Growth and glucose/insulin responses of draft cross weanlings fed Total Mixed Ration cubes versus hay/concentrate rations.

Sarah Ralston, Harlan Anderson, Roy Johnson

Rutgers, the State University of New Jersey, New Brunswick, NJ;  IdleAcres, Cokato, MN, USA; Minnetonka, MN

Introduction

Total mixed rations (TMR), wherein all the nutritional needs of the animals are met in a single feedstuff that is available ad libitum, have been used successfully for decades in growing food animals. However, in the equine industry, the traditional regimen is to provide weanlings concentrates separately and forage in limited amounts, resulting in high starch and sugar (NSC), low fiber meals that cause significant fluctuations in plasma glucose and insulin, which has been hypothesized to adversely impact growth. Increased insulin resistance (IR) has been documented in young horses fed high starch/sugar feeds (NSC = 20% or higher) and has been correlated with an increased incidence of developmental orthopedic disease (DOD) such as osteochondrosis (OCD), epiphysitis and flexure deformities.

It was hypothesized that rations with low NSC (<20), either as TMR cubes fed free choice or a meal fed concentrate with restricted NSC, would reduce IR and incidence of DOD in weanling horses while sustaining rapid growth rates in draft cross weanlings.

Materials and Methods

To test the hypotheses, growth rates, feed efficiency, insulin sensitivity, glucose/insulin responses to the feeds and incidence of DOD in draft cross weanlings were evaluated in a series of three trials conducted in three consecutive years (2004-2006). Each year 12 draft cross weanlings were fed either TMR cubes (Next Generation©, IdleAcres, Cokato, MN) free choice (TMR, n=6 per year) or hay/concentrate based rations of Nutrena ®(Minnetonka, MN) Life Design ®Youth ® (HS:2004, 2005, n=6 each year) or Nutrena ®(Minnetonka, MN) Safe Choice® (LS:2006, n=6) to provide 50% of the calories recommended for moderate growth with free choice grass/alfalfa hay for 6 weeks. In 2004 all weanlings were QuarterHorse/Belgian crosses. In 2005 and 2006 three and six of the weanlings, respectively, were more refined Hanoverian/QH/Percheron/TB crosses. Horses were fed in individual stalls overnight and turned out in dry lot paddock 0830-1600h daily. Orts were recorded daily. Horses were weighed and had wither and rump heights recorded weekly. Limbs were visually assessed for epiphysitis and flexure deformities, rated on a scale of 0 (no lesions) to 4 (severe lesions), with scores of 2 or higher considered to be physiologically significant. Radiographs were taken to confirm presence or absence of OCD in hocks and stifles. Insulin sensitivity was assessed with a low dose oral dextrose challenge (LDOD: 0.25 gm dextrose/kg BW) before treatments were initiated and after 6 weeks on treatments (PostTX). Glucose/insulin responses to equicaloric amounts of TMR and the concentrates were measured PostTX in all years. Glucose/insulin data were compared within and between years by ANOVA for repeated measures factoring effects of treatment, individual and year where appropriate (Statistixs for Windows, Analytical software). Student t-tests were used to compare feed efficiency (kg gain/Mcal consumed) and average daily gain (kg/day) between treatments within trials. Significance was set at P<0.05.

Results

             Nutrient content of the rations differed between and within years (Table 1). All weanlings maintained good general health in all years. No DOD>2 was observed in 2004. In 2005 three horses were IR relative to the others (Insulin responses to LDOD >25 µIU/ml), two of which had DOD>2 (flexure deformities and epiphysitis) before the treatments were initiated.  One DOD weanling was placed on TMR (flexure deformity scored 4), the other (epiphysitis 4 and flexure deformity 2) on HC. The weanling fed TMR had a DOD score of 1 within 2 weeks and 0 by te end of the trial, the one fed HC had persistant epiphysitis (scored 4) and flexure deformities (scored 2) throughout the trial. In 2006 two horses were IR, one of which had DOD>2 (Flexure deformity scored 4) that did not change when placed on TMR ration. The other IR weanling had no visual or radiographic lesions. Another weanling fed TMR that was not IR had OCD=4 in the right stifle which was diagnosed at the end of the trial, though it was not evident in the beginning of the study. All others had no significant lesions.

Glucose responses to the PostTX LDOD did not differ between treatments in the first two years but insulin responses tended (P<0.1) to be higher in HS fed horses, suggesting reduced insulin sensitivity though not statistically significant. Glucose/insulin responses to meals of HS were higher (P<0.05) than to TMR or LS. In 2006 TMR fed horses had higher (P<0.05) glucose and insulin responses than those fed LS but both tended (P<0.1) to be lower than in previous years.

In 2004 and 2005 horses fed TMR had higher (P<0.05) percent BW average daily gains (%BWADG) and feed efficiency than those on HS (Table 2), though wither and rump heights did not differ (P<0.05) between treatments. Average daily gain and feed efficiency were not as different between rations (P<0.1) in 2006.

    Discussion       

        Though the incidence of DOD was low, the data do suggest that horses with IR do not necessarily develop DOD. Genetic predisposition to DOD has been documented in several breeds, which may or may not be correlated with IR. It is of interest to note that three of the DOD horses were refined TB/ Hanoverian/Percheron crosses, the 4^th was a very refined ¾ Paint/Belgian cross whereas the non-DOD IR horses were QH/Belgian crosses from bloodlines used in 2004.

The TMR fed horses were consistently more efficient than horses on HG, consuming fewer calories per kg gain in all three years. There also was a tendency (P<0.1) toward higher ADG in the TMR fed horses which became significant when corrected for BW in 2005. It is of interest to note that the draft cross weanlings, regardless of dietary treatment, voluntarily consumed <90% of NRC(1989) recommended calories for moderate growth (>600kg mature weight) but sustained growth rates 90-143% of the 0.8kg/day predicted. In 2005 the quality of the hay was only moderate and the differences between treatment groups were greater than in 2004 and 2006. The lack of difference between rations in 2006 was probably due to the overall similarity of nutrient intake. It is interesting to note that %BWADG was lower and feed efficiency tended (P<0.01) to be lower in the years that more horses with a lower percent of draft blood were used (2005, 2006).

Both rations in 2005 and the TMR in 2006 had marginally lower phosphorus (0.34-0.37% DM) than recommended for growth (0.40% DM) but the apparent deficits were not clearly associated with the presence of DOD or reduced growth rates. Current recommendations may be in excess of actual needs.

Conclusions

Feeding TMR cubes formulated for growth free choice is an efficient alternative to traditional hay/high NSC concentrate rations under the conditions of these trials. However no clear benefit was seen if a low NSC concentrate was used with good quality hay as a comparison. The 1989 NRC caloric recommendations for growth may be in excess of the needs of draft cross weanlings. The responses seen in 2005 suggest that placing IR horses with flexure deformities and/or epiphysitis on low NSC rations may help to resolve the problems, but further research is necessary. Restriction of NSC will not prevent or resolve all cases of DOD and not all IR weanlings will develop DOD if fed rations formulated for growth.

Table 1: Nutrient Intake-100% DM basis of feed used in the 3 trials

Equine Science Center