Effects of protein supplementation frequency on metabolic responses associated with reproduction of beef cows

Abstract Text: This experiment evaluated if frequency of protein supplementation impacts physiological responses associated with reproduction in beef cows. Fourteen non-pregnant, non-lactating Angus cows were ranked by parity and BW, and allocated to 3 groups. Groups were assigned to a 3 × 3 Latin square design, containing periods of 21 d and the following treatments: 1) daily supplementation of soybean meal (7D), 2) soybean meal supplementation 3 times/wk (3D), and 3) soybean meal supplementation once/wk (1D). Within each period (d 0 to 21), cows were assigned to an estrus synchronization protocol; 100 μg of GnRH + controlled internal device release (CIDR) containing progesterone (P4) on d 1, 25 mg of PGF_2α on d 8, CIDR removal plus 100 μg of GnRH on d 11. Straw was offered for ad libitum consumption. Soybean meal was individually supplemented at 1 kg/cow daily. Moreover, 3D were supplemented on d 0, 2, 4, 7, 9, 11, 14, 16, and 18, whereas 1D were supplemented on d 4, 11, and 18 of each period. Blood samples were collected on d 11 and 18 from 0 to 72 h relative to supplement feeding, and analyzed for plasma urea N (PUN). Samples collected from 0 to 12 h were analyzed for plasma glucose, insulin, and P4 (d 18 only) concentrations. Uterine flush fluid was collected 28 h after supplementation for pH measurement. Data were analyzed with the MIXED procedure of SAS and cow as experimental unit. A treatment × hour interaction (P < 0.01) was detected for PUN, which peaked (P < 0.01) for 1D and 3D at 28 h after supplement feeding, whereas the same response was not detected in 7D. Moreover, PUN concentration at 28 h was greater (P < 0.01) for 1D compared with 3D and 7D (42.7, 34.4, and 25.7 mg/dL, respectively), and also greater (P < 0.01) for 3D compared with 7D. No treatment effects were detected (P ≥ 0.65) for plasma glucose and P4 concentrations, whereas mean plasma insulin concentrations were greater (P ≤ 0.02) in 7D and 3D compared with 1D (4.61, 4.76, and 3.74 μIU/mL, respectively). Uterine flushing pH tended (P ≤ 0.10) to be greater for 1D compared with 3D and 7D (6.204, 6.130, and 6.140, respectively). In conclusion, reducing frequency of protein supplementation to once/week impacted plasma insulin, PUN, and uterine flushing pH, which are known to modulate reproduction of beef cows.

Keywords: Beef cows, protein, physiology, reproduction