Development of tools for the nutritional management of dairy cows on silage-based diets

This thesis consists of five studies, of which the first three (I-III) were conducted to evaluate methods in predicting the organic matter digestibility (OMD) and D-value of grass silages. Studies IV and V examined the effects of silage digestibility and fermentation quality on voluntary intake (SDMI) and the relationships between feeding and milk urea concentration (MUC), respectively. The objectives of this work were to develop and evaluate methods for predicting silage OMD, D-value and intake potential on practical dairy farms and asses the use of MUC as diagnostic of protein feeding in dairy cows. Experimental grass silages (n = 52) for I-III were harvested from timothy-meadow fescue swards using standard harvesting techniques. The OMD of silages was manipulated by varying the harvesting date of primary swards and secondary regrowth. In addition, 42 grass silages were collected from Finnish dairy farms for study III, which were chosen from a larger collection of silages to cover a wide range in OMD. The OMD of experimental silages was determined in sheep fed at maintenance by total faecal collection. All silages were analysed for dry matter (DM), crude protein (CP) and cell wall parameters (NDF, ADF, lignin) by reference procedures. In vitro pepsin-cellulase solubility of organic matter (OMS) was determined by sequential incubation of samples in acid-pepsin and buffered commercial cellulase solutions for 24 and 48 h, respectively. Indigestible NDF (INDF) content of silages was determined by in situ incubation of samples for 12 d with forage-fed cows. Silages for study III were scanned with a spectrophotometer to examine the relationship between near infrared spectra (NIRS) and INDF content. Fixed or mixed regression analysis was used to assess the relationship between chemical content, OMS or INDF and OMD. Datasets comprising of treatment means from published milk production experiments were collected for studies IV and V. Experiments were predominantly conducted in Finland, but relevant studies from Sweden and the UK were also included. Data from 21 studies (n = 125) were used to estimate the relationship between D-value and SDMI. Silages were harvested at different maturities using the same ensiling techniques within each study. Relationships between silage fermentation parameters and SDMI were estimated from data of 47 studies (n = 234). Silages were simultaneously harvested from the same sward using various additives. In study V, mean treatment data (n = 306) from 50 production trials were used. Relationships between MUC and dietary factors were derived using either the entire data or subsets, in which data were separated into studies comparing the effects of concentrate level (n = 80), concentrate CP content (n = 188), silage fermentation quality (n = 55) or replacing grass silage with legume silage (n = 32). Data analysis in IV and V was performed by regression analysis of fixed factors or with mixed models that account for between-experiment variations. Concentrations of CP, ADF and lignin were significantly correlated with OMD, but the linear or non-linear prediction equations for OMD were inaccurate and are not recommended for use on-farm, due to a lack of biological significance or uniformity of the parameters in relation to OMD. In vitro OMS predicted OMD with a high accuracy for primary growth silages, but was less accurate with regrowths. Predictions could be improved if harvest-specific correction equations were used for OMS. In situ INDF content of grass silages was a universal determinant of OMD and accurate estimates of OMD were obtained with one general prediction equation for the combined dataset that included both primary and regrowth silages. Results of III demonstrated that reasonably accurate estimates of INDF content could be obtained with NIRS. The cell wall structure of primary growth and regrowth grass silages appears to differ leading to different digestion and clearance dynamics in the rumen. The most reliable method for NIRS calibration for the prediction of OMD and D-value is based on in vivo measurements in sheep, but simpler methods are required in practice. In vitro OMS and INDF content, in particular, can be recommended for this purpose. Silage DM intake was significantly affected by silage D-value and fermentation quality. A relative intake index was proposed for use in practice to assist on-farm ration formulation. The proposed index predicted SDMI with an acceptable accuracy within experiments. Variance in MUC was mostly attributable to differences in dietary CP content. The amount of variation accounted for by statistical models was not improved when excess ruminal ammonia (PBV) or dietary CP/ME –ratio were taken into account. Both excess ruminal ammonia and absorbed AA were significant sources of milk urea. Excess AA N (absorbed AA N – milk N) and PBV increased MUC in a similar manner. MUC can be used in practice to predict dietary CP and PBV content and monitor urinary N excretion. In contrast to previous interpretations, exact target values for MUC cannot be given. Adequacy of PBV in the diet corresponded to a MUC value of 160 mg/l. Milk and milk protein yield increased up to MUC levels of at least 450 mg/l, when high quality protein supplements are fed. Output of urinary N per kg milk increased with higher MUC values.