Milk production from cows grazing on tropical grass pastures. 2. Effects of stocking rate and level of nitrogen fertilizer on milk yield and pasture-milk yield relationships

Abstract

The effects of stocking rate and rate of nitrogen fertilizer application on milk yield from a Gatton panic (Panicum maximum cv. Gatton) pasture were studied over 3 years at Kairi Research Station on the Atherton Tablelands. Thirty-two Friesian cows were used in a 4x2 factorial experiment with treatments 2.0,2.5,3.0 and 3.5 cows/ha, each at 200 and 400 kg N/ha.year. Fat-corrected milk (FCM) yield per cow decreased (P<0.05) with increasing stocking rate according to the equation (¦ s.e.): Y=3476-276 ( ¦ 111) X (P<0.05), where Y is FCM yield per cow (kg), and X is the stocking rate (cows/ha). FCM yield per hectare increased linearly (Pt0.01) with increasing stocking rate in each year and was represented by the following equations ( ¦ s.e.) for nitrogen applications of 200 and 400 kg/ha.year, respectively: Y = 1584 + 1967 (¦ 289) X Y = 2366 + 1967 (¦289) X (P<0.01), and where Y is FCM yield per hectare (kg) and X is the stocking rate (cows/ha). Milk yield per cow and per hectare were significantly increased by the higher rate of fertilizer application (P<0.01) in year 3, but not in years 1 and 2. Mean FCM yields per cow across years were 2574 and 2858 kg from 200 kg N and 400 kg N pastures respectively (P<0.05). The pasture parameters which were most closely correlated with milk yield were green dry matter (GDM) on offer or its components, namely green leaf and green stem. The relations between FCM yield per cow and green dry matter on offer per cow and per hectare were represented by the equations ( ¦ s.e.): Y = 2211 +0.43 (¦ 0.12) X1 (P<0.01), And Y= 1656 + 0.35 (¦0.08) X2 (P<0.01), where Y is FCM yield per cow (kg/lactation), X1 is GDM per cow (kg) and X2 is GDM per hectare (kg). For this environment, it is concluded that a stocking rate of 2.5 cows/ha with 400 kg N/ ha.year can be safely employed to maintain cows on a pure grass pasture from the opening rains in summer until the end of winter.