Oorsig/Review
drop in body fat reserves post-partum and the
resumption of cyclicity in order to re-conceive.
To be able to efficiently utilize pasture, one
should take cognisance of the factors that
affect the non-structural carbohydrate (NSC)
concentration in grasses. The balance between
photosynthesis (during periods of sunlight) and
respiration (during dark and overcast periods)
determines accumulation of NSC by plants.
Absolute NSC concentrations are dependant on
plant growth rates and developmental stage, and
on prevailing environmental conditions.
On average, over 40-60% of NSC produced by
photosynthesis is consumed by respiration.
Most of the NSC is found in the bottom 5cm of
the grass plant. This however does not mean
that one should allow the plant to be grazed
at this length. Above optimum temperatures
for grass growth, provided there is sufficient
moisture and nutrients to sustain growth, will
lead to an increased rate of respiration resulting
in lower NSC concentrations, but if grass growth
is retarded during hot weather, less NSC will
be consumed by plant respiration. During
overcast weather, below optimum for grass
growth, generally increase NSC concentrations,
as growth and respiration are both reduced.
Following 6 days of cloud, NSC concentrations in
perennial rye grass were almost down to 50%
of the original concentration. The rate of NSC
accumulation during daylight is about a one half
percent increase per hour. Following extended
periods of overcast weather with much rain
and grass re-growth, observant stockmen have
noticed that the rate of intake was slower – as if
the grass was not that palatable as usual. As one
farmer said: “Die koeie vreet maar langtand aan
die weiding”. This may well be due to the lower
NSC concentration of the pasture.
Studies have found, in both northern and
southern hemispheres, taking all climatic factors
into account that NSC levels are highest in winter
– provided there is sunshine. NSC concentration
is also higher in more mature plant tissue (tiller
and leaf bases) compared to younger plant
tissue (tiller and leaf tips). A four-fold increase
in NSC concentrations has been noted from
the 1-leaf stage to the 3-leaf stage in perennial
8
rye grass within a growth cycle. High pressure
grazing during the preceding dormant season,
by promoting plant growth, initially lowers
plant NSC, but this is reversed as the larger
leaves produced by the added N (through feces
and urine) are capable of producing more NSC
through photosynthesis during daylight. Given
the factors that affect NSC in grass, one may
conclude that maximizing the intake of NSC from
pasture and using an energy supplement in a
strategic way early in the growth season, will
optimize production from pastures.
The ultimate objective is to achieve a synchrony
between energy (NFC) and non-protein nitrogen
(NPN) intake – resulting in the maximum rumen
microbial protein delivery to the small intestine
for efficient beef production. Movement to new
pasture should, in my opinion, preferably take
place in the afternoon, not in the morning; with
a second grazing cycle on the same pasture the
following morning.
Concentrate supplementation, however,
usually reduces herbage intake from pasture.
Substitution rate (kg reduction in herbage intake
per kg increase in concentrate DM intake) in
a French study averaged 0,5-0,6 (Peyraud and
Delaby, 2000). Substitution rate between grass
and concentrate is poorly related to the amount
of concentrate consumed and is rather a function
of the cow’s energy balance, which is affected by
the stage of lactation. Substitution rate will be
lowest when limitations for DMI pasture intake
is highest and the energy requirement cannot be
met from grass alone – such as during the first
few weeks of lactation during early spring when
the cow is in a negative energy balance.
When to move cattle to a new pasture
Several criteria may play a role in making this
decision.
• During the pasture growth season, short
duration grazing to allow for a relatively
long recovery period, is indicated. A plan
- taking into account the number of camps,
number of cattle, number of herds and the
required rest period for each camp to allow for
regrowth – should be drawn up for the farm
as a guideline for a planned grazing strategy