Nitrogen balance of Saanen goats in early lactation fed diets with different protein : energy ratio

Aim of the experiment was to evaluate the protein:energy ratio (MP/NEl) of the diet in early lactating goats on N balance. 8 Saanen goats were used in two consecutive N balance trials at 8-14 (P1) and 28-34 (P2) DIM. The dietary treatments were as follows: 56 (LOW) or 65 (HIGH) g MP/Mcal NEl, estimated with INRA model (2007). The CP contents of the diets were 15.1 (LOW) and 17.3% (HIGH) on DM. Both diets had 1.69 Mcal of NEl on DM. The forages fed (DM basis) were corn silage (28%) and dehydrated alfalfa hay (22%); in diet HIGH, soybean meal and corn gluten meal partially substituted corn grain. 4 d after kidding, goats were confined to metabolic cages. During each 6 d of trial, individual DMI, milk production and total collection of excreta were recorded. All data were statistically analyzed by GLM procedure (SAS, 2008) considering the diet and period effect and their interaction. DMI was influenced only by period, not by dietary treatment (2.06 and 2.97 kg/d, in P1 and P2 respectively; P<0.001); milk yield had the same trend increasing from 3.91 to 5.49 kg/d (P<0.001). Milk fat, crude protein and casein contents (%) were affected (P<0.01) only by period (4.77 vs 3.55, 3.67 vs 3.47, 2.63 vs 2.46, for fat, CP and casein, in P1 and P2, respectively). Milk urea nitrogen (MUN) was directly related to MP/ Mcal NEl ratio: on average 6.9 and 15.3 mg/dL with LOW and HIGH diets (P<0.001). The higher SBM inclusion in diet HIGH determined a higher CP digestibility (73.3 vs 67.8%, P<0.001); however, urinary nitrogen excretion resulted higher for this diet (19.7 vs 12.9% of N intake, P<0.01). N utilization for milk production (milk N, % of N intake) was higher in diet LOW (42.8 vs 36.7, P<0.01) and lower in the second period (41.4 vs 38.2, P<0.05) because of the higher DMI. These results suggest that a MP/NEl ratio of about 60 (with approximately 16% CP 486 J. Anim. Sci. Vol. 87, E-Suppl. 2/J. Dairy Sci. Vol. 92, E-Suppl. 1 on DM), should be adequate to meet the requirements of high yielding dairy goats during early lactation.