Factors affecting placental size in beef cattle: Maternal and fetal influences.

Our objectives were to determine the effects of dam body condition score (BCS), age of dam, and calf sex on placental size and the relationships between dam body weight (BW) and calf size with placental size. Expelled placentas and calf size at birth were collected from crossbred beef heifers and cows during four experiments (n = 22 to 39/experiment). Placentas deemed complete by visual inspection were dissected; dry weights were determined for cotyledonary and intercotyledonary tissues. Mixed linear models were used to individually determine main effects of peripartum BCS category [Thin (<5), Moderate (=5), or Fleshy (≥6)], age of dam category [Primiparous (2 yr), Young (3-4 yr), or Mature (≥5 yr)], and calf sex on placental measures. Correlations were determined for placental characteristics with prepartum dam BW, gestation length, and calf size. Thin BCS dams had lower (P ≤ 0.05) cotyledonary, total placental, and average cotyledon weights and greater placental efficiency (calf birth BW/placental weight) than moderate and fleshy dams. Intercotyledonary weight was lower (P < 0.01) in thin BCS dams compared with fleshy dams. Thin and moderate BCS dams had smaller (P ≤ 0.04) calf birth BW than fleshy dams. Primiparous dams had lower (P ≤ 0.05) total placental and average cotyledon weights than young and mature dams, yet calf birth BW was unaffected (P = 0.17). Male calves were heavier (P = 0.01) than females, yet there were no differences (P ≥ 0.59) in placental weights. Calf birth BW and heart girth had moderate positive correlations (P < 0.01) and shoulder to rump length and abdominal girth had weak positive correlations (P < 0.01) with all placental weights. Dam prepartum BW and calf flank girth had moderate positive correlations (P < 0.01) with total placental weights and weak positive correlations (P < 0.01) with cotyledonary and average cotyledon weights. Intercotyledonary weight had moderate positive correlations (P < 0.01) with gestation length and calf flank girth and a weak positive correlation (P < 0.01) with dam prepartum BW. Gestation length had a weak positive correlation (P = 0.02) with total placental weight. Number of cotyledons was not correlated (P ≥ 0.28) with any dam or offspring characteristics. In conclusion, these data suggest that both maternal age and BCS affected placental size. Calf size at birth and placental weight were positively correlated, but it is still unknown which controls and signals for the growth of the other.