Abstract :
Old man saltbush (Atriplex nummularia) is sown in saline farming systems worldwide, and
is commonly utilised as fodder for livestock. Saltbush contains very high concentrations
of NaCl and KCl, however, the complex interactions between minerals means that sheep
grazing saltbush may be susceptible to mineral deficiencies. The aim of this experiment was
to assess the effect of high NaCl and KCl intakes on the apparent absorption of some macrominerals
(Mg, Ca, P) by sheep fed formulated high-salt diets or old man saltbush to help
identify likely mineral deficiencies. Two experiments were conducted. In both experiments
18-month-old Merino wethers were offered one of 4 diets at maintenance: a control pellet
(no added salt), control pellet + 10% salt (low salt), control pellet + 20% salt (high salt), or old
man saltbush containing approximately 20% salt. The salt added to all diets was 2NaCl:1KCl.
Sheep were fed the diets for a minimum of 3 weeks before any measurements were taken. In
the first experiment we measured the accumulation of Na and K in the rumen of cannulated
sheep for 24 h following feeding. In the second experiment we measured the apparent
digestion and absorption of Na, K, Cl, Mg, Ca and P over 6 days. There was a net loss of Mg
(0.83 g/day), Ca (0.61 g/day) and P (0.46 g/day) from sheep fed the saltbush diet, but the
addition of salt to the control pellet and high levels of Na and K in the rumen had no effect
on the apparent digestion and absorption of Mg, Ca and P. The loss of Mg, Ca and P from the
gastrointestinal tract of sheep fed saltbush is likely to be due to a combination of reduced
availability and absorption of minerals from the saltbush leaves. Deficiencies in Mg, Ca and
P can reduce animal production and even cause stock deaths. These results indicate that
saltbush as a sole source of feed may be unsuitable for sheep with high nutritional demands,
and further research is required to fully assess the mineral balance of sheep grazing saltbush
pastures.
