Methods for assessing phosphorus overfeeding on organic and conventional dairy farms

Abstract

Phosphorus (P) losses from dairy farms can severely damage aquatic ecosystems, so it is important to have tools to assess overfeeding of P. This study screened P intake and faecal excretion of different P fractions in dairy cows on conventional and organic farms, compared the P feeding level of the herds against the recommendations and analysed different sampling and analysis methods for assessing the general status of P feeding on the farms. The organic (n = 14) and conventional farms (n = 15) were of comparable size and were located in southern Sweden. On each farm, feed intake was registered for 10 cows representing four different lactation stages and their P intake was calculated and related to current recommendations. Faecal samples taken from the same cows were analysed for total P (TP) and soluble P. Milk production data for the cows were obtained from the Swedish official milk recording scheme. TP was determined in one slurry sample per farm. More than 70% of the cows studied, representing both conventional and organic herds, consumed P in excess of the recommendations. Conventional herds had higher P content in the ration than organic herds, and lactating cows in conventional herds had higher faecal concentrations of total and soluble P than those in organic herds. However in dry cows, the P content of the ration and soluble P and TP in faeces did not differ between the two management systems. Soluble P was well correlated to TP in faeces, and both were good indicators of P overfeeding.Keywords: dairy cow, phosphorus, faeces, organic management ImplicationsThis field study on Swedish commercial dairy farms showed the differences in P feeding between organic and conventional systems (CS) and examined different sampling methods for assessing P overfeeding of dairy cows. The results showed an over supplementation of P in both systems, more significant in the CS. The methods that were used were analysis of the soluble P in faeces as well as total P in faeces.

Faecal excretion of total and acid extractable phosphorus in dairy cows fed rations with different levels of phosphorus

“…AEP is an appropriate indicator of P overfeeding since surplus P consumed is almost entirely excreted as acid soluble P . However, when it comes to the assessment of P overfeeding it was suggested that TP might be a more attractive alternative since the cost of analysis is lower and the handling of the samples is less complicated . Their suggestion is confirmed by the present results, since AEP was reflected by TP, and TP was slightly stronger related to P intake.…”

“…Furthermore, the rapidly depleting rock P reserves require a more equitable management of this essential and non‐renewable resource . Dairy cows in North America and Europe are often fed P in excess of their requirement . More than two‐thirds of the P consumed by cattle is generally excreted in manure, mainly as soluble inorganic P .…”

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

“…The potential for N and P to be environmental contaminants [44,45] resulted in more studies quantifying intake and excretion of these nutrients on commercial dairy farms compared with K, S, Ca and Mg. For instance, mean dietary P (4.5 g P/kg DM) of grazing herds on these study farms was greater and the ranges generally wider than Spanish (3.98, 2.8 to 5.4 g/kg DMg) [22], Swedish ([3.7, 2.1 to 5.2 g/kg DM) [46], northeast US (4.42, 3.6 to 7.0 g/kg DM) [9], and Wisconsin ([4.0, 2.3 to 8.5 g/kg DM) [47] commercial farms, and well in excess of NRC [35] recommendations of 3.2 to 3.8 g/kg DM for cows producing 22 to 55 kg milk/day. Mean P concentration of dietary components (0.47%, 0.01 to 3.99%) provided to the cows in this study was less than for Swedish rations (0.6%, 0.25 to 1.1%) [46], and Wisconsin (1..8%, 0.24 to 3.58%) [47]), and closer to that of Idaho (0.49%, 0.41 to 0.50%) [19]) farms. Arriaga et al [22] attributed high P intakes in their study mainly to grass silage, although concentrates had higher P concentrations.…”

Phosphorus feeding practices, barriers to and motivators for minimising phosphorus feeding to dairy cows in diverse dairy farming systems

“…However, the cost-effective solution to the challenge of managing P-rich manure remains the minimising of P feeding because in areas with a high soil P index, farmers may incur additional costs for the transportation of manure to further lands (Knowlton, 2011). However, encouraging manure P analysis remains important for minimising P feeding because it provides farmers with an indication of the relative degree of excess P feeding on their farms (Nordqvist et al, 2013). In the current study, farmers of smaller herds were less likely to analyse their manure P than larger herds.…”