Rapeseed meal as a supplementary protein for dairy cows on grass silage-based diet, with the emphasis on the Nordic AAT-PBV feed protein evaluation system

Authors

• Mikko Tuori University of Helsinki, Department of Animal Science, SF - 00710 Helsinki, Finland

DOI: https://doi.org/10.23986/afsci.72455

Abstract

The effect of rapeseed meal (RSM) supplementation on the performance of dairy cows on direct cut grass silage based diets was studied in five feeding trials. The proportion of RSM varied from 0% to 33% in the concentrate mixture (the grain was an oat-barley mixture of 1:1). In one experiment the treatments were RSM and soybean meal (SBM), while in another experiment forage was either grass silage or bam dried hay cut at the same maturity. In addition, this was compared to the data of other trials in Finland during the last ten years, in which RSM supplementation had been used. Using this data the response in terms of milk yield to RSM supplementation was estimated. The utilization of protein in milk production was estimated by the Nordic AAT-PBV protein evaluation system. During the experiments (1983-1990) the varieties of turnip rape were changed from high glucosinolate, containing single-zero, to low glucosinolate containing doublezero varieties, while the glucosinolate content was reduced from 40-50 μmoles to 14 μmoles per g of defatted meal. Heat-moisture treatment (™Öpex) further reduced the glucosinolate content by half. By replacing grain with RSM in the concentratemixture with ad libitum silage feeding, the silage intake increased by 0.43 kg per kg increase in RSM on the basis of dry matter (DM) (non significant). The response in increased milk production was 0.77 kg in milk or 0.70 kg in energy corrected milk (ECM) yield (P<0.02), and in protein yield 27 g/d (P<0.01) per kg increase in RSM DM. Although the linear effect of the RSM level was significant, the effect on the milk yield was reduced when the level of RSM was over 12-16% of concentrate mixture. The protein content of milk increased by 0.07 g/kg per MJ increase in metabolizable energy intake (P<0.02). Heat-moisture treatment of RSM increased milk production significantly in one experiment (21.9 kg vs. 23.9 kg milk or 23.4 vs. 25.2 kg ECM/d), (P<0.03). In two other experiments heat treatment had no noticeable effect on milk yield. In comparing SBM with RSM on the same crude protein basis in the concentrate, no difference in milk yield was observed. The goitrin content ofthe milk was reduced when the glucosinolate content RSM, or the level of RSM in the diet, was reduced. With Öpex-treated double zero RSM, the milk contained less than 10 μg/l (sensitivity of analysis 2 μg/l) goitrin. The utilization of AAT in milk production was also estimated using different constants in the calculations of AAT-PBV values of the feeds. When the proportion of AAT of microbial origin increased, the coefficient of variation of the AAT utilization reduced. This is affected by correcting the microbial-N contamination of the in sacco analysis, lowering the estimate for the rumen outflow rate (k-value) from 0.08 to 0.03, and changing the estimate for the efficiency of microbial protein synthesis (MPS). The best model was obtained using the method of Voigt and Piatkowski (1991) for calculating MPS.