Détermination des besoins azotés de la chèvre en lactation

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Abstract
Protein requirements of lactating goats were determined using 216 individual N balances measured in 67 goats between the 1st and the 40th week after kidding. Two methods were employed. Factorial method: protein requirement for maintenance was considered as the sum of endogenous urinary N (EUN) and metabolic fecal N (MFN) (EUN = 0.0434 g N/kg live weight; MFN = 0.0169 g N/g of nondigestible crude fiber intake). In the trials minimum N losses were EUN + MFN = 0.25 g N/kg W0.75. Overall method: digestible N represents N for maintenance (proportional to metabolic weight), for milk protein production (proportional to milk protein N) for increasing N pool (proportional to N balance); a part is lost in rumen metabolism (proportional to the difference between protein digestibility indices), and in urine when the level of dietary N is higher than milk protein production (proportional to milk urea content). Protein requirements for maintenance and milk production were calculated by repression. N requirement for maintenance equals 0.32 g N/kg W0.75. The efficiency of digestible N to produce milk protein N was between 0.72-0.81 when N and energy balances varied. The highest efficiencies were obtained with a positive N and a negative energy balance. These high values could be explained by a N saving (urinary N was low; 8.9 g N/day). In N deficient animals, the efficiency of digestible N to produce milk protein was equal to 0.56. It was lower than the efficiency of body N to produce milk protein N (0.71).

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