Glucose and protein metabolism during late pregnancy in triplet-bearing ewes given fresh forages ad lib. 1. Voluntary intake and birth weight.

Ewes of the Booroola X Romney genotype carrying triplet lambs were given fresh forages ad lib. in late pregnancy. In Expt 1, groups of three ewes were given kale (Brassica oleracea), perennial ryegrass (Lolium perenne) or perennial ryegrass (0.75)-barley (0.25). In Expt 2, groups of two or three ewes were given fresh perennial ryegrass and infused into the abomasum with iso-energetic quantities of casein and glucose in a 2 x 2 factorial arrangement. Post-lambing ewe live weights were 40-50 kg. Glucose irreversible loss (GIL) was determined from dilution of D-[U-14C]glucose. For ewes given kale, perennial ryegrass and perennial ryegrass-barley in Expt 1, mean metabolizable energy (ME) intakes were 0.50, 0.82 and 0.83 MJ/kg live weight 0.75 per d, GIL was 112, 142 and 157 g/d, and mean birth weight 2.22, 3.05 and 2.95 kg/lamb. In Expt 2, infusion of glucose, casein, and glucose + casein depressed herbage ME intake respectively by 1.6, 0.9 and 0.3 times the amount of ME infused. GIL (185-325 g/d) was increased by 800 and 350 g respectively for each kg of glucose or casein infused. Casein infusion increased calculated amino acid absorption from 0.18 to 0.36 of ME, increased wool growth and increased calculated maternal N balance. Birth weight was unaffected by nutritional treatment and averaged 3.29 kg/lamb. When values from both experiments were combined, birth weight was related to GIL by a hyperbolic relation, with maximum predicted birth weight being 4.1 kg/lamb. It was postulated that this value was never attained in practice, due to uterine expansion being restricted by the low maternal body size. Marked decreases in birth weight occurred when GIL decreased below 173 g/d. It was calculated that ewes in all treatment groups were in negative energy balance, and that glucose supplied by the kale and unsupplemented ryegrass diets were respectively below and equal to calculated conceptus uptakes of glucose necessary to maintain growth of triplet fetuses. It was further calculated that amino acid requirements of triplet-bearing ewes in late pregnancy were likely to exceed substantially net absorption from digestion of fresh forage diets, and that maternal tissues go into negative N balance to ensure fetal growth, thus explaining the lack of response to abomasal casein infusion.