Effect of water restriction on equine behaviour and physiology.

This study tested how much limiting drinking water affects the bodies and behavior of pregnant mares. As water was reduced over three weeks, mares became measurably dehydrated, ate less, lost weight, and spent less time eating, though the restriction was not life- or pregnancy-threatening.

What the researchers asked and why it matters

• Question: At what level does restricting water cause physiological or behavioral signs of stress in pregnant mares?
• Rationale: Horses—especially pregnant mares—depend on consistent water intake for digestion, thermoregulation, and fetal health; management errors or resource limitations can reduce access to water.
• Goal: Identify thresholds at which water restriction starts to impair welfare, to inform husbandry and prevention of dehydration-related problems (e.g., reduced feed intake, weight loss, colic risk).

Study design and methods

• Subjects: Six healthy, pregnant mares.
• Diet: Nonlegume hay offered ad libitum throughout (no limit on forage availability).
• Water access schedule:
  • Baseline (implied by comparison): Ad libitum intake averaged 6.9 L per 100 kg bodyweight (bwt).
  • Week 1: 5 L/100 kg bwt.
  • Week 2: 4 L/100 kg bwt.
  • Week 3: 3 L/100 kg bwt (≈42% of ad libitum).
• Measurements:
  • Feed intake (hay kg/day) and bodyweight change.
  • Daily blood variables: osmolality, plasma protein, packed cell volume (PCV), cortisol, and aldosterone.
  • Behavior: Continuous 24 h/day video/observation for 4 of the 6 mares across the study.

Key quantitative results

• Water intake: Ad libitum 6.9 L/100 kg bwt; restriction to 3 L/100 kg bwt reduced intake to 42% of ad libitum.
• Feed intake: Hay consumption fell significantly from 12.9 ± 0.75 kg/day to 8.3 ± 0.54 kg/day as water was restricted.
• Bodyweight: Significant cumulative loss of 48.5 ± 8.3 kg over 3 weeks.
• Hydration markers:
  • Plasma osmolality rose significantly from 282 ± 0.7 to 293.3 ± 0.8 mosmol/kg, indicating dehydration.
  • Plasma protein and PCV did not change significantly (no marked hemoconcentration detected).
• Hormones:
  • Cortisol decreased from 8.1 to 6.4 ng/mL over 3 weeks.
  • Aldosterone decreased from 211.3 ± 74.2 to 92.5 ± 27.5 pg/mL by the end of week 1.
• Behavior:
  • Time spent eating declined significantly with increasing water restriction from 46 ± 3% to 30 ± 3% of the day.
  • No other significant behavioral changes reported.
• Authors’ conclusion: Restricting water to 4 L/100 kg bwt dehydrates pregnant mares and may reduce welfare, but under these conditions did not pose a life- or pregnancy-threatening risk.

Physiological interpretation

• Rising osmolality:
  • Indicates net water deficit relative to solute load—consistent with true dehydration as access declined.
  • Magnitude of increase (≈11 mosmol/kg) is modest but biologically meaningful, aligning with reduced intake and weight loss.
• Stable plasma protein and PCV:
  • Suggests limited hemoconcentration; fluid loss may have been modest, compensated by shifts between compartments, or buffered by reduced feed/solute intake.
  • Highlights that osmolality can detect dehydration even when protein/PCV remain unchanged.
• Cortisol decrease:
  • Contrary to a classic “stress” spike; may reflect adaptation, sampling-time/diurnal effects, pregnancy-related HPA axis modulation, or reduced metabolic activity with lower intake.
  • Implies cortisol alone is an unreliable welfare marker for this scenario.
• Aldosterone decrease:
  • Unexpected under water restriction, as aldosterone typically supports sodium and water retention.
  • Possible explanations include pregnancy endocrinology, osmotic influences, dietary sodium context, or study timing; without renin/ACTH data, mechanism remains uncertain.

Behavioral interpretation

• Reduced time eating (from ~11 h/day to ~7 h/day):
  • Matches the drop in hay intake and likely reflects drier ingesta being harder to chew/swallow without water and reduced drive to eat when dehydrated.
  • Less chewing and saliva may increase gastrointestinal risk (e.g., impaction), even though no acute crises were reported.
• Limited changes elsewhere:
  • No significant shifts in other recorded behaviors, possibly due to small sample (n=4 observed continuously) and compensatory strategies.

Welfare and pregnancy implications

• Dehydration at 4 L/100 kg bwt:
  • Physiologically detectable and associated with reduced feeding and weight loss—clear welfare concerns.
  • Under controlled conditions it was not acutely life- or pregnancy-threatening, but longer duration or added stressors (heat, exercise) could escalate risk.
• Energy balance and fetal considerations:
  • Lower intake and 48.5 kg weight loss over 3 weeks reflect negative energy balance, which can be undesirable in pregnancy.
  • Maintaining ad libitum water access helps sustain forage intake and stable body condition.

Practical takeaways and example calculations

• Do not intentionally restrict water for pregnant mares; aim for continuous access to clean, palatable water.
• Threshold warning: At or below 4 L/100 kg bwt (e.g., 20 L/day for a 500 kg mare), measurable dehydration and reduced welfare are likely.
• Typical needs:
  • Ad libitum average in this study ≈ 6.9 L/100 kg bwt (about 34.5 L/day for 500 kg), within common husbandry ranges.
  • Heat, exercise, lactation, salty feeds, and dry forages can increase needs well above averages.
• Monitoring cues:
  • Watch for reduced eating time/forage intake, unexpected weight loss, drier feces, and subtle performance changes.
  • Blood osmolality (if available) is sensitive; protein/PCV may miss early dehydration.

Strengths and limitations

• Strengths:
  • Stepwise restriction allows identification of dose-response effects.
  • Multiple indicators (intake, weight, blood metrics, behavior) provide a multidimensional view.
• Limitations:
  • Small sample (n=6; behavior n=4) limits generalizability and statistical power.
  • Pregnancy stage, environmental conditions, and sampling times (hormonal diurnal rhythms) may confound interpretations.
  • No direct measures of thirst behavior, fecal dryness, urine output/osmolality, or cardiovascular responses.
  • Short duration; longer-term impacts on pregnancy outcomes were not assessed.

Bottom line

• Restricting pregnant mares to 4 L/100 kg bwt or less causes detectable dehydration, reduced forage intake, and weight loss, compromising welfare even without acute crises.
• Consistent ad libitum water access is the prudent management standard, with heightened attention under heat, exercise, or high-fiber diets.