Reproductive capacity of Merino ewes fed a high-salt diet

An option to increase the productivity of saline land is to graze sheep on salt-tolerant plants, which, during the summer/autumn period, can contain 20% to 25% of their dry matter as salt. This study assessed the impact of coping with high dietary salt loads on the reproductive performance of grazing ewes. From the time of artificial insemination until parturition, 2-year-old maiden Merino ewes were fed either a high-salt diet (NaCl 13% of dry matter) or control diet (NaCl 0.5% of dry matter). Pregnancy rates, lamb birth weights, milk composition and the plasma concentrations of hormones related to salt and water balance, and energy metabolism were measured. Leptin and insulin concentrations were lower (1.4 +/- 0.09 v. 1.5 +/- 0.12 ng/ml (P < 0.05) and 72 +/- 0.55 v. 8.2 +/- 0.83 ng/ml; P < 0.02) in response to high-salt ingestion as was aldosterone concentration (27 +/- 2.7 v. 49 +/- 5.4 pg/ml; P < 0.05), presumably to achieve salt and water homeostasis. Arginine vasopressin concentration was not significantly affected by the diets, but plasma concentration of T(3) differed during gestation (P < 0.02), resulting in lower concentrations in the high-salt group in the first third of gestation (1.2 +/- 0.18 v. 1.3 +/- 0.14 pmol/ml) and higher concentrations in the final third of gestation (0.8 +/- 0.16 v. 0.6 +/- 0.06 pmol/ml). T(4) concentration was lower in ewes ingesting high salt for the first two-thirds of pregnancy (162 +/- 8.6 v. 212 +/- 13 ng/ml; P < 0.001). No substantial effects of high salt ingestion on pregnancy rates, lamb birth weights or milk composition were detected.