Effects of excess nitrogen intake on nutrient utilisation and partitioning in lactating ewes

Abstract

The effects of different forms of excess nitrogen (amino acids vs ammonia) on the partitioning of nutrients between milk production and body weight change in lactating ewes suckling twin lambs were investigated. Effects on nutrient intake, ewe and lamb live weight changes, milk yield and composition, and nitrogen (N) balance were assessed.

Nine abomasally cannulated lactating 4 year-old Coopworth ewes with mean initial weight of 66.41 ± 5.90 kg each suckling twin lambs with initial litter weight of 5.72 ± 1.22 kg were used. All ewes were fed the same basal diets in 12 equals meals at 2-hour interval. Treatments consisted of abomasal infusions of water (control), casein (150 g/d), and ammonia (85 g/d) in two periods with a carry-over design. Lambs were fed with milk from their own mothers throughout the experimental periods and also given the same amount of 200 g/d of creep feed in the second period.

Ewe dry matter intakes and live weight gain (LWG) were not different between treatments. Lamb LWG was lowest in the ammonia group (p<0.05). Casein infusion tended to increase (p<0.10) mean milk yield by 33% while ammonia infusion tended (p<0.10) to reduce it by 15% of that of control. Milk fat yield was lower (p<0.05) in the casein group while no differences were found in milk protein, lactose, total solids, and solid non fat (SNF) yields. Ewes in the ammonia group had the lowest milk protein (p<0.05), highest fat (p<0.01), and lowest milk SNF concentrations. Milk lactose and total solids concentrations were not affected by treatments.

Dry matter and organic matter digestibilities, digestible dry matter and digestible organic matter intakes were not different between treatments. Casein and ammonia treatments increased N digestibilities (p<0.05) and slightly increased N intake (p<0.10).

Nitrogen retention per kg metabolic weight was highest in the ammonia group (P<0.05). The efficiencies of absorbed N utilisation for milk protein synthesis were higher in the control and casein groups (P<0.05) while that for body protein synthesis was higher in the ammonia group (P<0.05). Compared to ewes in the control group, those in the ammonia group showed a higher preference for partitioning the absorbed N into body protein synthesis by about 17% (P<0.05) and a lower preference for partitioning the absorbed N into milk protein synthesis by about 20% (P<0.05). No differences in the partitioning of absorbed N between body and milk protein syntheses were found between the control and casein groups.

It was concluded that the form of excess N (ammonia vs amino acids) differentially affects the partitioning of nutrients between milk production and body weight change in lactating ruminants.