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A linear programming (LP) model has been developed (CAP-DAIRY) to describe the
grass utilisation and feeding system on a dairy farm. It links several components of the
system and optimizes the system as a whole. The model links a grass utilisation model, a
feed ration model and a novel model which relates on-going milk yield to on-going feeding
level.
The main feature of the model is the approach adopted to relate feeding levels, milk
yield and weight changes. When cows are fed more than they require for maintenance and
the current level of milk yield, the excess energy becomes increased bodyweight and cause
an increase in milk yield. When fed less than they require, they mobilize reserves into
energy for milk production and lose weight and tend to reduce milk yield. At the start of the
lactation some weight loss is tolerated. This is treated in the model as a requirements for up
to 0.5 kg/day weight loss in addition to maintenance so that a lower weight loss is the
increase case. A linear mathematical model that represents this mechanism was developed
and incorporated into the LP and fitted to data which changed the level of feeding of dairy
cows during the lactation. This gives a greater flexibility to the LP and allows the model to
determine optimal feeding levels at all stages of the lactation and as a consequence optimal
milk yields and optimal stages for weight changes, which vary depending on calving date
and feed availability
The grass utilisation model permits the successive utilisation of grass for grazing or
silage making. Grazing can take place two, three or four weeks after the previous use and
silage making five, six or seven weeks after the previous use. To allow for the effect of
silage making on regrowth, use after this is delayed by one week. Data on energy and dry
matter yields at any time is required and the model determines the optimum schedule of use
and frequency. Silage is made in a number of separate silos but the feeding-out energy
value makes the model non-linear. This is solved by using a recursive approach in which
the initially unknown feeding value is calculated from successive solutions and the model reoptimized
to convergence.
The feed ration model determines the amount of grass, silage and concentrates
required based on the maximum dry matter intake, which is a function of yield and the
energy required for maintenance, milk yield and any weight change. The model could be
easily extended to also use protein given suitable data.
The LP determines the optimal land use for forage and cash crops, calving pattern
and feeding strategy according to specific farm conditions such as farm area, milk quota and availability of forage maize. Several scenarios were studied and the effects of changes of
different parameters analysed.
Results indicated that net margins increased with maize crop areas and gave higher
optimum milk yields replacing concentrates up to an optimum area of maize.
The seasonality of milk prices affected particularly calving pattern and milk yield and
the results suggested they led to more even milk production due to encouraging Autumn
calving.
Results also showed that the optimal feeding levels is different for cows calving in
different periods of the year resulting in different weight change pattern and milk yields.
Spring calving cows lost more weight than cows calving in any other period, but regained
the weight lost quickly. They also produced the lowest level of milk. Autumns calving cows
had the highest milk yield and the lowest weight losses, although a longer period to regain
that weight was optimal. Summer calving cows produced slightly less milk and lost slightly
more weight than Autumn cows.
Another important aspect that results showed was the influence that maize silage
has on farm decisions. The larger the maize crop area the higher the marginal price of milk
quota, showing that milk quota constraint was more severe for those cases and
consequently higher prices for extra milk quota could be paid.
CAP-DAIRY is suitable for analysing the impact that changes such as milk prices,
fertilizer prices or concentrates prices would cause on the optimal plans. The model is also
helpful to evaluate research priorities by analysing the effects caused by biological and
technical changes such as grass varieties and machinery. |
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