Effects of the stage and number of lactation on milk yield of dairy cows kept in open barn during high temperatures in summer months

Abstract. Objective of this paper was evaluating the effect of high temperatures on production of milk of dairy cows in southern Moravia (Czech Republic, East Central Europe). We tested hypotheses that the milk efficiency of dairy cows kept in free-stall open barn without supplemental cooling is influenced of a month of summer period, lactation stage (LS) and lactation number (LN) of dairy cows. 193 Holstein cows were used. Three stages were stated according to the days in milk at the July 1: Stage 1-peak (0-50 d); Stage 2-mid lactation (51-120 d); Stage 3-late lactation (121-200 d). We found 63 summer (SumD) and 14 tropical days (TropD), 86 days with the temperature-humidity index (THI) above 72.0, and 26 days with the THI above 78.0, from May to September. The average monthly milk yields were statistically differed (P<0.001) among LS, also among LN. We found significant interactions between LS and LN (May, P<0.05; June, P<0.01; July, P<0.001; August and September, P<0.01). The average monthly milk yields of Stage 1 were gradually increased from May to July (from 33.94 ± 8.99 kg on 36.62 ± 6.62 kg), then decreased to September (30.05 ± 5.69 kg). Depression of milk was significant between July and August (36.62 ± 6.62 kg vs. 32.26 ± 5.88 kg; P<0.01). Milk yield in the Stage 2 was the highest in May (41.55 ± 7.93 kg) and then gradually decreased until October (27.69 ± 5.12 kg). Comparisons of months, LS and LN for whole observed period from May to October differed very highly significantly, there were recorded interactions month:LS (P<0.001), month : LN (P<0.01), and LS : LN (P<0.001). Dairy cows of Stage 1 were affected by the high temperatures in milk production for 305 days lactation significantly more than cows from Stage 2 (8 954.4 ± 1 526.9 kg vs. 9 614.1 ± 1 488.6 kg; P<0.05).