Nutritional demands of pregnancy

By CS McConnel, Veterinary Medicine Extension

I am writing this after having watched Dr. Antonello Cannas from the University of Sassari, Italy, present a webinar on “Balancing Diets of Highly Productive Sheep and Goats: Combining Performances and Health.” He provided some interesting background on the differences in nutritional needs between small ruminants and cattle. For example, it is common for small ruminants to double (single fetus), quadruple (twins) or even sextuple (triplets) the fetal growth (grams/day) versus dam’s weight (kg) as compared to cattle during the last 30 days of pregnancy. Within sheep specifically, total fetal weight as a proportion of ewe weight declines from small to large breeds. Ultimately, fetal growth is exponential with more than 70% occurring within the last 30 to 40 days of gestation. These increased demands during a much shorter gestation means that small ruminants face a more “concentrated” nutritional challenge.

Dr. Cannas spoke about the most common nutritional disorders and diseases that frequently occur in late pregnancy to early lactation, such as ketosis (pregnancy toxemia), hypocalcemia, acidosis, protein imbalance, and immunosuppression. Pregnancy toxemia in particular was a focus and included an excellent reference entitled Pregnancy Toxemia in Sheep and Goats (Vet Clin North America: Food Animal Practice, Vol. 39, Issue 2, 2023, pp. 275-291). An interesting example was presented regarding the energy output of sheep and goats during lactation.

  • A 65 kg dairy goat producing 5 kg/d (11 lb/d) of milk with 3.5% fat, or a 65 kg sheep producing 3.7 kg/d (8.2 lb/d) of milk with 6.5% fat, has the same energy requirements for milk production per kg of body weight as a 650 kg dairy cow that produces 50 kg/d (110 lb/d) of milk with 3.5% fat.
  • However, the small ruminant has a higher maintenance requirement which means that the total energy requirement per kg of body weight for a goat producing 5 kg/d of milk (or a sheep producing 3.7 kg/d of milk) is equivalent to that of a cow producing 61 kg/d (134.5 lb) of milk.

As a consequence, without proper dietary oversight the risk of subclinical and clinical ketosis is elevated and can lead to immunosuppression, placental retention, metritis, clinical mastitis, and perinatal mortality. However, appropriate dietary interventions differ depending on the small ruminant (goat vs sheep). Per a study by Lunesu et al (JDS, Vol. 104, Issue 3, 2021, pp. 3617-3631) that conducted a simultaneous comparison between the two species, throughout the lactation period studied Saanen goats had a hormonal status (high growth hormone and low insulin concentration) that favored milk production, whereas Sarda ewes had a hormonal status more prone to favor body reserve accumulation. This was particularly clear in mid lactation, when high-starch and low-starch diets were compared in both species. Results of this simultaneous comparison under the same feeding and background conditions clearly showed that Saanen goats and Sarda dairy ewes have a different hormonal control of energy partitioning during lactation, with ewes being more insulinemic, especially during mid lactation, compared with dairy goats. These differences were suggested to have important implications in terms of optimal dietary carbohydrate sources and concentrations during lactation, which likely differ between the two species, especially in mid- and late lactation.

A follow-up study by Lunesu et al is in press in the Journal of Dairy Science and focuses on the effect of glucose infusion on glucose and insulin metabolism in early- and mid-lactation ewes and goats fed diets differing in starch and highly digestible fiber concentration. The take-home from this study is that compared with goats, ewes showed a higher peak insulin, insulin increment, linear insulin area under the curve, insulin resistance index, and lower insulin sensitivity indices. Despite the limitations associated with the use of an intravenous glucose tolerance test to assess glucose regulation mechanisms, this study indicated large species differences in both early- and mid-lactation and a more evident anabolic status in the ewes compared with the goats.

Regardless of these species’ differences, nutrient balance is a function of dry matter intake and dietary nutrient composition. If dry matter intake declines in late gestation, appropriate modifications to nutrient density will be necessary to ensure adequate nutrient intake. Per the Vet Clin North America article referenced above, gestation diet protein content needs to be considered when one increases grain to accommodate intake. The authors of that article purport that based on their experience in feeding sheep and goats, dietary protein should be considered a critical factor in preventing pregnancy toxemia. Current NRC recommendations do not account for the greater fetal mass, intake reduction, and greater nutrient requirements for prolific ewes that would be more susceptible to pregnancy toxemia. Feeding higher dietary protein content (∼15% crude protein), which is above current recommendations for ewes with twins, has shown improved ewe and lamb performance, colostrum quality, and lamb growth via greater milk yield. In fact, the authors believe that prolific ewes and does (≥3 fetuses) should be fed a minimum of 15% crude protein diets to meet increased needs and prevent pregnancy toxemia. Yearling ewes or does that are also pregnant have a higher protein requirement (suggest 16.5% in a close-up diet) to support both fetal growth and structural growth of the dam. Overall, the authors point out that:

  1. Forage maturity, recognized as increased NDF content, is an important regulator of DMI. Forages exceeding 55% NDF should not be fed to late pregnant ewes or does and an NDF intake of 0.65% to 0.8% of live body weight is targeted for the late gestation diet.
  1. Essential nutrient content, especially energy and protein, need to be adjusted relative to intake rate. A minimum dietary crude protein of 12% and 65% total digestible nutrients is recommended for prevention of pregnancy toxemia.