and Whiteman,