Review/Oorsig Volume 22, Issue 01 - Page 8

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