Biological effects of plant extracts containing essential oils and polyphenols have been extensively studied for humans and nonruminant production animals, but in vivo studies with ruminants in field conditions are relatively scarce. This work aimed to investigate the effect of addition of Origanum vulgare or Camellia sinensis extracts upon average daily weight gain (ADG) of dairy heifers. Two trials were conducted for 70 d to test the levels of Origanum vulgare (0, 2.5, 5, 7.5 g/heifer/day) and Camellia sinensis (0, 1, 2, 3 g/heifer/day) extracts. Trial 1 used 32 Holstein heifers with initial overall BW 442.1 ± 96 kg and trial 2 used 34 heifers: 11 Jersey and 23 Holstein, with initial overall BW 305 ± 42 kg. Heifers grazed range pasture (trial 1) and Tifton -Cynodon dactylon - (trial 2) and were individually supplemented with 2.5 and 1.5 kg of concentrate once a day in the morning, in trials 1 and 2, respectively. Heifers were weighed on day −14 (used as covariate), and afterward, on d 1, 28 and 56 after the inclusion of the extracts into the concentrate. Data of each trial was submitted to variance analysis, using the Mixed procedure of SAS according to a completely randomized design, considering the effect of diet (n = 4), period, interaction diet x period, breed and interaction diet × breed (just for trial 2) and the initial body weight as a covariate. Heifers supplemented with Origanum vulgare extract did not change daily weight gain and overall ADG was 0.56 kg. Heifers supplemented with 1 g of Camellia sinensis extract had greater ADG (P < 0.05) than those fed with zero and 3 g of Camellia sinensis extract. Inclusion of 1 g of Camellia sinensis extract increased ADG 11%. In this study feeding Carmllia sinensis at 1g/heifer daily improved ADG of dairy heifers. Table 1.

Item	Trial 1 – Origanum vulgare (g/heifer/day)				P > F	Trial 2 – Camellia sinensis (g/heifer/day)				P > F
0	2.5	5.0	7.5	0	1	2	3
ADG (kg/d)	0.62a	0.68a	0.66a	0.73a	NS	0.55a	0.61b	0.57ab	0.52a	*

*P < 0.05.