The effects of long-term feeding of extracted rapeseed meal and whole rapeseed

Abstract

Rapeseed meal (RSM), a traditional ingredient in dairy cow diets, is a protein rich feedstuff with an energy concentration similar to that of wheat. UK dairy compound feeds contain on average 150 gkg-1 RSM, which has remained relatively static in recent years. This reflects the fact that many dairy farmers and their nutritional advisors remain cautious about increasing the levels of RSM in cow diets, regarding RSM as a 'cheap' protein which is thought to lead to problems such as poor fertility, lameness and reduced performance. More recently, some dairy farmers have begun to feed small amounts of whole rapeseed (WRS) as a cost-effective energy source, and as a means of reducing the concentration of butterfat or modifying the fatty acid composition of milk fat.

The aim of this project was firstly, to test the hypothesis that RSM could replace soyabean meal as the main protein source in the diet of dairy cows and secondly, to test the hypothesis that WRS could be used as an energy source for the dairy cow, without any impact on herd performance, fertility or health over the duration of a lactation.

Sixty freshly calved cows were allocated to one of three treatment diets, control, RSM (360 gkg-1 of the concentrate) and WRS (147 gkg-1 of the concentrate). Diets were formulated to be isonitrogenous and isoenergetic with grass and maize silage as the forage portion of the diet. Diets were offered as a total mixed ration (TMR) for 28 weeks, after which the cows were introduced to the grazing diet. The RSM and WRS were then offered as part of a buffer diet consisting of maize silage, molassed sugar beet feed, wheat and either RSM or WRS (3.00, 1.78, 0.86, 0.45 and 0.45 kg DM per day respectively). The control group were given the basal diet plus equal amounts (0.22 kg DM per day) of maize gluten and soyabean meal instead of the rapeseed products. After 36 weeks, the cows were removed from treatment into one group for the dry period. Dry matter intake, liveweight and condition score, milk yield and composition, the concentrations of urea, ammonia and thyroxine in the blood, fertility, lameness and general herd health were recorded throughout the study. The financial performances of the three groups were monitored throughout the lactation.

There was a significant reduction in dry matter intake for the RSM and WRS treatment compared with the control (20.8, 20.2 and 21.9 kg DM/animal/day respectively). This was not reflected in any significant effects on mean milk yield, liveweight or body condition score. Milk fat content was significantly reduced on the WRS diet and the milk fatty acid composition was altered with a reduction in the concentration of saturated fats and an increase in the concentration of monounsaturated fatty acids (MUFA). The ratio of saturates to MUFA was 2.90 and 2.15 in milk fat for the control and WRS diets respectively. There was also a significant increase in conjugated linoleic acid content of the milk fat for the WRS diet, although the highest concentration was observed, regardless of treatment, when the cows went out to grass. There was no significant effect of either rapeseed treatment on herd health (incidence of lameness or mastitis) nor was there any effect on fertility. However, there was a numerical increase (though not significant) in the calving interval and the number of days to conception with cows fed WRS. The blood thyroxine content was significantly reduced for this treatment, which suggests that the glucosinolates in the WRS were having a mild suppressing effect on thyroid function. The overall financial performance (margin over all feed per cow) of the three treatments was £837, 855 and 877 for the control, RSM and WRS respectively. These figures were greatly affected by differences in the milk hygiene quality between the three groups (the milk from cows fed WRS achieved a premium more often than the other treatments) and the cost of the main feed ingredients (soyabean meal, maize gluten feed, megalac, RSM and WRS).

It was concluded that RSM could be included at 360 gkg-1 of the concentrate throughout the winter feeding period with no adverse effects on herd performance and some improvement in margins. WRS can be included at 147 gkg-1 of the concentrate, but should only be considered if there is a requirement to reduce milk fat or alter the fatty acid composition of the milk fat to bring about financial benefits, as there were some indications of a negative impact on herd fertility.