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The Utilization of energy by lactating dairy heifers Fasuyi , Gabriel Oluwadare

Abstract

Six lactating Ayrshire heifers were used to study the utilization of energy and the energy requirements for milk production at different stages of lactation and at different levels of production. The heifers produced an average of 5175 kg 4% FCM during lactation. Gross energetic efficiency of milk production declined from 47.58% at the beginning of lactation to 29.93% at the end of lactation. The over-all gross energetic efficiency was 36.42 ±5.55%. There was a highly significant (P < .01) positive correlation (r = 0.91) between gross energetic efficiency and 4% FCM production. High net energetic efficiencies of milk production were associated with early stages of lactation or high levels of production. The overall net energetic efficiency was 64.22 ± 5.20%. This was equivalent to a requirement of 1.187 ±0.089 megacalories digestible energy /kg 4% FCM or 270 ±20 grams TDN/kg 4% FCM. These requirements were significantly (P < .01) lower than NRC recommendations. There was a highly significant (P < .01) difference between heifers in their daily net energetic requirements. A highly significant (P < .01) positive correlation (r = 0.88) was found between net energetic efficiency and 4% FCM production. Total energy balance trials were conducted. By using an assumed maintenance requirement of 131 kcal ME/Wkg.75 to calculate the efficiency of ME utilization for milk production, the efficiency with which ME was converted to milk decreased gradually from 55.37% in early lactation to 52.11% in late lactation. Higher efficiencies of ME utilization in early stages of lactation were attributed to tissue mobilization. A significant (P < .01) difference between heifers in their efficiency of ME utilization for milk production was observed, while period effect was non-significant. By either simple linear regression analysis of ME available for milk plus maintenance on milk energy or multiple regression analysis of dietary ME on milk energy, tissue loss and metabolic body size, the efficiency of ME utilization for milk production was estimated to be 69.2 to 70.0% with a maintenance requirement of 183.5 to 184.5 kcal ME/kg .75. Multiple regression analysis showed that tissue energy was utilized for milk production with an efficiency of 98.5%.

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