Volatile fatty acids

Volatile fatty acids are produced in the rumen as the end-products of microbial fermentation and they represent the major energy source absorbed by ruminants, accounting for 50 to 80% of total metabolisable energy supplied to ruminants (Merchen, 2002). Overall acetate was the main VFA produced during the incubations of this study, which is in agreement with France and Dijkstra (2005), who reported that during most of the times acetate is the most abundant VFA produced in the rumen when roughage based diets are fed.

Independent of the type of pasture used in this experiment the mixtures with PKE up to 75% improved in vitro VFA production (mmol/ g DM or mg/ g DM). This result is unexpected as the quality of both pasture samples was greater than PKE, and therefore a decrease in the VFA production should be expected. Additionally, the results of NH3 production showed that after 6h of incubation N was restricted, which should

affect VFA production, as carbon “skeletons” from the degradation of proteins contribute to the VFA pool in the rumen (Waghorn et al., 2007).

The in vitroincubations also showed that after 24 h of incubation the addition of PKE into the mixes increased the molar proportion of butyrate at the expense of acetate, independent of the grass type incubated with it. However, the response in the production of propionate was inconsistent when PKE was added to the different grasses throughout the incubations. According to Van Soest (1994) the distribution of the microbial population that digests feed nutrients in the rumen is usually determined by dietary composition. Therefore, the shift in favour of butyrate production due to the addition of PKE may suggest changes or interactions among microbial species in the rumen.

According to Rook (1964), the addition of concentrates to grass diets can lead to the development of either organisms that produce high proportions of propionate or high proportions of butyrate, and this result will depend on the type of supplement, rumen flora developed with the diet and physiological factors related to the animal. Additionally, the bottles with only PKE have shown lower VFA production than any other treatment tested; this result could be due to the fact that excess of lipids in the diet, more than 5-6% of DM, inhibit microbial activity and rumen fermentation (Jenkins, 1993). The average PKE fat content (9% of DM) is well above this limit.

The molar proportions of VFA produced in the rumen affected milk fat concentration (Sutton, 1989); therefore, the increased proportion of butyrate in the rumen, as a consequence of PKE supplementation, has a direct effect on milk fat concentration. Miettinen and Huhtanen (1996) reported that increased intraruminal infusions of butyrate resulted in an increase in the milk fat yield of dairy cows. After the

DEVRUSWLRQRIEXW\UDWHE\UXPHQHSLWKHOLXPȕ-hydroxybutyrate is produced, and used

for the synthesis of fatty acids in the mammary gland through de novo synthesis (Bauman and Griinari, 2003). The increase in the milk fat production that occurred due to PKE supplementation in Chapter 6 and was reported by Dias et al. (2008), was possibly a consequence of the high proportion of butyrate produced in the rumen and the higher fat content of the diet.

The lower amount of VFA produced from each kg of DM incubated in experiment 1 compared with 2, is probably due to the lack of rapidly degradable energy to accelerate the fermentation process in the incubations of experiment 1. The unusual small quantities of available soluble carbohydrates, such as soluble sugars and starch, found in RW samples and also from PKE (Table 4.1) may suggest that energy was limiting initial microbial growth, which is essential in the maintenance and development of the microbial biomass (Nocek and Russel, 1988).

The different diets fed to the cows, also had an effect on the VFA production of RW, Ryegrass and its mix with PKE. The greater VFA production from DM during the first 6 h of the first incubation suggested that the microorganisms were more adapted to that environment because the inocula was obtained from cows fed a diet containing PKE. Weimer et al. (1999) reported that the diet has an influence on the rumen environment (microbial population and chemical environment), and consequently on the VFA production. According to Huntington and Givens (1998) the inoculumsused in the

in vitro incubations should be obtained from host animals fed diets similar to the test substrate.

4.5. Conclusion

In general, when PKE was mixed with pasture (ryegrass and white clover or ryegrass-only) net NH3 production was reduced, with PKE only being able to maintain

the net NH3 production for just six to eight hours. In contrast, the addition of PKE with

pasture resulted in greater VFA production, which is unexpected, and further studies are necessary. Acetate was the main VFA produced at all times, but the inclusion of PKE in the mix with pasture resulted in an increase of butyrate, with propionate production not changing during the incubations. In practice these results showed that PKE can affect nitrogen digestion of pasture and increased amounts supplemented in the diet can influence the availability of nitrogen in the rumen. Additionally, the mix between PKE and pasture can lead to a change in the VFA profile, which could affect the fat metabolism of the cow.

CHAPTER 5:

The effects of four levels of palm kernel expeller plus molasses

(PKEM) supplementation on apparent digestibility, intake and

nitrogen balance of sheep fed fresh pasture (ryegrass plus white clover)

Abstract

The effects of four levels of palm kernel expeller (PKE) plus molasses (PKEM) supplementation on apparent digestibility, intake and nitrogen balance were measured, when fed with fresh pasture (ryegrass and white clover) to lambs during two experimental periods. The pasture harvested in period 1 was of better quality than the pasture harvested during period 2, with a higher crude protein (CP; 20 vs. 13%) and lower neutral detergent fibre (NDF; 44 vs. 49%) content, respectively. Consequently, in period 1 the quality of pasture was higher than that of PKEM, but in period 2 pasture and PKEM were of similar qualities. In both periods no significant differences were found between treatments for total dry matter (DM) intake per lamb and per kg of metabolic liveweight (LW0.75), except for total DM intake per kg of LW0.75 at the ad libitumfeeding level in period 1, which was significantly lower (P<0.05) than the other three diets containing PKEM. However, daily pasture DM intake was reduced as the amount of PKEM offered increased in both periods. Substitution rates ranged from 0.25 kg pasture DM/kg PKEM up to 1.05; these were lower at 15% of PKEM than at 45% of PKEM in the diet. At similar intakes, there was a linear decrease in the apparent digestibility of DM and CP with the addition of PKEM into the diet in both periods. However, NDF digestibility of pasture was significantly (P<0.001) decreased only in period 1 when PKEM was fed with good quality pasture, with similar values of NDF digestibility measured for PKEM and for the lower quality pasture fed in period 2. The estimated apparent digestibilities of DM, CP and NDF for PKEM were around 63%, 52% and 68.5%, respectively, measured in periods 1 and 2, and estimated by regression analyses from data for all lambs, fed at maintenance and ad libitum. The concentration of digestible energy (DE) of the diet decreased linearly when PKEM supplemented good quality pasture, whereas when PKEM was added to lower quality pasture, there was a linear increase in the DE concentration of the diet, with an estimated DE of

PKEM in both periods of approximately 12.8 MJ DE/kg DM. The intake of nitrogen (N) decreased linearly (P<0.001) as the amount of PKEM increased in the diet of good quality grass (period 1), while the opposite result was found when PKEM was fed with low quality pasture (period 2). In both periods, faecal N was significantly higher (P<0.001) when increasing amounts of PKEM were fed. In contrast, there was a gradual linear decrease in urinary N excretion in both periods when PKEM was fed in increasing amounts, with significant differences in period 1 (good quality pasture) (P<0.01). The results of N retention showed that PKEM supplementation to good quality pasture (period 1) led to a lower retention of N, although when supplemented to low quality pasture (period 2) there was a gradual increase in the retention of N. Volatile fatty acids (VFA) concentration in the rumen was reduced with PKEM supplementation to good quality pasture, but increased when fed to low quality pasture, either at maintenance or ad libitum. Palm kernel expeller supplementation can be used to improve diet quality of more mature pasture, despite the low DM and CP apparent digestibility. However, due to the large differences in digestibility between PKEM and high quality pasture, its supplementation decreased DE of the diet, retention of N and VFA concentration, but at the same reduced urine excretion.

Keywords: apparent digestibility, pasture, palm kernel expeller, nitrogen balance, intake, sheep.

5.1. Introduction

The incorporation of supplements into the diet of grazing dairy cows in NZ has increased in the last few years. Firstly, to fill short term feed deficits on the farm created by seasonal changes in pasture growth or by an increase in stocking rate; secondly to meet the feed requirements of high genetic merit dairy cows (Kolver et al., 2004); and thirdly to extend lactation. However due to the increase in the costs of feeds used in the farm (eg. pasture and supplements; Clark et al., 2007), interest in the feeding of lower- cost by-products from other industries has increased.

During the last eight years palm kernel expeller (PKE), which is a solid residue of the palm oil industry, has become a major feed used on NZ dairy farms, with imports of PKE increasing a thousand-fold between the year 2000 and 2008, with 800 000 tonnes being imported in 2008 (MAF, 2008). By-products, such as PKE, are generally

characterized by great variability in their chemical composition (de Ruiter et al., 2007), and overseas studies have shown that PKE quality can also vary significantly (Moss and Givens, 1994; Hindle et al., 1995; O’Mara et al.; 1999). According to O’Mara et al. (1999) PKE can be characterized as medium quality energy feed containing moderate concentrations of protein (around 16% of CP) and high fibre content (around 70% of NDF).

There is insufficient literature concerning the nutritive value of PKE for grazing dairy cattle. Based on the chemical composition of PKE coming into NZ, it can be concluded that PKE has reasonable amounts of protein (14-16% of CP) and energy (11 MJ ME/kg DM) (de Ruiter et al., 2007). Additionally the fat content in PKE (8% of DM) suggests that mechanical extraction is the main process used for its production. However chemical composition does not take into account possible interactions between PKE and the animal, and also between PKE and other feeds. Most of the published data has been measured with animals that have not been grazing on pasture.

Overseas studies have measured the in vivo organic matter digestibility (OMD) of PKE when mixed with conserved forages (eg. hay), giving estimated values of 650 g/kg (O'Mara et al., 1999), 750 g/kg (Moss and Givens, 1994), and 790 g/kg (Carvalho et al., 2005). According to Carvalho et al. (2005) these differences between digestibility values could be due to differences the NDF content of PKE, which was higher in the study of O’Mara et al. (1999) and similar to the other study. Moreover, O’Mara et al. (1999) found a negative relationship (increasing fibre content significantly reduced OMD) between crude fibre content and the digestibility of PKE, but the overall relationship between OMD and chemical composition was poor.

The amount of PKE in the diet could also have an effect on its digestibility. Carvalho et al. (2005) found a linear increase in the OMD with the addition of increasing amounts of PKE into a diet composed of dehydrated alfalfa. In vitro

procedures can also be used to predict OMD of PKE; however most of the common enzymatic methods underestimate the digestibility of PKE (O’Mara et al., 1999).

Therefore, PKE is not like any other feed currently used on farm in NZ and an understanding of how it is digested by the ruminant in a pasture-based system is important. However, PKE cannot be fed as a sole feed to the ruminants and its own nutritional characteristics have to be estimated, when fed as supplement to other as the basal diet (e.g. pasture). We hypothesized that PKE would reduce the overall digestibility of the diet and digestibility would be decrease steadily as increasing

amounts of PKE were added to the diet. Based on the data from chapter 4, we would also expect a decrease in the concentration of VFA in the rumen with the addition of increasing amounts of PKE, with relatively more butyrate and acetate than propionate, and that the output of urinary N would also decrease.

Thus, the objective of this study was to measure the effects of four levels of PKE supplementation on apparent digestibility, intake and nitrogen balance when fed with fresh pasture to lambs.