Nutrient Intake, Excretion and Use Efficiency of Grazing Lactating Herds on Commercial Dairy Farms

Abstract

Estimating excreted nutrients is important for farm nutrient management, but seldom occurs on commercial grazing system farms due to difficulties in quantifying pasture intake. Nitrogen (N), phosphorus (P), potassium (K), sulphur (S), calcium (Ca) and magnesium (Mg) intake, excretion and use efficiency of 43 commercial dairy herds grazing pasture were calculated to understand the range in nutrient intake and excretion in these systems. Milk production, feed (grazed and supplement), as well as farm and herd management data were collected quarterly on representative farms located in temperate, arid, subtropical and tropical regions of Australia. Lactating herd sizes on these farms averaged 267 (30 to 1350) cows, with an average daily milk yield of 22 (9 to 36) kg/cow per day and the herds walked from <0.01 to 4 km/day on a variety of terrains. The mean total metabolizable energy (ME) required by cows in the herds was estimated to be 195 (116 to 289) MJ/cow per day. Although these farms are considered grazing systems, feeding strategies ranged from total dependence on pasture to total mixed rations (TMRTMR) and consisted of a wide variety of nutrient and energy contents. Mean pasture dry matter intake (DMI) (9 kg/cow per day, from 0.1 to 22 kg/cow per day) was just over half of total DMI. Dietary concentration of crude protein, P, K, S, Ca and Mg concentrations were, on average, 19%, 0.45%, 2.1%, 0.29%, 0.65%, and 0.3%, respectively, for all herds and, except for N, supplement nutrient concentrations were always more variable than pasture. Approximately 72% and 88% of diets provided greater than recommended P and N intakes, respectively. Calculated mean N, P, K, S, Ca and Mg excretions were 433, 61, 341, 44, 92 and 52 g/cow per day, respectively. Of the farm characteristics examined, residual maximum likelihood (REML) analysis indicated that daily excreted N, P and S were significantly related to per ha milk production, and excreted P, K and Mg were related to percentage of herd DMI provided as supplement. Mean use efficiencies by cows of N, P, K, S, Ca and Mg were 21%, 25%, 9%, 16%, 23% and 4%, respectively. These estimates of nutrient excretion and feed nutrient use efficiencies can be used to improve nutrient management on grazing system commercial dairy farms.

Estimating Excreted Nutrients to Improve Nutrient Management for Grazing System Dairy Farms

“…Significant effects of interview date (0.017 ≤ p ≤ 0.022) and season (0.024 ≤ p ≤ 0.041) were observed in REML analysis of excreted S, Ca and Mg loads, while excreted N, P and K loads were similar on all visits and in all seasons. These results were unlike previous [ 27 ] REML excretion (g cow −1 day −1 ) analyses, in which significant interview date and season effects were observed for N, P, S and Mg excretion but not K and Ca. Aarons et al [ 16 ] also reported a significant effect of season was only observed for percent time lactating cows spent in either the dairy shed or yards.…”

“…Nitrogen, P, K, S, Ca and Mg excreted by the lactating herds were estimated using dietary intake data and feed and milk samples collected at five occasions on each farm [ 22 ]. On each visit to a farm, face-to-face interviews were held, during which the farmers provided details about their lactating herds, including milk production, stage of lactation, days in milk, and their dietary intake (pasture and supplements).…”

“…On each visit to a farm, face-to-face interviews were held, during which the farmers provided details about their lactating herds, including milk production, stage of lactation, days in milk, and their dietary intake (pasture and supplements). A modified back calculation method [ 22 , 23 ] based on the metabolisable energy (ME) required for the lactating cows (maintenance, pregnancy, milk production, grazing and activity) and that provided in supplements fed to the animals was used to calculate the ME consumed in pasture. The calculated pasture ME consumed was used in conjunction with pasture sample ME measured in the laboratory to give the pasture DM consumed.…”

“…This method was devised to enable nutrient excretion estimation for commercial grazing system farms with a variety of lactating breeds, at various stages of lactation and parity, and which use a diversity of dietary strategies. For more details, see Aarons et al [ 22 ].…”

“…The fodder and milk samples were analysed by Westons Laboratories (Sydney, NSW), for ME, N, P, K, S, Ca and Mg concentrations. Full details of the analytical methodology are given by Aarons et al [ 22 ].…”