Heat Stress in Sheep and Goats: Thermoregulation, Yield, Reproduction and Management

Thanks to their small body mass and low water and feed requirements, sheep and goats are more heat-tolerant than cattle (Joy et al., 2020). This tolerance is not unlimited, however; heat stress reduces milk and growth yield, reproductive performance and welfare in small ruminants (McManus et al., 2022). This article examines how small ruminants keep cool, THI thresholds, effects on yield and reproduction, the special situation of lambs and kids, and practical management on farm and pasture in the light of current scientific literature.

1. How Do Small Ruminants Keep Cool? Thermoregulation

With rising heat, the first and most obvious physiological response in small ruminants is an increase in respiratory rate (panting); this is followed by changes in sweating, heart rate and metabolic rate together with behavioural responses (seeking shade, increased water intake, reduced feed intake, standing) (Berihulay et al., 2019; McManus et al., 2022). Cellular and genetic responses such as heat-shock proteins (HSP) also come into play (McManus et al., 2022).

Wool is an insulating layer and acts as a barrier against radiant heat gain. A counterintuitive but important finding: under tropical conditions, shearing did not improve thermoregulation in crossbred sheep — shorn animals showed higher rectal and skin temperatures, while unshorn animals had a higher sweating rate and more effective homeothermy (Pulido-Rodríguez et al., 2025). In other words, the assumption that "shearing always cools" does not hold under all conditions; wool also provides protection against external heat.

2. THI Threshold and Yield Loss in the Dairy Goat

For goats, the critical threshold is reported to be approximately THI 80. In Saanen goats under summer heat stress, pulse and respiratory rate rise and milk yield falls; at THI 81 and 89, reductions in daily milk yield of approximately 3% and 13%, respectively, have been reported (Akkaya et al., 2024). High-yielding dairy breeds such as the Saanen are more sensitive to heat, and part of the yield loss is due to reduced feed intake.

3. Effects on Reproduction and Pregnancy

In the pregnant ewe, maternal hyperthermia reduces uterine blood flow and placental development and restricts oxygen and nutrient transfer to the fetus; the result is intrauterine growth restriction and low-birth-weight lambs (Limesand et al., 2018). Heat stress in early pregnancy causes an approximately 19% reduction in placental weight (a decrease in cell number and total DNA); this early placental suppression forms the basis of fetal growth restriction in late pregnancy (Vatnick et al., 1991).

In dairy goats, heat stress lowers conception rates in females and increases embryonic loss; in males it adversely affects libido, testosterone, ejaculate volume and sperm motility. Crossbred goats may show better reproductive performance under hot conditions than purebred Saanen (Adjassin et al., 2022).

4. Heat Stress in Lambs and Kids

In newborn lambs and kids, thermoregulation is not yet mature; during the perinatal period body temperature is largely regulated by brown adipose tissue (BAT) and energy metabolism (Bienboire-Frosini et al., 2023). For this reason the young are vulnerable to both cold and extreme heat, and microclimate management is critical.

Heat stress during pregnancy and after birth reduces the lamb's growth up to weaning: in Spanish Merino lambs at THI 65 and above, reductions of approximately 17% in birth weight, 4-8% in live weight at days 15-30 after birth, and approximately 8% in daily weight gain have been reported (Molina et al., 2022). Similar growth losses have also been demonstrated in Tunisian local kids (Atoui et al., 2024); in a dry-hot climate, the thermoregulatory capacity of kids develops from birth to weaning (Gonçalves et al., 2023). In weaned lambs, a heat stress threshold based on physiological and growth indicators has been defined (Zhao et al., 2023).

From a practical standpoint, two interventions stand out: (1) Shade: for newborn lambs, even tree shade improves environmental conditions and heat stress indicators (Cloete et al., 2021). (2) Water and cooling: rising temperature and water restriction together impair lamb physiology, welfare and meat quality (Lowe et al., 2002); the young must therefore be guaranteed access to shade and continuous clean water. Planning weaning to avoid the hottest part of the day or season reduces the total stress load on the young.

5. Why Are Small Ruminants More Tolerant Than Cattle?

Small ruminants have higher thermotolerance than cattle thanks to their low body mass, low water and feed requirements, and good feed conversion (Joy et al., 2020). There is also variation within species: desert goats are more resistant to heat stress than desert sheep, and desert-adapted breeds dissipate heat by accelerating their respiration (Aboul Naga et al., 2021).

6. Practical Management (General)

• Shade: erect shade structures that do not block airflow, preferably oriented north-south (Most et al., 2023).
• Airflow and cooling: apply fan-driven air circulation and evaporative cooling indoors (Most et al., 2023).
• Water: provide continuous clean, cool water; allow adequate trough space (at least approximately one linear foot/30 cm of trough perimeter per 20 head) (Most et al., 2023).
• Feeding: since feed intake declines in the heat, offer the bulk of the ration during the cool hours of the day; prioritise energy density and water access.
• Shearing: the cooling effect of shearing is condition-dependent; under intense radiant heat, wool may also provide protection (Pulido-Rodríguez et al., 2025).

7. Management on Pasture and Mountain/Grazing Areas

Under extensive (pasture and highland) conditions, in-barn cooling is often not possible; therefore shade, water distribution and grazing timing are the three most powerful tools.

7.1 Shade — The Single Most Effective Intervention on Pasture

In grazing sheep, shade markedly lowers midday respiratory rate and stress biomarkers such as cortisol and improves meat quality; although no difference is seen in live weight gain, the welfare gain is clear (Liu et al., 2012). On extensive pastures, animals consistently seek shade during heatwaves (above 40 °C) and shift their activity to dawn and dusk; use of shade improves body condition and consequently reproductive success (Leu et al., 2021). For this reason shade should be provided abundantly and dispersed — multiple shaded areas should be created to prevent crowding at a single point.

7.2 Water Point Distribution and Walking Distance

Pasture sheep and goats graze on average 0.6-1.1 km (up to approximately 2.4 km) from water; as temperature rises, animals move closer to water and the daily distance travelled and activity decrease — that is, the fringe pasture far from water goes unused (Meat & Livestock Australia, 2022). Distribute water points so that no grazing area is more than approximately 1 km away; use portable additional water points during hot periods. In summer, water intake can rise to 19-25% of live weight; keep the water cool and clean (Schoenian, Maryland Small Ruminant).

7.3 Grazing Timing

Animals naturally graze in the early morning and the cool of the evening and rest during the hottest part of the day; shift grazing to the cool hours in line with this rhythm and avoid droving, handling and transport during the hot hours (University of Missouri Extension, 2023; Schoenian, 2019). A rectal temperature exceeding 40.6 °C (105 °F) is a sign of danger, and exceeding 41.7 °C (107 °F) is a sign of a fatal condition (Schoenian, 2019).

7.4 Browsing and Energy Deficit in Goats

In Mediterranean forest rangeland, 64-90% of the goat's diet is shrubs and trees; on restricted rangeland the goat compensates by extending grazing time and increasing bite rate (Chebli et al., 2020). However, in the hot season walking time increases (approximately 36% in summer versus approximately 23% in spring), grazing time decreases (approximately 27% versus approximately 48%) and the energy balance turns to a deficit (approximately 24% deficit) (Chebli et al., 2022). Therefore, in the hot-dry period, provide goats with adequate browsing access and supplementary feeding when needed, and move grazing to the cool hours.

7.5 Breed, Coat and Colour

Hair sheep are more heat-tolerant than wooled sheep, and goats more than sheep; light-coloured and horned animals dissipate heat more efficiently (Schoenian, Maryland Small Ruminant). In hot regions, breed and coat selection is a management tool.

8. A Brief Note on Beef Cattle

In beef and feedlot cattle, heat stress can lead to increased morbidity and mortality associated with ambient temperature (Broadway et al., 2020). THI thresholds, shading and ration management in beef cattle are covered in detail in the articles Heat Stress in Cows (THI) and Dairy Cow Rations Under Heat Stress.

9. References