Genetic grazing system.pdf


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INVITED REVIEW: PASTURE-BASED DAIRY GENETICS

cows depending upon their personal preference, cost
of concentrates, and length of grazing seasons. Shorter
growing seasons, as seen in the northern US, require
growing, harvesting, and storing forage or purchasing
forages for use during the nongrazing season. Good
pasture management is essential for the bottom line of
any pasture-based dairy but graziers may choose to add
concentrates to increase milk production, especially
for cows that cannot reach their genetic potential on
pasture alone. Also, use of concentrates or other supplements can allow increased stocking rates and increased
productivity per unit of land (Macdonald et al., 2008a;
Baudracco et al., 2011; Macoon et al., 2011; McCarthy
et al., 2012; Vibart et al., 2012). Insufficient DMI of
pasture is certainly a limiting factor to milk production
by high-producing dairy cows, as reviewed by Bargo et
al. (2003). They noted that “milk production [of grazing
cows] increases linearly as the amount of concentrate
increases from 1.2 to 10 kg DM/day, with an overall
milk response of 1 kg milk/kg concentrate.” However,
for each kilogram of supplemental concentrate, grazing
time decreased 12 min/d (Bargo et al., 2003) and, at
higher rates of supplementation, incremental milk yield
responses are expected to be less (Vance et al., 2013).
Milk production per cow is typically lower in grazing herds with minimal to moderate supplementation
compared with herds consuming a TMR in barn confinement in North America (Kolver and Muller, 1998;
Soriano et al., 2001; White et al., 2002; Boettcher et
al., 2003; Fontaneli et al., 2005). At relatively high levels of supplementation such as 1 kg of concentrate for
each 3 kg of milk, high-yielding HO cows in Canada in
intensively managed grazing systems had similar milk
yield over 2 yr compared with confined, TMR-fed HO
(Fredeen et al., 2002). However, this may be a function
of the short grazing season as well as the difference in
diet composition. Substituting part of the TMR of HO
cattle with high-quality pasture did not adversely affect
milk production in North Carolina with up to 34% of
the total diet as annual ryegrass pasture (Vibart, 2006)
or in Louisiana by allowing early- to mid-lactation cows
~2.7 kg of DM/day of oat and ryegrass pasture in late
fall (McCormick et al., 2011). Therefore, moderate to
high supplementation in pasture-based systems or using pasture as a supplement to TMR can be used to
maintain high levels of milk production, especially from
cows bred to perform in a TMR feeding system.
High-producing cattle may need time to “learn to
graze” before decisions about the efficacy of grazing can
be made. For example, mid-lactation HO transitioned
from a TMR to grazing either native grasses or a mixed
pasture sward plus supplementation had lower production and estimated DMI than expected in a Wisconsin
study (Wu et al., 2001), likely in part because the cows

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were used to a TMR and may not have had time to
adapt before the beginning of the grazing experiment.
Providing TMR in the pasture rather than in a separate feeding facility could reduce pasture intake: when
given the choice of eating TMR indoors or on pasture,
late-lactation HO consumed 2.2 kg/d more TMR in the
pasture (Charlton et al., 2011).
Cows that can maintain a higher BCS may have an
advantage in pasture systems because they can draw
upon body reserves if feed is limited, resulting in higher
total lactation yields of milk solids as well as good fertility (Pryce and Harris, 2006). It has been documented
that North American (NA) HO require more supplementation than other strains and breeds to maintain
body condition in pasture systems (Roche et al., 2006;
Macdonald et al., 2008b).
In a short-term study of high-producing early lactation HO in Pennsylvania (Kolver and Muller, 1998) on
a 100% pasture diet or a 100% TMR diet, pastured
cows had lower daily milk production (29.6 kg. vs. 44.1
kg), weighed 35 kg less, and averaged 0.5 lower BCS
(5-point scale of Wildman et al., 1982) than cows consuming a TMR. Also in Pennsylvania, a 21-wk study by
Bargo et al. (2002) reported that HO cows consuming
pasture plus concentrate lost BCS (−0.20), cows consuming pasture plus TMR maintained BCS (+0.01),
whereas cows consuming a TMR gained BCS (+0.19).
Soriano et al. (2001) also observed lower BCS in HO
cows on pasture versus cows in TMR-based feeding
systems in Virginia.
In Florida, HO cattle grazed on 2 combinations of
cool season and warm season pastures with concentrate
supplementation had a greater postpartum loss of BW
than TMR-fed cattle, and BW remained significantly
lower through much of the lactation period (Fontaneli
et al., 2005). In that study, confinement-fed cattle also
produced more milk but milk composition was similar
across treatments.
Both JE and HO cows had lower BCS on pasture
than TMR-fed cows, in a 3-yr systems study of seasonally calved pasture-based and confined cattle (Washburn et al., 2002b). Clearly, grazing cattle in systems
that derive much of their nutrients from pasture tend
to have lower BCS than cattle in TMR-based systems
and lower overall milk production. However, in situations where limited amounts of high-quality pasture are
fed as a supplement to a TMR, both body condition
and milk production can be maintained (Vibart, 2006).
For systems in which pasture is used as a supplemental
feed to a TMR ration, there is likely less overall energy
expenditure for walking and grazing compared with
systems in which pasture makes up most the diet.
Pryce and Harris (2006) estimated the heritability
of BCS in NZ first-parity cows to be between 0.25 and
Journal of Dairy Science Vol. 97 No. 10, 2014