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Sea­son of birth can af­fect foal size; stress in­flu­ences per­for­mance

Ahorse’s me­tab­o­lism can slow down when the weather gets cold. It’s one way the horse’s body can re­align re­sources to stay warm. With mod­ern breed­ing prac­tices of­ten ma­nip­u­lat­ing mares’ nat­u­ral re­pro­duc­tive cy­cles to de­liver win­ter foals (born in Fe­bru­ary to early March), a group of re­searchers won­dered if that meta­bolic re­duc­tion could af­fect fe­tal devel­op­ment.

The re­searchers, led by Chris­tine Aurich, DVM, PhD, Dipl. ECAR, of the Univer­sity of Ve­teri­nary Medicine Vi­enna, and Elis­a­beth Beythien, Tier­arzt (a ve­teri­nar­ian from a Ger­man univer­sity) of the Graf Lehn­dorff-In­sti­tute for Equine Sci­ence, stud­ied 27 brood­mares and their foals. All the mares had sim­i­lar di­ets and a sim­i­lar num­ber of pre­vi­ous foal­ings. Ges­ta­tion length and the sex ra­tio of foals did not dif­fer.

The mare–foal pairs were grouped by foal­ing date: Group 1 foals were born be­tween Fe­bru­ary and early March; Group 2 be­tween early March and early April; and Group 3 from mid-April to May.

The re­searchers mea­sured the size and weight of each mare’s pla­centa at birth. The mea­sure­ments were smaller in the win­ter­foal­ing mares than mares de­liv­er­ing later. This in­di­cates a re­duc­tion in nu­tri­ent trans­fer to the fe­tus, which could in­flu­ence fe­tal growth.

A num­ber of size pa­ram­e­ters were also recorded for each foal weekly from birth to 12 weeks of age. Re­searchers found that most pa­ram­e­ters—in­clud­ing height at with­ers, dis­tance from fet­lock to carpal joint and length from poll to nose— were lower in the ear­li­est foal­ing group com­pared to the other groups. Foal weight did not dif­fer among the groups.

“The size dif­fer­ence of the early-born foals was present over the whole ob­ser­va­tion pe­riod, i.e., un­til 12 weeks of age,” says Dr. Beythien. Af­ter that time, the mares and foals were sent out to pas­ture and could not be eas­ily mea­sured.

De­spite the ini­tial size dis­par­ity, Dr. Beythien notes that early­born foals will be taller and larger than later-born foals dur­ing the first few months of life sim­ply due to the dif­fer­ence in age. “If you imag­ine a foal pre­sen­ta­tion sched­uled in June, foals born in March will be taller than foals born in May be­cause they are ap­prox­i­mately eight weeks older at this time,” she ex­plains.

For this rea­son, breed­ers of­ten feel early-born foals have an ad­van­tage in young-horse com­pe­ti­tions at 3 to 4 years of age. Ul­ti­mately, though, breed­ing mares too early in the year could be detri­men­tal to the foals’ ma­ture size, says Dr. Beythien. Based on other stud­ies, she adds, the lower en­ergy sup­plied to the win­ter foals dur­ing ges­ta­tion may also con­trib­ute to health prob­lems dur­ing adult life. There­fore, the team con­cluded, breed­ers may be bet­ter off aim­ing for foals to be born dur­ing the nat­u­ral foal­ing sea­son (typ­i­cally be­tween April and Septem­ber) when they’ll “ex­pe­ri­ence a health­ier en­vi­ron­ment dur­ing ges­ta­tion,” says Dr. Beythien.

Yes, Stress In­flu­ences Per­for­mance

You may have won­dered whether stress can re­ally de­tract from your horse’s per­for­mance. Now there’s sci­en­tific proof that it can, ac­cord­ing to a study con­ducted by Alek­san­dra Gorecka-Bruzda, PhD, DSc, and col­leagues at the In­sti­tute of Ge­net­ics and An­i­mal Breed­ing of the Pol­ish Academy of Sciences.

The sci­en­tists set out to ob­jec­tively de­ter­mine if there was a re­la­tion­ship be­tween sporthorse per­for­mance and be­hav­ior and phys­i­o­log­i­cal stress in­di­ca­tors. The study in­cluded a test group of 19 equine show-jump­ing com­peti­tors, six of whom were housed at the in­sti­tute and 13 of whom were trans­ported. An­other five horses re­mained in their home train­ing rou­tine and served as con­trols.

The test horses were grouped ac­cord­ing to jump height: “light,” with ob­sta­cles equal to or less than 100 cm (about 3-foot-3) and “dif­fi­cult,” with ob­sta­cles over 100 cm. The re­searchers mea­sured sali­vary cor­ti­sol con­cen­tra­tion, a phys­i­o­log­i­cal in­di­ca­tor of stress, be­fore a first jump­ing round as well as 20 min­utes and one hour af­ter a sec­ond jump­ing round.

For both rounds, they also eval­u­ated con­flict be­hav­iors—signs of re­sis­tance in-

di­cat­ing men­tal or phys­i­cal dis­com­fort. These in­cluded head­shak­ing, yank­ing on the reins and tail-swish­ing. Faults, i.e. re­fusals and knock­downs, of each horse were to­taled for both rounds and the an­i­mals were clas­si­fied in “less faults” class if they made less than or equal to 1 (me­dian value) fault, or as “more faults” if they ex­ceeded this value (more than one fault).

The re­searchers found that sali­vary cor­ti­sol con­cen­tra­tion at the 20-minute post-jump­ing mark was higher in the “more faults” group and among horses who had been trans­ported. Con­flict be­hav­ior was ob­served more fre­quently in horses with more faults and among those jump­ing the taller fences. Also, horses who waited longer to jump their sec­ond round showed more con­flict be­hav­iors.

In short, horses show­ing more stress re­sponses were less suc­cess­ful per­form­ers. Thus, the team con­cluded, find­ing more ef­fec­tive ways to re­duce the stress of trans­port and com­pe­ti­tion it­self could both im­prove your horse’s wel­fare and his com­pet­i­tive per­for­mance. —— Sushil Du­lai Wen­holz

Re­searchers have found that win­ter foals (those born in Fe­bru­ary to early March) tend to be smaller at birth than those born dur­ing the nat­u­ral foal­ing sea­son (be­tween April and Septem­ber).

In a re­cent study, sci­en­tists mea­sured signs of re­sis­tance in horses, in­clud­ing head­shak­ing, yank­ing on the reins and tail-swish­ing, to de­ter­mine the ex­tent that stress has on their per­for­mance.

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