The plasma and urine mineral status of lambs offered diets of perennial wheat or annual wheat, with or without lucerne

Abstract

The development of perennial wheat is proposed to increase resource-use efficiency and sustainability of crop production. Annual dual-purpose winter wheat and perennial wheat offer grazing potential, however they both have mineral imbalances due to excess forage K and deficient Na, which may lead to metabolic diseases such as hypocalcaemia and hypomagnesaemia. Notwithstanding a low forage Na concentration, it was hypothesised that the addition of forage lucerne (alfalfa), high in forage Ca and Mg, may overcome calcium and magnesium deficiencies of the wheat varieties. A pen feeding study was conducted using 10-month-old Poll Dorset × Merino ewe lambs (46.4 ± 1.8 kg) to test the effect on plasma and urine mineral status with the addition of lucerne to perennial wheat or annual wheat forage. The lambs (n = 48) were individually fed one of four diets, including perennial wheat (PW), annual wheat (W), perennial wheat + lucerne (alfalfa) (PW + L) or annual wheat + lucerne (W + L) and feed intake was calculated daily. Individual blood and urine sampling commenced on entry to the pens and were repeated at 7-day intervals for 3 further weeks. The addition of lucerne increased the dietary supply of Ca, Mg and Na and decreased K and P. When compared to W, PW offered more Ca, Mg, K, and less Na and P. There were mineral intake differences over time (P < 0.001) for all minerals. All diets offered high dietary cation anion difference (45.0 – 94.9), K: (Na + Mg) (7.3 – 12.8), K: Na (305 – 1137) ratios, while the addition of lucerne increased the Ca: P ratio and improved the K: (Mg + Ca) (Tetany index). Plasma Mg, Na, K and P declined significantly throughout the study, under all diets, to varying degrees. In Week 1, plasma Mg was significantly lower in PW lambs when compared to all other diets, but no further diet effects were observed thereafter. By contrast, plasma Ca increased throughout the study, but without dietary effects. The urinary excretion of Mg was higher with the addition of lucerne in the PW + L and W + L diets after 1-week, although this effect declined in subsequent weeks. The excretion of Ca, Na and P was very low in urine and decreased, while K excretion increased during the study. Taken together the results indicate that the greater dietary intake of Ca, Mg and Na due to the addition of lucerne was insufficient to overcome the primary mineral imbalance of high dietary K and low Na in the cereal forages. Furthermore, the addition of lucerne to the diet further decreased P intake and greatly increased the Ca: P ratio. Our calculations suggest lambs grazing any of the diets in the present study still required supplementation with Na and Mg, but the addition of lucerne alleviated the need for Ca supplementation. The need for supplementation with a source of dietary P requires further investigation.