# Late gestational nutrient restriction decreases placental size and calf birth weight without altering uterine blood flow in primiparous beef females

## TL;DR

Reducing nutrients in pregnant first-time beef cows during late pregnancy lowers calf birth weight and placental size without affecting blood flow to the uterus.

## Contribution

This study shows that late gestational nutrient restriction in first-time beef cows reduces calf birth weight and placental size without altering uterine blood flow.

## Key findings

- Nutrient-restricted cows had lower circulating glucose, urea N, and triglycerides but higher non-esterified fatty acids.
- NR cows had smaller placentas and calves with 14.4% lower birth weight and smaller heart girth and volume.
- Uterine blood flow was unaffected, but cotyledon number increased and average cotyledon weight decreased in NR cows.

## Abstract

To investigate impacts of late gestational nutrient restriction in first-parity beef females on prenatal nutrient availability to calves, fall-calving heifers [body weight (BW): 472 ± 33 (SD) kg; body condition score (BCS): 5.4 ± 0.5] were individually-fed 100% (control; CON; n = 13) or 70% (nutrient restricted; NR; n = 13) of metabolizable energy and metabolizable protein requirements for maintenance, pregnancy, and growth from day 160 of gestation to calving. Maternal measures were determined every 21 d (BW and metabolites) or 42 d (BCS, backfat, and longissimus muscle area) during gestation and post-calving. Doppler ultrasonography of both uterine arteries was conducted every 21 d until day 244 of gestation. At birth, calf BW and size were measured, and expelled placentas were dissected and dried. Targeted messenger ribonucleic acid (mRNA) expression was determined for cotyledons. Data were analyzed with nutritional plane, treatment initiation date, heifer’s sire, and calf sex (when P < 0.25) as fixed effects. Metabolites and uterine blood flow included day and nutritional plane × day as repeated measures. Circulating glucose was less (P = 0.05) for NR dams than CON. Circulating urea N and triglycerides were less (P ≤ 0.05), but non-esterified fatty acids were greater (P ≤ 0.05), for NR at most timepoints after treatment initiation. Post-calving, NR dams were 62.1 kg and 2.1 BCS less (P < 0.01) than CON. Moreover, NR had less (P < 0.01) backfat and longissimus muscle area, but similar (P = 0.72) shoulder height compared with CON. Heart rate was less (P < 0.01) for NR dams than CON after treatment initiation. Nutritional plane did not affect (P ≥ 0.15) ipsilateral, contralateral, or total uterine artery blood flow. Number of cotyledons was greater (P = 0.03), average cotyledon weight was less (P = 0.04), and total placental weight tended to be less (P = 0.10) for NR than CON. Cotyledonary relative mRNA expression of GLUT1 was greater (P = 0.04) and SNAT2 tended to be greater (P = 0.07) for NR, but other nutrient transporters, angiogenic factors, and PAG2 were not affected (P ≥ 0.13). Calves born to NR dams weighed 14.4% (P = 0.03) less at birth and had smaller (P ≤ 0.03) heart girth and volume than CON. Despite catabolizing maternal tissue stores, beef heifers experiencing late gestational nutrient restriction had altered circulating metabolites and smaller placentas, independent of a reduction in uterine blood flow, which compromised fetal growth.

First-parity beef females that were nutrient restricted during late gestation lost maternal body weight and mobilized adipose and muscle tissue stores. Nutrient restricted females had altered circulating metabolites and smaller placentas, independent of a reduction in uterine blood flow or major changes in cotyledonary nutrient transporter expression, which collectively led to decreased calf birth weight.