Forage fibre quality as a determinant of nitrogen use efficiency in dairy cows

[摘要] ENGLISH ABSTRACT: Pollution from dairy farming, in the form of nitrogen emissions, is becoming an increasing concern globally due to greenhouse gasses and global warming. Many countries around the world, especially in the European Union (EU), have already put in place regulations regarding treatment of dairy farm effluent in an attempt to reduce nitrogen emissions. Many studies are also focussing on possible ways in which nitrogen excretion from cows can be reduced. Although none of these regulations are currently in place in South Africa it is likely that they soon will be as our agricultural practices follow EU regulations closely.Four lactating Holstein cows were used in a 4 x 4 Latin square design balanced for carryover effects with a 2 x 2 factorial arrangement of treatment. Treatments were low crude protein (CP) concentration with high neutral detergent fibre (NDF) digestibility (LpHd), high CP concentration with high NDF digestibility (HpHd), low CP concentration with low NDF digestibility (LpLd) and high CP concentration with low NDF digestibility (HpLd). Crude protein concentrations for the rations were formulated to be around 18% for Hp and about 15% for Lp. The indigestible NDF, as % of the NDF, of the two oat hays used were 40.8% for Ld hay and 35.54% for the Hd hay. Wheat straw was included in the Ld diets to obtain iso-NDF diets of different quality. Cows were fed ad libitum for 14 days with data collection taking place over the last 4 days of each period.The aim of this study was to improve nitrogen use efficiency (NUE) and to investigate the possible economic benefits for dairy farmers presented through better nutritional management by optimizing the use of dietary protein by using better quality forages (with regard to the digestible NDF) and by reducing CP intake by formulating the diets to meet metabolisable protein (MP) requirements.Dry matter intake (DMI) and milk yields (MY) were recorded daily and DMI for 3 of the diets was found to be similar, with the exception of the LpLd diet (i.e. LpLd had lower DMI than the other 3 diets) showing how protein availability can counteract the lower forage quality, by stimulating fibrolytic bacteria. Energy corrected milk yield (ECM) was found to drop 2.46kg/d for Hd diets and 3.00kg/d for Ld diets with Hp having higher production then Lp levels. Nitrogen use efficiency was found to improve by 3.04% when protein was reduced in combination with Hd forages and by 5.63% for Ld forages. Dry matter intakes and milk yields were used to determine daily feed costs and income respectively. These were used to calculate income over feed cost (IOFC). It was seen the higher protein diets had a higher cost per day but also resulted in higher milk production. The impact of better quality forages can also clearly be seen, especially on the lower protein levels. Statistically diet had no effect on IOFC, with IOFC being the same across all treatments.We concluded that lowering protein improved NUE significantly with forage digestibility contributing to the level of improvement. However, a consequence of reducing CP was a corresponding decrease in production. Although no statistical difference was found for IOFC, numerical differences that would be considered significant on farm level were however observed. The lack of statistical significance is a possible consequence of high standard errors of the mean (SEM) resulting from limited data point and thus we recommend performing this study on a larger herd to improve statistical variation.

[发布机构] Stellenbosch University