Sheep Blood Reveals Hidden Metabolic Strain of Late Pregnancy and Lactation

In the rolling hills of Lika, Croatia, a team of veterinary scientists has been quietly drawing blood from a flock of indigenous sheep to answer a deceptively simple question: what exactly happens inside the body of a ewe as she approaches lambing and then begins producing milk? The answer, published in the journal Archives Animal Breeding, is a remarkably detailed portrait of physiological strain. Researchers from the Faculty of Agrobiotechnical Sciences Osijek, led by Zvonko Antunović, tracked thirty clinically healthy adult Lika Pramenka sheep through late pregnancy and across the first hundred days of lactation, measuring an unusually broad panel of blood indicators. Their findings show that the final weeks of gestation and the earliest phase of milk production push the ovine body through measurable shifts in red blood cells, energy metabolites, proteins, minerals and liver enzymes — shifts that could easily be mistaken for disease if the animal’s reproductive stage were ignored.

The study focused on the Lika Pramenka, a hardy Croatian breed traditionally managed under extensive farming conditions. The thirty ewes selected from a herd of roughly eight hundred animals were, on average, five years old and in their fourth lactation, with a lambing index of 1.2. Their diet was deliberately simple and consistent: 1.8 kilograms of meadow hay per day supplemented with 800 grams of oats. At the start of the trial the ewes averaged 53.36 kilograms in body weight and scored 3.75 on the standard five-point body condition scale. Blood was drawn from the same animals on four occasions — twenty days before lambing, and then on the 40th, 70th and 100th days of lactation — yielding 120 samples in total. This repeated-measures design allowed the team to compare each ewe against her own baseline across the demanding transition from pregnancy to milk production.

The laboratory work was exhaustive. Complete blood counts were determined the same day using an automated three-differential hematology analyzer, capturing white blood cells, red blood cells, platelets, hemoglobin, hematocrit and the erythrocyte constants known as MCV, MCH and MCHC. Differential leukocyte counts — the proportions of neutrophils, lymphocytes, basophils, eosinophils and monocytes — were read from blood smears stained with a Pappenheim stain. Serum was separated by centrifugation, frozen at minus 80 degrees Celsius, and later analyzed on an automatic chemistry platform for minerals including calcium, inorganic phosphorus and iron; metabolites including glucose, cholesterol fractions, triglycerides and non-esterified fatty acids; total protein, albumin and urea; and the activities of five enzymes associated with liver and tissue metabolism. Statistical analysis using general linear models and Tukey comparisons identified which changes across the four sampling points were genuinely significant rather than random variation.

The red blood cell results tell a story of oxygen transport under pressure. Compared with lactating ewes, sheep in late pregnancy carried significantly higher counts of red blood cells and higher concentrations of hemoglobin and hematocrit, along with a greater proportion of lymphocytes but fewer platelets and a lower share of neutrophils. Once lactation began, the picture reversed: red blood cell counts, hemoglobin and hematocrit fell significantly, with the lowest values recorded at the second measurement early in lactation, while platelet counts, MCH and neutrophil proportions rose. The authors note that these early-lactation declines could indicate anemia, and indeed the second measurement met commonly used diagnostic thresholds for slight anemia in ruminants, with hemoglobin at 67.70 grams per liter and hematocrit at 0.24 percent. In the other measurements, no such indications appeared.

Why would a milking ewe become temporarily anemic? The researchers point to a mechanism described in earlier literature: hemodilution driven by the physiological adaptations of early lactation, when large amounts of water are mobilized toward the mammary gland to support milk synthesis. Some authors have also argued that elevated red cell destruction in mammary tissue contributes to the reduced hematocrit. Early lactation is additionally a period of energy deficit in ruminants, because fat mobilization peaks precisely when milk production is highest, and this high-intensity metabolism generates oxidative stress and exposes animals to infection. Related work in dairy cattle has shown that cows in early lactation can have compromised immune systems that make them susceptible to bacterial infection and anemia, and that good feeding management can prevent these effects. The team also observed macrocytosis and hypochromia — larger-than-normal red cells with reduced hemoglobin concentration — relative to reference values for sheep.

The biochemical panel revealed an equally dramatic metabolic pivot. Late-pregnant ewes had significantly higher blood concentrations of glucose and triglycerides and a higher albumin-to-globulin ratio, but significantly lower concentrations of urea, total protein, albumin, globulins, calcium and iron, together with reduced activity of the enzymes GGT, CK and AST. During lactation the pattern flipped: glucose and the albumin-to-globulin ratio fell while urea, total protein, albumin, globulins, HDL-cholesterol, calcium, inorganic phosphorus and CK activity rose. The elevated glucose in late pregnancy aligns with earlier findings in sheep and cattle, where increased hepatic gluconeogenesis in the final weeks of gestation raises blood glucose, which then declines as the demands of milk synthesis take over after parturition. The highest concentrations of non-esterified fatty acids appeared in early lactation, a classic signature of body fat being mobilized to cover the energy deficit when glucose supplies fall short.

The protein and mineral data exposed a nutritional weak point in the flock’s diet. Urea concentrations in the first lactation measurement sat at the lower limit of reference values, which the authors interpret as a sign of insufficient protein in the feed — a conclusion reinforced by total protein and albumin concentrations that were at or below the lower limits of reference ranges in the first two lactation measurements. These changes indicated dysproteinemia, an imbalance in blood proteins. Meanwhile, the reduced calcium and iron concentrations in late-pregnant and lactating ewes reflect the intense consumption of these minerals during fetal growth and milk secretion; calcium is deployed from blood to milk, and iron is excreted in milk, explaining the significant drop in serum iron during lactation. Earlier studies have shown that reduced blood calcium during late pregnancy and early lactation results from the combined calcium demands of fetal and milk secretion.

Liver enzyme activity added a final layer of evidence for the metabolic burden. Significantly lower activity of GGT, CK and AST in late-pregnant ewes, followed by increases during lactation, indicated an overload of the organism and of the liver itself with protein deamination, linked to the increased demands of milk secretion. Elevated activities of ALT, AST and GGT in lactating sheep are associated with more intense hepatic metabolism and the body’s response to negative energy balance. The authors suggest these enzyme changes could be connected with reduced dry matter intake and increased fat mobilization around lambing, which can lead to hepatic lipidosis and disturb normal liver function. Notably, most enzyme activities remained within reference limits except GGT during lactation, suggesting the strain was significant but not yet pathological.

Beyond the individual measurements, the correlation analysis demonstrated how tightly these blood indicators are interwoven. The team found very strong positive correlations between red blood cells and hemoglobin (r = 0.98), red blood cells and hematocrit (r = 0.97) and hemoglobin and hematocrit (r = 0.95), which is physiologically expected since hematocrit measures the fraction of blood made up of red cells. A very strong negative correlation emerged between neutrophils and lymphocytes (r = −0.94), a ratio widely used to assess stress in animals. Among the biochemical parameters, very strong positive correlations linked cholesterol with HDL-cholesterol (r = 0.87), cholesterol with LDL-cholesterol (r = 0.77), total protein with albumin (r = 0.70) and total protein with globulins (r = 0.91), while a strong negative correlation appeared between globulins and the albumin-to-globulin ratio (r = −0.60). These interdependencies confirm that blood indicators respond as a coordinated system to the ewe’s metabolic state.

The practical message for shepherds and veterinarians is clear: blood values cannot be interpreted in a vacuum. The study demonstrates that late pregnancy and lactation significantly reshape most indicators of the metabolic profile and complete blood count, with the most pronounced changes occurring in late pregnancy and the earliest weeks of lactation, and that feed quality — particularly protein and mineral content — plays a decisive role in shaping those values. The authors recommend that any assessment of a sheep herd’s metabolic profile should account for the animals’ physiological status and the quality of their nutrition in order to obtain the most accurate picture of herd health, breeding quality, possible nutritional errors and welfare. For extensive farming systems like the one in Lika, where veterinary intervention is often minimal, a single blood draw interpreted without this context could mislabel a healthy, hard-working ewe as sick — or, more dangerously, miss a genuine problem hidden behind what looks like a normal result.

Subject of Research: Changes in the blood metabolic profile and complete blood count of ewes during late pregnancy and lactation

Article Title: Blood metabolic profile and complete blood count of sheep during late pregnancy and lactation

Article References: Antunović, Z., Kučan, J., Klir Šalavardić, Ž., Djidara, M., & Novoselec, J. (2026). Blood metabolic profile and complete blood count of sheep during late pregnancy and lactation. Archives Animal Breeding, 69(2), 337-346. https://doi.org/10.5194/aab-69-337-2026

Image Credits: AI Generated

DOI: 10.5194/aab-69-337-2026

Keywords: sheep, Lika Pramenka, blood metabolic profile, complete blood count, late pregnancy, lactation, anemia, non-esterified fatty acids, glucose, liver enzymes, calcium, animal nutrition