The stress response of foals during weaning was examined in terms of a behavioral protocol and the responses of plasma ascorbate, serum cortisol, and the serum cortisol response to an ACTH challenge. The experimental plan was a 2 x 2 factorial of two preweaning diets and two stall weaning methods. Dietary groups included foals raised on pasture supplemented with hay and a pelleted concentrate (PHC) and foals raised on pasture supplemented with hay only (PH). Stall weaning methods included foals placed in stalls singly or in pairs. Sex influences were also examined. The foals exhibited characteristic behavioral and physiological responses to weaning stress. Behavior scores indicated fewer outward signs of stress in single than in paired foals (P = .008) and tended to indicate fewer signs of stress in PHC than in PH foals (P = .15). No differences in plasma ascorbate concentrations were found among treatments. Responses of serum cortisol to an ACTH challenge were lower (representing adrenal depletion arising from stress) in PH than in PHC foals (P = .001) and in paired than in single foals (P = .058). Behavior scores were positively correlated with the response of serum cortisol to ACTH. Both behavioral data and the ACTH response indicated that foals may cope better with weaning when supplemented with concentrate before weaning or when placed singly in stalls rather than in pairs.

Increases in plasma H+ and K+ concentrations in this endurance ride were moderate and unlikely to contribute to signs of muscle fatigue and hyperexcitability in horses.

The objectives of these studies were to compare effects of diet and weaning method on stress responses in Thoroughbreds raised on pasture. Ethograms showed that foals weaned abruptly were more stressed than foals weaned gradually, and foals supplemented with the fatand-fiber diet were less stressed. Cortisol levels in foals supplemented with a corn-based diet were higher before and after weaning compared to foals fed a fat-and-fiber supplement. These studies indicate that foals weaned gradually were less stressed and that diet may afiect a toals abrlity ro handle the stress of weaning.

Forty-five Thoroughbred mares used in an 8-mo depletion study were kept for an additional 20 mo on the same three forage diets (15 mares each): 2-yr-old orchardgrass hay and vitamin A-free concentrate on a drylot (HC); pasture, orchardgrass/alfalfa hay, and vitamin A-free concentrate (PHC); or pasture and orchardgrass/alfalfa hay only (PH). Each diet group was divided into three subgroups, and mares (n = 5) in each group were given either retinyl palmitate (A) at twice the NRC (1989) recommended daily intake, the equivalent amount of vitamin A in the form of water-dispersible beta-carotene (B), or the vehicle (C). Vitamin A status was monitored with serum retinol and a relative dose response (RDR) test every 60 d. In the C subgroups, retinol concentration was 18.65 +/- .84 micrograms/dL (mean +/- SE) and the RDR was 16.26 +/- 1.72% over the 20 mo. Retinol and RDR fluctuated seasonally regardless of supplementation. Vitamin A status, based on serum retinol (P = .001) and RDR (P < .001) values, was lower in the HC than in the PH and PHC. Vitamin A status, based on retinol (P = .05) and RDR (P = .013) values, was improved by retinyl palmitate supplementation in all diet groups, but not by water-dispersible beta-carotene supplementation. Supplementation of the HC mares with vitamin A matched the serum retinol, but not the RDR, of the two pasture, control subgroups. Thus, replete vitamin A status in previously depleted mares was barely obtained by supplementation with twice the currently recommended daily intake of vitamin A.

Some of the clinical signs seen in horses during endurance races may result from increases in neuromuscular excitability and are related to plasma [K þ ] and [Ca þ þ ]. The present study aimed to test the following hypotheses:(1) Potassium supplementation will affect plasma [K þ ] and may result in clinical signs related to neuromuscular hyperexcitability during an 80 km endurance ride.(2) Plasma [Ca þ þ ] will reflect dietary cation-anion balance (DCAB) and calcium intake. Feeding with a high DCAB and high dietary calcium content (1.5% total calcium of daily ration) diets would lead to higher plasma [Ca þ þ ] during an endurance race than on feeding high DCAB diets with a moderate dietary calcium content (1% of total calcium of daily ration). The current study was undertaken during the 80 km endurance research ride in 2002 in Virginia, USA. Forty volunteer rider-horse pairs participated in the race. During the race, electrolyte mixtures with (EM þ K) and without (EM 2 K) potassium were supplied to 18 and 22 horses, respectively. After the race, the horses receiving EM 2 K during the race were supplied with a recovery formula containing potassium (EM-REC). The horses were fed in addition to their own forage (hay and pasture) either their own commercial concentrate (CC; 1% calcium, n ¼ 11) or one of two research-supplied concentrates during 3 months preceding the research ride, one concentrate rich in sugar and starch (SS; 2% calcium, n ¼ 15) and the other rich in fat and fibre (FF; 2% calcium, n ¼ 14). Peripheral blood samples were taken the day before, within 3 min of the arrival at the vet checks at 27, 48 and 80 km, and after 3 h of recovery. Plasma samples were analysed for pH, haematocrit (Hct), [Na þ ], [K þ ], [Cl 2 ], [Ca þ þ ], [Mg þ þ ], total protein (TP) and albumin [alb]. Effects of sampling times, treatments and interactions were evaluated by ANOVA in a mixed model with repeated measures and applied to the 25 horses that completed 80 km. Eliminated horses had their blood sampled before entering the elimination vet check and 3 h after elimination, and were compared with finishing horses by t-test. As the ride progressed, significant increases were found in plasma pH, [Na þ ], ½PO 2 4 , [TP], [alb], Hct and osmolality; and decreases in [K þ ], [Mg þ þ ], PCO 2 , [Ca þ þ ] and [Cl 2 ]. Horses supplied with potassium-free, sodium-rich electrolyte formulae (EM 2 K) had 12.5% lower (P ¼ 0.001) mean plasma [K þ ], 7.8% lower (P ¼ 0.024) TP and 8.4% lower (P ¼ 0.004) albumin at 80 km, and at 3 h after the race they had 6.8% lower (P ¼ 0.045) TP, when compared with EM þ K supplemented horses. Horses fed with SS and FF had higher [Ca þ þ ] at 27 (P ¼ 0.027), 56 (P ¼ 0.006) and 80 km (P ¼ 0.022) when compared with horses fed with CC. The lower [K þ ] in the EM 2 K group, and the higher [Ca þ þ ] in the SS-and FF-supplemented horses may help prevent increases in neuromuscular excitability and related clinical signs. The lower TP and albumin indicate less dehydration in the EM 2 K group and could help prevent re...