Effects in pregnant beef heifers grazing fungus-infected tall fescue on birth weight, milk yield and calf growth

Cattle grazing certain pastures of tall fescue grass infected with endophytic fungi (fungus-infected fescue) exhibit reduced feed intake, depressed growth rate, reduced milk yield and lowered plasma prolactin (PRL). This study determined whether lowered plasma PRL in beef heifers resulting from consumption of fungus-infected fescue during gestation would be followed by reduced postpartum milk production or with reduced growth of their calves. To avoid confounding any negative effects on milk production with reduced feed intake that sometimes results from ingestion of toxic compounds contained in fungus-infected fescue, heifers were pastured on fescue only during gestation. Beginning on about 155 d of gestation, 55 pregnant beef heifers (about 370 kg) were removed from a corn silage diet and assigned randomly to 0.5 ha paddocks planted with varieties of tall fescue known to be fungus-infected: G1-307 and Kentucky-31 infected (KY-31I) or varieties of tall fescue known to have little or no fungus infection (fungus-free): Kentucky-31 (KY-31N), Kenhy or Johnstone. Within 6 h of calving, heifers and their calves were moved from the respective fescue paddocks to a drylot and fed corn silage. Milk production by the heifers was determined once every two weeks over a 10 week period by the weigh-suckle-weigh method. Birth weight of the calves whose dams grazed the two fungus-infected lines of fescue was lower (P less than .02) than that of calves from heifers on the three non-infected varieties of fescue. However, body weight gain over the 10 week test period was not lower for calves from dams that grazed fungus-infected fescue prepartum. Neither milk production at the first postpartum measurement nor mean milk production overall was reduced in heifers that had consumed infected fescue before parturition. During the portion of the study when heifers were pregnant, plasma prolactin was significantly lower (P less than .0001) for heifers on the fungus-infected fescue paddocks. After parturition when heifers consumed only corn silage, the level of prolactin was not different for either the lactating heifers or their calves. Results indicate that consumption of fungus-infected fescue from day 155 of gestation to parturition reduced both calf birth weight and plasma PRL in heifers, but did not reduce milk production of heifers or impede body weight gain of calves.