Dairy cattle breeding performance when grazing the high protein pastures of Uruguay

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Tosi, Hugo Ricardo
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Management and breeding information of 602 dairy cows and 3696 dairy heifers was obtained from the 1993, 1994 and 1995 records of four farms to study the dairy cattle breeding performance under Uruguayan commercial farming conditions. When diet was based on an unsupplemented legume pasture first service conception rates of lactating cows were 43.2 and 50.3%, services per conception 2.20 and 2.25 and overall pregnancy rates (% of bred) 82 and 88% on two dairy farms, respectively. Lactating cows, from one farm, fed legume pasture supplemented with corn silage and cereal grains showed better performance with first service conception rate being 62.7%, services per conception 1.63 and overall pregnancy rate (% of bred) 90.1%. Dairy heifers grazing legume pastures had better performances than lactating cows grazing legume pasture, but the response was variable among seasons and years. First service conceptions ranged from 63.5 to 75.1, services per conception from 1.27 to 1.81 and overall pregnancy rates from 78.6 to 96.1. Although many factors could have determined the differences observed a dietary effect related to the high CP content of legume pastures was not dismissed. Based on the results obtained from the farm survey two grazing trials were conducted to evaluate the breeding performance of nulliparous Holstein heifers under three dietary treatments with different levels of crude protein. Heifers grazed a red clover ('Trifolium pratense ') pasture as the basic component of the diets. One diet consisted of pasture alone while for the other two corn silage was utilized to supplement pasture and to reduce CP intake. Diet did not affect the breeding performance of heifers as measured by pregnancy rate (% of bred) during the two breeding seasons evaluated (winter and spring) and after two chances of AI. Supplementation with corn silage at 2.0% of BW (DM basis) resulted in a lower proportion of heifers observed in estrus (60.1% vs. 82.6%) and in a lower first service pregnancy rate (50.0% vs 71.4%) during spring. Reduced SUN and increased progesterone levels during the periovulatory phase on heifers fed the high silage diet may have determined the differences observed. It is concluded that high CP content of legume pastures did not affect the breeding performance of Holstein heifers and that supplementation of legume pastures with moderate amounts of silage resulted in heifers with similar chances to become pregnant as herdmates fed on pasture alone. However, when pastures are supplemented with large amounts of silage special strategies for estrus detection should be considered.