Milk production and nutrient partitioning as measured by C-13 enrichment of milk components during C-3 and C-4 plant feeding in purebred Holstein and in Charolais x Holstein F-2 crossbred cows

摘要

Nutrient partitioning was investigated in cows with different genetic merits for milk production by measuring C-13/C-12 ratios (reported by delta values delta C-13) in milk components in response to C-3 (grass silage) and C-4 diets (corn silage). We hypothesised that changes of delta C-13 in milk differ between Holstein (HOL; high milk production) and Charolais x Holstein cows with medium (CHM) and low (CHL) milk production. Changes of delta C-13 (Delta delta C-13) in milk components were estimated by calculating differences of delta C-13 due to switch from C-3 to C-4 feeding. After switch to C-4 feeding, Delta delta C-13 of lactose was greater in HOL than in CHL. Immediate Delta delta C-13 of milk fat was the lowest in CHL. The maximal Delta delta C-13 of casein was the lowest in HOL. The proportion of carbon in milk derived from diet increased with milk yield, indicating the main impact of the milk production level, but minor impact of breed, on nutrient partitioning towards the mammary gland.