Models for lucerne quality grading

English: A study was conducted to develop and identify an evaluation system (model) to predict lucerne hay quality. Two hundred and ten lucerne hay samples for chemical analyses and in vitro digestibility determinations were obtained from different cuttings during two seasons (100 samples for 1998/1999 and 110for 1999/2000), at different times in a season and from different lucerne producing areas (sites) in the Northern Cape, South Africa. One hundred and eighteen of these samples were used for essential amino acid analysis. Thirty of the 210 samples and an additional 42 lucerne hay samples were obtained during one season (1999/2000) to estimate protein degradation (in sacco). Twelve Dorper lambs fitted with a rumen cannula were used in the degradation study. The variation in nutritive value of South African lucerne hay was evaluated as an initial study. The highest moisture content recorded (9.35%) was safely below the critical moisture level of 15% for effective storage. The coefficient of variation (CV = ± 15%) of crude fibre (CF), acid detergent fibre (AD F) and neutral detergent fibre (NDF) emphasized the need for analysis to ensure accurate diet formulation. This also applied for ash (CV = 22.11 %), crude protein (CP) (CV = 11.6%), non-fibre carbohydrate (NFC) (CV = 34.0), effective ruminal protein degradability (ERPD) (CV = 18.9%) and essential amino acids (EAA) (CV = 14.5% - 27.1%). The effective protein degradation parameters a, b and c showed the highest CV (106.8% - 306.4%). The mean in vitro organic matter digestibility (IVOMD) value (67.6%) seems to be representative (CV = 5.4%) of the IVOMD for the lucerne hay population used. Lower ERPD values (average 47.7%) than usually reported, were observed. The influence of acid detergent fibre nitrogen (ADF-N) on the rumen undegradable protein (RUP) of lucerne hay was negligible. Heat damage occurred to a small percentage (3.5%) of the samples. Lucerne hay had a high lysine (4.55g/100g CP) and low methionine (0.44g/100g CP) content. Methionine, lysine and isoleucine were, in a descending order, the least detectable in lucerne hay for milk synthesis. A second study was conducted to include protein quality, according to the United Kingdom metabolizable protein system (MP), into the total forage index (TFI) system to determine the quality of lucerne hay more accurately. This model (lucerne quality index) was compared to the chemical analysis and existing models for the determination of lucerne hay quality. The following models were used to estimate lucerne hay quality: a) Relative feed value (RFV) = % digestible dry matter (DDM) x dry matter intake as a % of body weight (DMI) x 0.775 b) Total forage index (TFI) = RFV + % CP x 6 c) Adjusted total forage index (ATFI) = RFV + adjusted crude protein (ACP) x 6. d) Lucerne quality index (LQI) = IVOMD x DMI + MP x 6, expressed as an index. NDF (r2 = 0.57) compared to ADF (l = 0.48) was a more accurate estimator of IVOMD. The NDF content of lucerne hay also played an important (P<0.0001)role in regression equations to predict the quality models RFV (r2 = 0.86), TFI (r2 = 0.82) and ATFI (r2 = 0.84) respectively as well as IVOMD (r2 = 0.57) and ERPD (r2 = 0.50). A high relationship occurred between these various models (r2>0.90). A moderate correlation (P<0.0001) was however observed between LQI and respectively ADF (r2 = 0.50), NDF (r2 = 0.50), IVOMD (r2 = 0.56), CP (r2 = 0.50), RFV (r2 = 0.51), TFI (r2 = 0.56) or ATFI (r2 = 0.55). No significant (P>0.05) predictor for the ERPD fractions (a, b and c) were observed. LQI could be predicted from ADF (r2 = 0.50) or ADF, ERPD and CP (r2 = 0.94) in a multiple regression equation. The results of the present study clearly indicated that a large variation occur in the energy and protein composition as well as the utilization of nutrients in South African lucerne hay. This emphasizes the need for a rapid and accurate quality evaluation system for lucerne hay in practice. LQI seems to be a better quality evaluation system than to those currently available. The relationships with LQI in the present study is based on a relatively small number (n = 30) of lucerne hay samples. Further research with a larger number of lucerne hay samples is needed to confirm these results. The usage of near infrared reflectance spectroscopy (NIRS) for the rapid nutritional evaluation of lucerne hay (LQI) also needs urgent investigation.