Variations δ 1 3 C values in faeces, milk and blood of lactating cows in grass-legume pastures

Abstract

The trial was carried out at the Estação Experimental Central of the Instituto de Zootecnia, in Nova Odessa. State of São Paulo, Brazil. The ratio of natural ¹²C and ¹³C isotopes n the feed, faeces, milk and blood of lactating cows were determined in the Centro de Energia Nuclear na Agricultura - Piracicaba, State of São Paulo. Using this technics it was. evaluated the proportion of C-3 species (tropical legumes) and C-4 species (tropical grasses) selected by lactating cows grazing grass-legume pastures and the correlation of the δ ^{1 3 C} values determined in the faeces, milk and blood. The work was carried out in two steps: step - lactating cow were penned and fed com silage alone or perennial soybean hay substituting for corn silage; step II - the lactating cows were mantained in different pasture treatments as follows: A) green panic with nitrogen fertilization; B) green panic mixed with perennial soybean; C) setaria cv. Kazunyula with nitrogen fertilization; D) setaria cv. Kazungula mixed with perennial soybean. The results showed that: 1) The δ ^{1 3 C values in the faeces and in the milk of the penned lactating cows decreased quickly in the first three days after changing com silage by perennial soybean hay, while €˜,ad less variation in the blood; 21 The} Î´ ^{1 3 C} values in the faeces, milk and blood of lactating cows grazing grass-legume pastures were more negative than those of the cows grazing grass alone, demonstrating to have an effective intake of perennial soybean by the cows grazing the mixed pasture; 3) There was more intake of perennial soybean by the cows during April (autmn) than January (summer). This was showed by the results of the δ ^{1 3 C} values in the faeces; 4) The correlation coefficients were highly significative (P < 0.01) between faeces and milk = 0.90, blood and milk 0.61, and significative (P < 0.05) between faeces and blood = 0.48; 5) The crude protein contents either of the forage available or the diet selected by esophageal fistulated steers were higher in the grass-legume pastures than in the single grass pastures.