68 Late Gestational Nutrient Restriction Decreases Blood Flow to the Mammary Gland in Lactating Primiparous Beef Females

Abstract

Abstract To determine the effects of late gestational nutrient restriction on mammary blood flow during lactation, fall-calving Hereford-SimAngus heifers (single-sired; BW: 451 ± 28 [SD] kg; BCS: 5.4 ± 0.7) bred to a single sire were individually-fed 100% (control; CON; n = 10) or 70% (NR; n = 13) of NASEM net energy and metabolizable protein requirements for maintenance, pregnancy, and growth from d 160 of gestation to calving. Post-calving, all females were individually fed chopped tall fescue hay supplemented to meet lactational nutrient requirements in Calan gates. Transrectal color Doppler ultrasonography of both external pudendal arteries was conducted every 21 d from d 23 to 107 of lactation. Total mammary blood flow was calculated and hemodynamics from both sides averaged. Data were analyzed with fixed effects of nutritional plane, day of lactation, and their interaction, using repeated measures. Calving date and calf sex (if P < 0.25) were fixed effects, and Doppler angle of insonation was included in the model for flow-related variables. We have previously reported that CON cows had greater (P ≤ 0.003) BW, BCS, and backfat from calving to d 147 of lactation, and NR cows had 15% less (P < 0.04) milk yield. External pudendal artery pulsatility index tended to be greater (P = 0.06) and resistance index, peak systolic velocity, and cross-sectional area were greater (P ≤ 0.04) in CON cows. Heart rate was greater (P = 0.008), yet mammary blood flow was 19% less (P = 0.004) in NR cows. Mammary blood flow/cow BW and blood flow/milk yield were not affected (P ≥ 0.13) by nutritional plane. All hemodynamic variables were affected by day (P ≤ 0.04) except resistance index (P = 0.11). In summary, late gestational nutrient restriction decreased mammary blood flow similarly to the reduction observed in milk yield.