Methane emissions from sheep grazing Pearl millet (Penissetum americanum (L.) Leeke) swards fertilized with increasing nitrogen levels

Abstract

This study aimed at quantifying methane emissions from sheep grazing pearl millet swards (Penissetum americanum (L.) Leeke) as affected by nitrogen (N) fertilization doses (50, 100, 200 and 400 kg N ha−1). The experimental period was 70 days of pasture use, from February to April 2011. The grazing method was continuous stocking with variable stocking rate, so pasture structure was intended to be similar (30 cm sward height) among treatments. Thirty six tester animals aging 5 months and initially weighing 20 ± 1.6 kg were used for evaluations. Intake (OMI) was estimated by their relationship with N content on faeces. The sulfur hexafluoride (SF6) technique was used for two sampling periods of five consecutive days each to quantify daily methane (CH4) production. Parameters related to sward structure was not affected by N fertilization, but herbage accumulation increased linearly (P < 0.05). Greater (P < 0.05) stocking rate, weight gain per area and CP content were observed with higher fertilization levels. N doses did not affect (P > 0.05) OMI, and methane emissions expressed as a ratio of OMI. Results indicated that increasing nitrogen doses decrease methane emissions per animal. When expressed as grams of methane per kilogram of organic matter ingested or energy converted into methane, no difference was observed. However, emissions per unit area increase as a consequence of higher stocking rates allowed by the increase in forage production. Sheep emitted 16.3 g of methane per kg of ingested organic matter and converted 5.1% of ingested gross energy into methane.