Factors Affecting Foal Birth Weight in Thoroughbred Horses C

Available online at www.sciencedirect.com Theriogenology 71 (2009) 683–689 www.theriojournal.com Factors affecting foal birth weight in Thoroughbred horses C. Elliott a,*, J. Morton b, J. Chopin c a Main Ridge Veterinary Clinic, 334 Main Creek Road, Main Ridge, Victoria 3928, Australia b School of Veterinary Science, The University of Queensland, St Lucia, Queensland 4072, Australia c Coolmore Australia, Denman Road, Jerrys Plains, New South Wales 2330, Australia Received 6 May 2008; received in revised form 24 August 2008; accepted 7 September 2008 Abstract Foaling data from 348 Thoroughbred foals born on a commercial stud were analysed to investigate interrelationships among mare age, parity, gestation length, foal sex, placental weight, and foal birth weight. Placental weight was positively correlated with foal birth weight up to a threshold of 6.5 kg; above this, placental weight was not significantly associated with foal birth weight. Placental weight was assessed, including the amniotic membranes and umbilical cord as well as the allantochorion. Using path analysis, parity was positively associated with foal birth weight both directly and through increased placental weights, but age was not directly related to foal birth weight. Over the range of gestation lengths observed, gestation length was not significantly associated with foal birth weight. We conclude that, in populations represented by this study population, either placental weights up to 6.5 kg are rate-limiting for foal birth weight or placental weight increases with foal birth weight up to this threshold. However, further increases in placental weight are not associated with additional increases in foal birth weight. The positive association between parity and foal birth weight is mediated through increased placental weight as well as other pathways. Age is not directly related to foal birth weight and gestation length is not strongly associated with foal birth weight. # 2009 Elsevier Inc. All rights reserved. Keywords: Foal; Birth weight; Mare parity; Gestation; Placenta 1. Introduction The general industry perception is that larger yearlings usually sell for higher prices than smaller yearlings, Foal birth weights are perceived to be important because of expected superior racing performance. within the commercial Thoroughbred industry of Withers height and hip height of yearlings are positively Australia, as foal birth weight is commonly thought to correlated with lifetime earnings, Standard Starts Index be positively associated with size as yearlings. Although and winning percentage [2] and body length and heart the relationship between foal birth weight and birth girth of yearlings are positively correlated with Standard height has not been described, Reed and Dunn [1] Starts Index and winning percentage [2]. reported a significant positive correlation between birth Placental and foal birth weights are routinely recorded height and mature height and concluded that birth height on many stud farms. However, the relationship between could be used as an accurate predictor of mature height. these two measurements has not been investigated in Australian Thoroughbred horses. Positive linear relationships between placental weight and foal birth weight * Corresponding author. Tel.: +61 3 5989 6232; have been reported in horse populations from the United fax: +61 3 5989 6366. Kingdom and the USA [3–6]. Both age and parity have E-mail address: [email protected] (C. Elliott). been positively correlated with both foal birth weight and 0093-691X/$ – see front matter # 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.theriogenology.2008.09.041 684 C. Elliott et al. / Theriogenology 71 (2009) 683–689 placental weight [3–8]. However, the effects of gestation New South Wales, Australia (328300 latitude; 1508500 length on foal birth weight and placental weight have longitude approximately) during the 2006 breeding received little attention. In a study conducted in southern season (August–December). All foals that were born Brazil, there was no relationship between gestation alive and did not die within 48 h after birth during the length and foal birth weight [9]. Other than this report, all 2006 breeding season were enrolled. This resulted in a other studies were conducted in the northern hemisphere. total of 348 foals and their respective mares being Due to differences in climate, rainfall, seasonality and included in the study. Long-term average annual rainfall management procedures, the applicability of these for this region of Australia was 639.4 mm, with monthly relationships to Australian Thoroughbreds requires averages for the breeding season ranging from 36.6 mm investigation. in August to 67.5 mm in December. Average maximum Most studies assessing placental and birth weights daily temperatures for the breeding season ranged from have used univariable analyses [3–12]. Results of 19.4 8C in August to 31.4 8C in December. Average univariable analyses can be biased due to confounding. minimum daily temperatures for the breeding season Multivariable techniques, such as stratification and ranged from 4.4 8C in August to 15.7 8C in December. multivariable regression, can be used to control for Photoperiod for the breeding season ranged from an confounding [13]. Control of confounding is essential average of 7.3 h of sunshine daily in August, to an for correct understanding of determinants of placental average of 8.7 h of sunshine daily in December [16]. weight and foal birth weight. Path analysis, an extension Each foaling was supervised by an experienced of multivariable regression, could also result in better attendant. Foals were allowed to suckle and to remain understanding of the likely complex interrelationships with their dam for at least 2 h to allow bonding and were between mare age, parity, gestation length, foal sex and weighed when between 2 and 12 h of age. The foal was placental weight and foal birth weight. This technique held by the foaling attendant whilst standing on a set of has not been previously applied in this area but has scales (Tru-Test scales, Model 702, Tru-Test Limited, proven useful for understanding causation of multi- Auckland, New Zealand). This gave a combined weight, factorial conditions such as lameness [14] and from which the attendant’s weight was subtracted reproductive events [15] in dairy herds. giving the actual foal birth weight, which was recorded The most widely used method for determining to the nearest kilogram. All placentas were retrieved by placental weight within the Australian Thoroughbred the foaling attendant at the completion of Stage 3 of industry involves weighing the entire placenta, along labour. The entire placenta including the umbilical cord with the amnion and umbilical cord. This differs from and amnion was placed into a bucket and weighed to the the accepted research method of removing the amnion nearest half kilogram. The placental weight was then and umbilical cord prior to weighing, i.e. only weighing calculated by subtracting the weight of the bucket. the allantochorion. Most studies assessing placental Data collected about each mare included service weight have used the latter method [3–6,11,12]. The date, foaling date, mare age at foaling and parity at nature of the association between placental weight and foaling (when mares produced their first foal during the birth weight may differ between the two methods of study period, this was recorded as Parity 1). Gestation assessing placental weight. If the relationship between length for each foaling was calculated in days as foaling placental weight and birth weight was described using date minus the last recorded service date. Service dates the standard industry method of weighing placentas, were confirmed against the official records of the stud veterinarians may be better able to relate the results Australian Stud book, as were mare age and parity. of research studies to field observations. Descriptive statistics were calculated using Microsoft The aims of the current study were to investigate the Excel 2003 Office edition (Microsoft Corporation, interrelationship between placental weight and foal Redmond, WA, USA). Putative associations were then birth weight, and to investigate interrelationships assessed using univariable and multivariable linear among mare age, parity, gestation length, foal sex regression using Stata version 9.2 (StataCorp, College and placental weight and foal birth weight in horses at a Station, TX, USA). For path analysis, a null hypothesis commercial Thoroughbred stud in Australia. model was developed (Fig. 1) based on previous research results [3–12] and plausible temporal sequences of 2. Materials and methods events. Foal sex was not included in the null model, as sex was not significantly associated with either foal birth A retrospective cohort study was conducted on one weight or placental weight in the univariable analyses. large Thoroughbred stud located in the Hunter Valley of Each variable in the null hypothesis model was, in turn, C. Elliott et al. / Theriogenology 71 (2009) 683–689 685 birth weight nor placental weight were significantly associated with foal sex on univariable analysis (P = 0.166 and 0.559, respectively). 3.2. Univariable relationship between placental weight and foal birth weight The relationship between placental weight and foal Fig. 1. Null hypothesis of path model for interrelationships among birth weight is shown (Fig. 2). There was a strong mare age, parity, gestation length, placental weight,

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