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has historically been fed in excess of requirements for a number of reasons, including the provision of a safety factor to ensure no animal in a group is underfed protein. Increases in diet protein concentration typically increase feed dry matter intake (DMI) and thus milk yield, with diminishing response at higher protein concentrations. This is in part due to effects on rumen fermentation that increase dry matter (DM) digestibility. Olmos-Colmenero and Broderick (2006) cited previous meta-analyses of published data that reported maximal milk and milk protein yield at 21-23% CP concentration and maximal digestibility of DM and NDF at 16.5% CP concentration. Because of the economic risk that a reduction in diet CP concentration can potentially decrease milk yield, due to digestive or post-absorptive effects of protein and amino acid supply, there is considerable reluctance to reduce diet CP concentration in practice. While there may be clear benefits of increased NUE with lower CP diets, the strategy will only be acceptable if it can be achieved without large reductions in milk yield or detrimental effects on health and fertility. Ultimately, dietary protein efficiency may only be increased in practice in response to drivers other than economics (e.g. government legislation or milk buyer requirements). There are innumerable studies reporting effects of varying dietary

protein type and amount on NUE of lactating dairy cows, and recent interest in the potential benefits of feeding rumen-protected protein or essential amino acids to maintain milk protein yield when lower dietary protein concentration diets are fed. In a meta-analysis of published studies where rumen protected methionine and lysine were fed in diets containing less than 15% CP, there was huge variation in response across studies, from negative to positive, but overall there was a very small increase in milk protein yield when protected methionine and lysine were fed (Sinclair et al., 2014). There are also numerous studies examining the effects of dietary starch concentration on NUE. For example, replacing NDF with starch in diets containing 12 or 16.5% CP increased NUE by 11% (Cantalapiedra-Hijar et al., 2014). Whilst there have been thousands of papers published reporting dietary factors affecting NUE in lactating dairy cows, with very few exceptions the data are from experiments that have not allowed sufficient time for expression of long-term effects. Published studies are typically short-term studies with change-over designs and treatment periods of weeks that do not allow time for full adaptation of body N pools to treatments. For example, in two studies using the same diets in the USA (Lee et al., 2012 and 2015), when MP deficient diets were fed to lactating dairy cows in a change-over design study with 28-day periods plasma histidine concentration was not affected by diet MP supply, but plasma histidine concentration was reduced when the MP deficient diet was fed for 70 days. We are aware of only 5 studies published in the literature where the effects of dietary protein on production of dairy cows have been measured for a full lactation. It is possible that effects of sub-optimal MP supply may be cumulative

over multiple lactations, as has been observed when reduced lysine concentration diets were fed to pigs over multiple farrowings (Pinder Gill, personal communication).

Long-term effects of feeding lower protein concentration diets In a previous study funded by Defra (Reynolds et al., 2010), we measured effects of diets formulated with 3 dietary protein concentrations (14, 16, and 18%) to provide MP below, at, or above estimated MP requirement on DMI, milk production, and NUE of lactating dairy cows. Diets were each formulated as a total mixed ration (TMR) with the forage component containing high concentrations of either maize silage or grass silage to give 6 dietary treatments. In this study decreasing diet CP concentration from 18 to 14% increased NUE by 29%, regardless of the forage mixture fed, without significantly changing milk yield. However, the study was conducted as a change-over design experiment with 5-week treatment periods and thus there was insufficient time for full adaptation of the cows to the dietary treatments. This study was the foundation for a Defra funded project on protein efficiency in dairy systems that began in 2012 and is finishing in 2018. The project is a collaboration between the University of Reading’s Centre for Dairy Research (CEDAR) who are conducting a long-term lactation trial, Aberystwyth University who are conducting a trial with growing heifers to determine effects of reduced protein diets on growth and subsequent lactation performance, Rothamsted North Wyke who are modelling effects of increased protein efficiency on environmental impacts, the University of Reading’s Applied Economics, Marketing and Development Research Division who are modelling economic impacts of feeding reduced protein diets, and SRUC Crichton who are leading a demonstration trial in lactating cows fed grass silage- based diets and grazing grass. For the study at CEDAR, 215 first calf Holstein heifers were

assigned to 3 maize silage-based diets (71 or 72 heifers each) similar to the higher maize silage diets used in the previous study for Defra (Reynolds et al., 2010), but without the use of rumen-protected protein to achieve an oversupply of MP. The treatment TMRs were formulated to contain 14, 16, and 18% CP and provide MP below, at, and above predicted requirements, using incremental substitution of wheat, wheat feed, and soy-hulls with soybean and rapeseed meal. The TMR contained 35% maize silage, 15% grass silage, and 1.5% chopped straw on a DM basis, and the concentrates were formulated as a high and low CP concentration blend. Heifers were assigned to treatments in blocks based on genetic merit and body weight (BWT), body condition score (BCS), and age at calving. After calving they were fed a herd TMR until 7 days in milk and then maintained on treatment diets and individually fed until dry off. Dietary treatments were maintained for 3 lactations except during the dry period, when


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