Feeding for Growth

• issues arise as combination of multiple factors = causal componenets • most issues require multiple components to interact • Nutrition is one of those components in conditions such as DOD ~ Osteochondritis dissecans (OCD) = thickened articular cartilage and reduce blood supply to base layer of bone = causes cracking and flaking of bone which can break off or flap = inflammation of joint = lameness +/- arthritis ~ Epiphysitis = inflammation of the growth plate (physis), most commonly in fetlock joint ~ Bone cysts ~ Juvenile arthritis ~ Wobbler syndrome (Cervical Vertebrae Stenotic Myelopathy - CVSM) • other components = genetics, exercise, hormones, growth, conformation • rapid growth associated with DOD, want to grow slowly = grow for profit but lead to skeletal issues, hard to sale

= rapid growth ~ Genetic disposition, conformation and environment increase risk - DODs occur due to abnormalities of endochondral ossification (replacement of cartilage within bone) of the epiphyseal growth plates = dont grow properly ~ Alterations in bone mineralisation ~ Cartilage becomes thickened ~ Blood supply altered ~ Inflammation / necrosis of the tissues - rapid and varied growth rates predispose to DOD

• High energy diets are most commonly associated with DODs, not excess protein • Research is conflicting – high starch and fat diets • One theory is high circulating insulin may affect chondrocyte maturation ~ Chondrocytes = cartilage cells which forma matrix. • Alters matrix and cartilage growth, and mineralisation of bones • Nutrition of broodmare how lead to decreased insulin sensitivity = could be due to that rather than what foal is being fed • Rapid growth leads to skeleton growing more rapidly than other nutrients are able to match • Affected youngsters should have energy supply reduced whilst maintaining all other nutrients

• mineral imbalences associated with DOD • need to ensure correct ratio given ~ Ca:P = 2:1 (range 1.5:1 and 2.5:1) ~ Cu:Zn = 1.3 to 1.4 • Feed fortified feed or vitamin and mineral supplement or a balancer • Weigh and condition score foal/youngster regularly • Movement is needed for normal musculoskeletal development

• DE – month 5 onwards • CP – month 5 onwards • Vitamins A & E – raised throughout pregnancy • Ca & P – small ↑ months 7-8, larger ↑ months 9-11 • Cu & I – months 9-11 • Fe, Mn, Zn, Se – no increases

• 80% of placental growth first third of preg - 80% foetal growth and wt gain in final third of preg

• Nutrition of the foal begins before birth, making nutrition of the mare a priority • Early pregnancy placental growth takes place • Final 3-4 months of pregnancy the majority of foetal weight gain takes place

• BW and BCS • most broodmares managed on pasture to satisfy DE/CP requirements but need vit/mineral source to meet maintenance requirements ~ as preg develops = add conc feeds fortified with vit/min or balancer which provides vit/min ~ Excess fat stores increase demand on joints and body systems and may increase difficulty with labour ~ Inadequate fat stores due to inadequate DEI to support foetal growth and maintain mare = mare utilising internal energy stores to support foetal growth ~ In late pregnancy vDMI may reduce due to less capacity in the GIT due to foetal growth = increase energy density of feed

• Moderate (80% of DE requirement) – no effect in utero or post-natal growth of foal ~ Placenta adapts to compensate = vascularisation and changes in amino acid and vitamin metabolism • Severe energy deficit affects in utero growth of foal and reduces birthweight • following effects are reported compared to foals born to mares receiving adequate energy ~ Delayed testicular maturation at 12 months old ~ Decrease in insulin sensitivity at 19 months old ~ Decrease in cannon bone width at 19-24 months old

• Less clear understanding of effects of excess energy • Obesity during early gestation has strong effects on embryo development due to changes in the uterus Increased inflammatory markers and influence on lipid regulation • Obesity in dams did not affect foal birthweight but the following effects reported compared to foals born to mares receiving appropriate energy ~ Increased systemic inflammation in foals until 6 months old ~ Decrease in insulin sensitivity at 6 and 18 months old (more likely insulin resistant) ~ Increase in development of osteochondrosis lesions

• growth = increase in size • optimum growth = increase in size of the horse from birth that allows for maximum health and performance at designated stage of life • influenced by: ~ Genetics – growth pattern and overall size and shape of horse ~ Environment – nutrients available, climate, competition for food (short term) ~ Nutrition – impacts on genetically determined long-term growth pattern (short term) • most foals managed outside = season impacts nutrients and weather conditions • once weaned, compertition with field mates can influence feed intake (dominant more, submissive less)

• Growth occurs through increases in the size of cells and the number of cells • Differences in size between mature horses due to differences in the total number of cells • Growth rate decreases as horses approach maturity, mainly due to decrease in cell proliferation (cell division) • Horses classed as mature when 5-6 years ~ BW ~ Wither height ~ Body length ~ Metacarpal circumference

= range of breeds and sizes - no breed standards = standard growth curves challenging - Gro-Trac software tracking growth of TB foals accross world = allows comparisons - classed as rapid moderate or slow - subjective = diff management systems, enviro etc ~ 2 month old rapid = 1.2kg ADG, moderate = 1.0kg ADG ~ 6 month old = 0.9kg ADG rapid, 0.8kg ADG moderate ~ ADG reduces as get older - body wt overall perameter but no account of differences in tissue development ~ bone then muscle then fat

• The diet of the growing horse comprises of ~ colostrum, ~ milk, ~ water, ~ forage ~ concentrated feeds • The proportion of each component changes with the horses age and its circumstances • Diet still needs to provide all of the nutrient groups CHOs, proteins, fats, vitamins, minerals and energy

• Energy is the first limiting factor influencing growth rate • Excess energy only optimises growth rate to its genetic potential • Need other nutrients to be supplied in adequate quantities • Energy above requirements = stored as fat and is associated with developmental issues • Continue to calculate energy requirements using DEm plus DEg ~ Most accurate for 0-18 months of age, can be used up to 24 months old • When very young the proportion of the DE required for growth reduces as growth rate slows • Initially energy provided by milk, then pasture/forage intake increases ~ Energy provided from VFAs • Upon weaning VFAs should provide majority of energy ~ Some also from concentrates • research shows TB foals fed fibre based feed at weaning had similar growth rates to foals fed cereal-based stud feed ~ Also supported GIT health compared to cereal-based feed (Moore-Colyer et al. (2020))

• Continue to calculate CP requirements using CPm plus CPg • Research on utilisation of dietary protein for gain in foals <4 months old and above 12 months of age is extremely limited • Consider protein quality as well as total CPI ~ Quality is based on essential amino acid content of feed • Lack of essential amino acids in the diet can decrease growth rate • Supplementation with free lysine and threonine = rapid growth rate if energy is not deficient • ensure that other nutrients are being fed at this rapid growth rate also ~ Lysine is first limiting amino acid ~ Threonine is second limiting amino acid • Feeds high in lysine and/ or threonine: ~ Soya bean meal ~ Brewer’s yeast ~ Alfalfa • Recommended feeds 13-18% CP • Cereal grains low in essential amino acids = low quality protein

• requirements of vits/min based on calcs using ADG ~ rapid growth youngster = higher req than same wt slow growing youngster • appropriate intake = critical for optimum growth • main considerations = Ca, P, Mg, Cu, Zn and Mn

• Ca and P are vital components of bone and cartilage along with Mg ~ 99% Ca, 80% P and 62% Mg stored in bones ~ Ca:P compete for absorption in the small intestine • Ideal ratio for growing horses is 2:1 ~ Even if the diet contains enough Ca, if ratio of P is higher then Ca absorption is decreased • Causes Ca to be resorbed from bones into the blood to maintain plasma Ca homeostasis ~ Skeletal and joint issues develop due to weakening of the skeletal structures • Cereal grains are high in P and low in Ca • Forage high in Ca and low in P

• important roles ~ Mg – bone structure, muscle contraction, enzymes, increases Ca absorption ~ Cu – collagen and elastin in connective tissue (bone & cartilage), nerve conduction, haemaglobin production, competes with Zn for absorption ~ Zn – enzymes for skeletal metabolism, competes with Cu for absorption ~ Mn – formation of chondroitin sulfate and cartilage, can reduce P absorption Cu:Zn ratio ideally 1:3 or 4 • consider vitamins – usually sufficient supply in fresh pasture ~ Levels reduce considerably once cut and preserved ~ Horse’s seem less sensitive to vitamin levels when growing = less affected by reduced vitimins, minerals very affected if below needed ~ B vitamins produced by microbial fermentation in hindgut