408 The effect of melatonin supplementation during mid-late gestation in dams grazing toxic, endophyte-infected tall fescue on stocker phase growth performance and feed efficiency of offspring.

Abstract Melatonin was supplemented to multiparous crossbred beef cows during mid-late gestation to investigate its influence on subsequent growth and feed efficiency of resulting calves within a stocker phase. Male calves were born to dams from 1 of 3 treatments: 1) non-toxic, novel, tall fescue pastures without melatonin (NE+/NM), 2) endophyte-infected, toxic, tall fescue pastures with melatonin (E+/M), and 3) toxic tall fescue pastures without melatonin (E+/NM). Dietary melatonin was supplemented (100 µg/kg BW) over a 70-d period (d 147 to d 217 ± 1 of gestation). Following the 70-d treatment period, pregnant cows remained as a single group on E+ free pastures through calf birth and weaning. At 53 d post-weaning, steers were housed on concrete in 2 pens in an open-sided barn and offered a total mixed ration (as fed 66% warm-season forage balage, 24% corn, 8% distillers’ grains and 2% additional ingredients to meet nutrient requirements) with ad libitum intake using C-Lock SmartFeed Pro controlled feeding units for a 30-d adaptation period. Following adaptation, steers (n = 43, initial BW = 353 ± 8 kg) were used in a 56-d feed intake and efficiency trial. Steers were weighed before feeding on two consecutive days at the beginning and end with an interim weight on d 28 (Period 1 = d 0 to 28, Period 2 = d 29 to 57), and daily feed intake was recorded for each steer. On d 30, TiO2 was incorporated into the diet (1.5 g TiO2 /kg of dry matter) and fecal samples were collected twice daily between d 35 and 42 to determine fecal output and calculate diet digestibility. All variables were analyzed using the MIXED procedure of SAS, with treatment as a fixed effect and pen as random. BW and average daily gain (ADG) were analyzed using repeated measures. During this 56-d trial there were no differences among treatments for calf BW (P ≥ 0.24), overall ADG (P ≥ 0.22; NE+/NM = 1.22 ± 0.05 kg/d, E+/M = 1.16 ± 0.05 kg/d, E+/NM = 1.28 ± 0.05 kg/d), or daily dry matter intake (DMI; P ≥ 0.40). Gain:feed ratios were not different for Period 1 or Period 2 (P ≥ 0.28); however, there was a tendency for calves that had the treatment of E+/NM to have the greatest overall gain:feed ratio (P = 0.09; NE+/NM = 0.138 ± 0.011, E+/M = 0.137 ± 0.012, E+/NM = 0.152 ± 0.011) for the 56-d period. There were no differences among treatments for fecal output (P = 0.65) or digestibility (P = 0.74). To conclude, melatonin supplementation in multiparous crossbred beef cows during mid-late gestation did not influence stocker calf growth performance measurements, feed efficiency, or diet digestibility within this 56-d trial.