Economic
values for dry matter intake capacity (DMIC) ranged
from 18 to 40 ⁄ kg ⁄ cow ⁄ year in Danish production
systems and depended on the difference between
marginal costs of roughage and concentrates
(Koenen et al. 2000). In the USA, Dado et al. (1994)
estimated the feed costs per kilogram of milk component,
expressed as kilograms of standardized milk
with equivalent value, as 1.00 for lactose, 1.89 for
fat and 3.49 for protein. This approach was used as
the national averages for ratio of shelled corn to
milk price and soybean meal to milk price were stable
over time, permitting estimation of feed costs
from milk price as prices inflate.
Multiple trait selection indexes should account for
different markets and production systems. Breeding
programmes should estimate future, rather than current,
costs and prices (Van Raden 2004). The REVs
for some traits or trait components of dairy sires differ
substantially between purebred and crossbred
dairy systems. There are also differences among the
REVs of traits for beef sires, depending on whether
these bulls are used for terminal crossing with F1
females in the cow–calf pasture system (backcrossing),
for crossing in dairy herds producing slaughter
animals, or for crossing in dairy herds producing F1
females for the cow–calf pasture system (Wolfova
et al. 2007).
Hoekstra et al. (1994) found that phenotypic correlations
between milk production traits and fertility
traits were negative ()0.05 to )0.18), with genetic
correlations being more negative ()0.14 to )0.62).
High correlations have been found among production
levels across different lactations. Genetic correlations
between second and third lactations are close to one,
indicating that these can be considered as repeated
observations of the same trait. Genetic correlations
between yield on days within lactation are high
except between extreme parts of the lactation (Druet
et al. 2005). In the UK, lactation persistency is defined
as the rate of decline in daily yield after the peak
within lactation and is often defined as a ratio of yield
at 2 points in lactation. The lower the rate of decline
after the peak, the higher the lactation persistency. A
cow with a higher lactation persistency can make better
use of inexpensive forage depending on season of
calving, may suffer less stress from high peak yield, is
more resistant to disease, shows an increased conception
rate and probability of pregnancy and is therefore
more likely to be profitable. Sensitivity analyses have
shown that an increased probability of pregnancy, an
increased persistency of milk yield and a smaller
replacement heifer cost greatly reduce the average
cost of a pregnancy (De Vries 2006). Appuhamy et al.
(2007) found that increased lactation persistency (uncorrelated
with 305-day yield) was associated with
lower levels of mastitis. Despite these effects, most
selection indices treat protein yield as the main trait
of interest without reference to increased lactation
persistency, as the impact of persistency on total
economic merit (TEM) is usually relatively minor