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all cows were fed the same far-off and close-up dry period diets fed to the commercial herd at CEDAR. Cows were inseminated between 50 and 200 days in milk (DIM) and any cow not pregnant by 200 DIM left the study after 305 DIM. Measurements include daily milk yield and feed dry matter intake (DMI) and weekly milk composition, BWT, and BCS.

Variation in diet N concentration. To monitor variation in diet CP concentration, daily samples of diet components were bulked weekly and analysed for N concentration by Kjeldahl procedures and results used to calculate weekly CP concentration of the treatment diets. It was quickly apparent that there was considerable variation in diet CP concentration, primarily due to fluctuations in grass silage and concentrate blend N concentrations that resulted in lower target protein levels than planned. Therefore, in order to maintain treatment CP concentrations diets were supplemented with variable amounts of soyabean meal based on 3 week rolling averages of forage and concentrate CP concentrations. The variation observed in part reflects variation due to sampling error, especially for the concentrate blends, as well as analytical error, but also reflects true variation in the composition of the feeds used (St-Pierre and Weiss, 2015). To date we have focused on weekly variation in N concentrations of the treatment diets and their components, but we will also examine variation in other diet components (e.g. NDF and starch). The impact of this variation is not certain, but raises questions about the ability to precision feed dairy cows based on estimates of metabolizable essential amino acid supply if drivers of microbial protein synthesis are subject to fluctuation due to variation in silage quality over time. Ultimately it is the longer-term average diet composition and nutrient supply that will determine the level of production and energy and protein balance, as long as cows are fed ad libitum and can adjust their daily DMI as needed (Reynolds and Kristensen, 2007; Yoder et al, 2013).

Production responses. Reductions in milk yield for the 14% CP diet were less than expected based on previous studies, but in line with percentage reductions (10%) in predicted metabolizable protein supply relative to requirement. Results for first lactation heifers (parity 1) are presented in table 1 (Reynolds et al., 2016). Milk protein yield for the 14% CP treatment was 94, 91, and 93% of milk protein yield for the 16% CP treatment for parity 1, 2 and 3, respectively. The response of DMI to treatments, and magnitude of differences in milk yield due to treatment, differed in multiparous vs primiparous cows, but DMI was always higher for cows fed the 18% CP diet. For parity 2 and 3, cows fed the 14% CP diet had 1.4 kg less daily DMI compared to cows fed the 18% CP diet. Body weight and estimated energy balance were always higher for cows fed the 18% CP diet as these cows consumed more feed DM but did not produce more milk compared to cows fed


the 16% CP diet. Body condition score was slightly higher for cows fed the 18% CP diet in parity 1, but was never a concern and was not affected by treatments in parity 2 and 3.

Nitrogen use efficiency. Across all 3 parities lower protein diets produced remarkably similar improvements in dietary NUE, but with large individual animal variation. This variation maybe due to variation in diet composition and quality over time (cows began the study over an 18 month period), variation in milk production level due to genetic or environment effects, and resulting differences in digestion or post-absorptive metabolism between cows. Some of these differences in NUE between cows may be due to differences in rumen microbial communities and their functions. We are also investigating the extent to which differences in NUE are related to genomic or cow family differences. We will also investigate the extent to which feed conversion efficiency varies among individual cows, but in a companion trial we did see large animal variation in feed DM and nitrogen digestibility. Numerically NUE was slightly higher in primiparous cows.

Longevity. Calving interval and days to first progesterone rise (return to oestrus) were not affected by treatments, but days to first milk progesterone concentration rise was lower in parity 1 cows. The risk of not being pregnant was numerically greater for the 14% CP diet in parity 3 and over the life of the study the survival of these cows was numerically reduced compared to the cows fed the other 2 diets (36 vs 47%). On a numerical basis, the cows fed the lowest protein concentration diet, which did not meet their predicted metabolizable protein and essential amino acid requirements, had a greater incidence of stealing other cow’s feed, had more abortions and embryo losses, and more lameness. There was also preliminary evidence of epigenetic effects that could influence the production of the daughters of cows fed the lowest protein diet, which could represent a long-term impact for the herd.


There is considerable loss of N from dairy production systems that has environmental and economic consequences that need to be considered and may lead to increased pressure to increase the efficiency of protein utilization in dairy production systems. Cows fed lower protein concentration diets are more ‘nitrogen efficient’, but we need to consider effects of lower protein diets, both positive and negative, at a systems level. The risk of potential loss of milk yield if dietary protein levels are reduced is a constraint on the use of lower protein diets in practice. There are potential economic and animal health implications of feeding cows diets deficient in metabolizable protein, and effects on longevity observed in mature (parity 3) cows are a particular concern. The large variation in diet protein concentrations

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