STUDY OF THE NUTRITIONAL CAUSES OF MILK FAT DEPRESSION IN GRAZING SHEEP: THE CASE OF WATER-SOLUBLE CARBOHYDRATES OVERLOAD FROM GRASSES

Porcu, Maria Angela

The decrease in milk fat content observed in dairy sheep farms in Sardinia is an important issue of the last decades, particularly during winter and spring months. In grazing conditions, the common use of modern tetraploid annual ryegrass varieties rich in water soluble carbohydrates (WSC) may be one of the causes of milk fat depression (MFD) due to sub-acute ruminal acidosis in ewes grazing on these pastures. The findings of the first study, in which the milk fat content of ewes fed unshaded grass with higher WSC content was lower compared to those fed shaded grass with lower WSC content, especially during afternoon milking (three hours after grass feeding), suggested that WSC overload from grasses may be a cause of MFD. The results of the second study indicated that sodium bicarbonate may be used to prevent ruminal sub-acidosis and MFD from WSC overload in lactating grazing ewes, because of improved total DMI, herbage intake, milk fat percentage, and milk fat yield, especially in afternoon milking when this buffer was supplemented. The third study showed that diet composition and grazed herbage influenced milk composition, and particularly milk fat concentration. The forage to concentrate ratio is confirmed to be as a parameter strongly and positively associated with milk fat content, whereas diets high in starch and NFC may negatively affect milk fat content and, consequently, cheese yield. Negative relationships were found between milk fat content and herbage NFC, whereas the NDF content of herbage did not influence this milk component. In conclusion, WSC overload from grasses in grazing dairy ewes represents a new nutritional issue that may contribute to MFD. Moreover, the relationship found between milk fat content and other dietary and herbage compounds indicates that the decrease in milk fat percentage in Sardinia over the last decade, especially during winter and spring months, represents a multifactorial nutritional issue, in addition to genetic factors.

The decrease in milk fat content observed in dairy sheep farms in Sardinia is an important issue of the last decades, particularly during winter and spring months. In grazing conditions, the common use of modern tetraploid annual ryegrass varieties rich in water soluble carbohydrates (WSC) may be one of the causes of milk fat depression (MFD) due to subacute ruminal acidosis in ewes grazing on these pastures. The findings of the first study, in which the milk fat content of ewes fed unshaded grass with higher WSC content was lower compared to those fed shaded grass with lower WSC content, especially during afternoon milking (three hours after grass feeding), suggested that WSC overload from grasses may be a cause of MFD. The results of the second study indicated that sodium bicarbonate may be used to prevent ruminal sub-acidosis and MFD from WSC overload in lactating grazing ewes, because of improved total DMI, herbage intake, milk fat percentage, and milk fat yield, especially in afternoon milking when this buffer was supplemented. The third study showed that diet composition and grazed herbage influenced milk composition, and particularly milk fat concentration. The forage to concentrate ratio is confirmed to be as a parameter strongly and positively associated with milk fat content, whereas diets high in starch and NFC may negatively affect milk fat content and, consequently, cheese yield. Negative relationships were found between milk fat content and herbage NFC, whereas the NDF content of herbage did not influence this milk component. In conclusion, WSC overload from grasses in grazing dairy ewes represents a new nutritional issue that may contribute to MFD. Moreover, the relationship found between milk fat content and other dietary and herbage compounds indicates that the decrease in milk fat percentage in Sardinia over the last decade, especially during winter and spring months, represents a multifactorial nutritional issue, in addition to genetic factors