The effect of pregnancy on visceral growth and energy use in beef heifers.

Beef heifers (24 mo; 378 +/- 32 kg of BW; 22 pregnant, PR; 17 nonpregnant, NP) were grouped in common pens and fed corn silage- and hay-based diets formulated to provide an ADG of 0.45 kg in NP heifers. Both PR and NP heifers were slaughtered on d 40, 120, 200, and 270 of the study. Intestinal and hepatic tissues were analyzed for protein, DNA, RNA (mg/g of fresh tissue), and in vitro oxygen use. Jejunal samples were analyzed for cellular proliferation via immunohistochemical analysis. For ileum, DNA, which provides an estimate of cell number per unit of tissue, revealed an interaction (P = 0.06) between pregnancy and slaughter day; both PR and NP decreased with time, but NP increased on d 270 (P = 0.09). Cell number in the ileum was reduced at d 200 and 270 in the PR heifers (P < 0.09). Liver protein concentration was less (P = 0.07) in PR than in NP heifers (NP = 291.1 vs. PR = 210.5 +/- 33.9 mg/g). Hepatic protein:DNA ratio was not affected (P > 0.10) by pregnancy or day. Energy use (kcal/d) of duodenum and jejunum, calculated from in vitro oxygen consumption, increased linearly (P < 0.02) with time for both PR and NP. Pregnant and NP ileal energy use increased linearly (P < 0.01), but ileal energy use by PR was less throughout gestation (P = 0.07) than ileal energy use by NP. Cellular proliferation in the crypt region of the jejunum was decreased on d 120 and 200 (P < 0.02). These data indicate that the small intestine and liver of PR heifers may conserve energy expenditure compared with NP heifers. Energy conservation can partially be explained by differences in growth and cell proliferation and by energy use of the liver and small intestine.