Advances in Horse Health and Management: Estimating Bodyweight and Grazing Legumes

Abstract

The role of the horse (Equus caballus L.) has evolved since it first appeared four million years ago (Hunt, 1995). According to a survey conducted by the United States Department of Agriculture (USDA), 45.7% of farms use horses for pleasure, 24.8% use horses for farm or ranch work, and 15.9% use horses for breeding (USDA, 2007). Within the sector of horses used as pleasure, workloads can vary drastically from minimal work (maintenance) to intense work (horses participating at the highest levels of competition). Within this range of workload, there are also horses described as hard keepers or easy keepers. The range of energy output of horses varies drastically; therefore, there is not a single ratio or feedstuff that applies to all horses. These different categories of horses have led to two different management problems; how to keep bodyweight (BW) off easy keepers and maintenance horses, and how to keep BW on hard keepers and performance horses. The objectives of the following studies were: 1. to determine the forage nutritive value, yield, and preference of legumes when grazed by adult horses and 2. to assess the accuracy of previously derived BW estimation equations, and if warranted, develop new BW estimation equations for adult draft and warmblood horse breeds using morphometric measurements. To determine objective 1, research was conducted in 2014 and 2015 in St. Paul, MN. Legumes were established as monocultures and in binary mixtures with cool-season grasses in a randomized complete block design with four replicates. Stands were established on May 16, 2014 and April 27, 2015. Adult horses grazed eight alfalfa (Medicago sativa L.) varieties, one red clover (Trifolium pratense L.), and one white clover (Trifolium repens L) when legumes reached the pre-bud stage. Legumes were measured for yield and samples to determine forage nutritive values were harvested prior to grazing. Plots were visually assessed for the percentage of forage removal on a scale of 0 to 100 to determine horse preference. White clover had the greatest amount of equine digestible energy (DE; 2.58 to 2.75 Mcal/kg) in monocultures and mixtures in 2014 and in monoculture in 2015. Digestible energy of all legumes exceeded equine DE requirements for adult horses at maintenance. In both years, alfalfa varieties yielded more compared to white clover (P < 0.0001). The top alfalfa variety yielded 17.4 and 12.9 Mt/ha in 2014 and 2015, respectively. In both years, horses had similar preference for all legumes and removed between 72 to 99% of available forage. This research helps to confirm that legumes are a nutrient dense, high yielding and preferred forage when grazed by adult horses. To determine objective 2, morphometric measurements were collected on adult (≥3 yr), non-pregnant draft (n = 138) and warmblood (n = 89) horse breeds at two separate shows in Minnesota in 2014. Trained personnel assessed body condition score (BCS) on a scale of 1 to 9, measured wither height at the third thoracic vertebra, body length from the point of the shoulder to the point of the buttock (BL wrap), body length from the point of the shoulder to a line perpendicular to the point of the buttock (BL straight), neck circumference at the midway point between the poll and the withers, and girth circumference at the third thoracic vertebra. Each horse was weighed using a portable livestock scale. Individuals were grouped into breed types using multivariate ANOVA analysis of morphometric measurements. Bodyweight estimations equations were developed using linear regression modeling. For estimated BW, the model was fit using all individuals and all morphometric measurements, except BL wrap. For ideal BW, the model was fit using individuals with a BCS of 5 and morphometric measurements not affected by adiposity; BL straight and height. Mean (± SD) BCS was 6.3 (± 0.9) and 5.2 (± 0.6) for draft and warmblood horses, respectively. BW (kg) was estimated by taking [girth (cm)1.528 x BL straight (cm)0.524 x height (cm)0.246 x neck (cm)0.261] / 1,181 (draft) or 1,209 (warmblood)] (R2 = 0.96; rMSE = 28 kg). This is an improvement over the previous BW estimation equation for light-breed horses, which utilized BL wrap and girth circumference to estimate BW (R2 = 0.94; rMSE = 34 kg). Ideal BW (kg) was estimated by [(4.92 x BL straight (cm)) + (4.64 x height (cm)) – 951 (draft) or 1,016 (WB)] (R2= 0.90 and rMSE = 33 kg). Morphometric measurements were successfully used to develop new and improved BW-related equations for draft and warmblood horses. The equations will assist draft and warmblood horse owners and professionals with managing horse BW, nutrition and health.