When lucerne is grazed rotationally, it is quite common for high priority stock to be left on a plot for a longer period than the feed supply can support high growth rates. Whereas all the leaves on a lucerne plant maintain a high quality, only the top 10cm or so of the stem material is of high quality. Leaf material will range from 80% to 70% in digestibility but stem material will range from 80% to 40%., from top to bottom of the plant. This means that once most of the leaves are eaten, and despite the fact that a lot of green material remains, the plot of lucerne can no longer maintain reasonable growth rates. If possible, high priority stock should be moved to the next subdivision at this point, leaving the stems to be cleaned up by lower priority animals. A further penalty for leaving prime stock on a plot for too long , is that the feed in the next plot may be of lower quality by the time the animals graze it.

 

In this first example Hereford type steer weaners are shown on their first day in a plot of lucerne, with 1.5 tonnes dry matter (DM)/ha of green material of average digestibility 71%. They can select a diet that is 78% digestible and achieve 94% of their potential intake. This intake is more than twice their maintenance requirements of metabolisable energy (ME) and is used efficiently (0.45) for weight gain. As a result, the predicted live weight gain is 1.5 kg/day.

 

Herbage:            Green                     1.50 t DM/ha        71% digestible

                           Dead                       0.50 t DM/ha        42% digestible

                           Legume 75%

 

Animals:                       Weight           240 kg     Age      8 months

Intake of pasture          8.47 kg

Weight gain                 1.50 kg

 

The second example shows the situation some days later; only one third of the total green material has been removed but as this was leaf and stem tips, the digestibility of the remaining green material has fallen to 60%. The selected diet is now only 67% digestible and the steers are eating only 73% of their potential intake. Their intake is only 41% above their maintenance requirements and this surplus ME is used less efficiently (0.38) so the predicted weight gain is now down to 0.47 kg/d.

 

 

By recognising the rate at which changes in the quality of the green material are occurring during grazing, the grazier can, with GrazFeed, determine quite precisely the point at which stock should be removed from a plot if a target pattern of weight gain is to be achieved. Seasonal conditions will often prevent the target being reached but GrazFeed should enable the best compromise to be reached and available feed used to the best advantage.