e.1 Forage chemical composition
The crude protein (CP) content of Napier grass of 56.3 gkg− 1 DM was within the ranges of 56–79 gkg− 1 DM reported by Muinga et al. (1995) and the 42.0–67.0 gkg− 1 DM reported by Gemiyo et al. (2017). However, the CP content was lower than the range of 68–112 gkg− 1 DM reported by Muia et al. (2000); and the 66.3 gkg− 1 DM reported by Njarui et al. (2003). The variation could be associated with differences in soil fertility (Kariuki 1998; Rengsirikul et al. 2013), climate (Mureithi et al. 1998), and harvesting stage (Kariuki 1998; Rusdy 2016). The CP contents for the grass was within the range of 20.0-270.0 gkg− 1 DM reported by Skerman and Riveros (1990) for tropical grasses. The CP contents for the grass was below the minimum needed for effective rumen function (70.0 gkg− 1 DM) (Minson 1981), ruminant growth (113.0 gkg− 1 DM) and for lactation (120.0 gkg− 1 DM) (ARC 1984). With fibre levels of Napier grass of 686.5, 412.5 and 48.4 gkg− 1 DM for neutral detergent fibre (NDF), acid detergent fibre (ADF) and acid detergent lignin (ADL), respectively, the values were higher than the 603.0-614.0, 311.0-318.0 and 28.0–29.0 gkg− 1 DM, respectively, reported by Kariuki et al. (2001). The NDF content was higher than 600 gkg− 1 DM, the critical level beyond which the forage is characterised as poor quality (Van Soest 1982). The ADL content for Napier grass of 48.4 gkg− 1 DM was below the 60 gkg− 1 DM level above which digestibility is negatively affected (Van Soest 1982). Minson and Milford (1967) noted that at farm level, CP content of Napier grass does not always satisfy the 60–80 gkg− 1 DM, the minimum needed for optimal rumen microbial activity. Further, even with a CP content of 70 gkg− 1 DM necessary for optimal rumen function (Minson, 1981), grasses alone are inadequate for dairy goat maintenance (Yates and Panggabean 1988; Njarui et al. 2003), due to their high fibre content, poor digestibility (Njarui et al. 2003), and low protein content when mature (Minson 1988). Together with the crop residues, grasses require supplementation with high protein forages such as legumes, or concentrates when fed.
e.2 Feed intake
Goats supplemented under T3 and T4 consumed more (P < 0.05) total dry matter (TDMI) by 15 and 33%, respectively, than the unsupplemented goats. The decrease in basal DMI from 0.985 kg for T1 to 0.899 kg for T4 (0.409 kg DM concentrate) and the increase in concentrate:forage ratio from 0:100 for T1 to 31:69 for T4 with increasing concentrate levels depicts a decreasing concentrate substitution rate for grass whereby more of the concentrate was substituting less of the basal DM. The substitution rates of 0.65 (T2), 0.44 (T3), and 0.21 (T4) kg DM, implying the respective reduction in forage consumption for every 1 kg DM increase in concentrate intake, compared with the reducing substitution rate observed by Otaru et al. (2020) for hay with high level of concentrate, of 0.30 and 0.05 kg at 560 and 700 g concentrate, respectively. Increasing concentrate from 150 g (T2) to 300 g (T3) increased TDMI significantly by 0.104 kg and milk production significantly by 56.5 ML, but increasing concentrate by the same amount beyond 300 g to 450 g (T4) decreased milk yield significantly by 136.7 mL. The optimal amount of concentrate for the highest milk production (476.9 mL) was therefore under T3 (300 g), which would be cheaper than 450 g. The observed increase in TDMI with increasing level of concentrate or energy supplement was in agreement with findings by Tolemariam et al. (2009); Donnem et al. (2011); and Otaru et al. (2020) using various goat types on various types of basal feed. The observed increase in TDMI may have been due to the progressive increase in consumption of protein and energy from the concentrate which provided readily fermentable organic matter (FOM), as it also contained maize meal which has high degradability, leading to increased feed intake and volatile fatty acids (VFA) energy, thereby increasing the efficiency of microbial protein synthesis in the rumen (NRC 2007). The improved efficiency of microbial protein synthesis led to greater supply of post-ruminal protein (NRC 2007; Migwi et al. 2013; Leng et al. 2020) to the goats, for an improved protein to energy (P/E) ratio of absorbed nutrients (Migwi et al. 2013; Leng et al, 2020). This has an effect of further increasing the feed intake (Preston 1995; NRC 2007; Migwi et al. 2013). The ratio of absorbed propionate or glucogenic to acetogenic (acetate) substrates, both emanating from the improved energy digestion of forages or concentrates in the rumen fluid, contributes to the efficiency of energy utilization (NRC 2007) at tissue level metabolism, leading to increase in voluntary feed intake (Migwi et al. 2013). DMI usually increases with CP intake from a concentrate (Otaru et al. 2020), but there may be no significant effect if CP content of the basal forage is over 100 g kg− 1 DM (Otaru et al. 2020).
For the low TDMI by the goats consuming unsupplemented Napier grass (T1), this was most likely caused by the diet’s failure to meet the nutrient requirements for the rumen microbes, resulting in low fibre digestion in the rumen and also an imbalance of the P/E ratio of the nutrients absorbed from the small intestines. With less than 100 g kg− 1 DM CP, the Napier grass required N and energy sources, which are readily degradable for optimal rumen microbial protein yield (Muia et al. 2000). Kariuki et al. (2001) reported that unsupplemented Napier grass could result in NH3-N concentration below the requirements for microbial synthesis, due to reduced protein supply which reduces rumen fermentation, thus such a diet is likely to be poorly digested and therefore inefficiently utilized. Van Eys et al. (1986); Yates and Panggabean (1988) and Njarui et al. (2003) showed that goats fed sole Napier grass diet lost weight due to negative nutrient balance. The efficiency of protein utilization can, however, be improved by supplementation with concentrates or tree legumes (Otieno et al. 2016). It is noted that protein accounts for a higher limitation for ruminant production on tropical grasses than the ME content (Hennessy 1980; Preston 1995; Migwi et al. 2013). With the optimum dietary NDF proportion for maximum DMI being 350 g kg− 1 DM (West et al. 1997), the DM intake was less than optimal as the mean NDF content of the treatment diets ranged from 686.5 g kg− 1 DM for T1 to 598.9 g kg− 1 DM for T4. The low digesta clearance of low protein high fibre feeds such as the Napier grass is likely to lead to low voluntary intake and therefore low animal production (Migwi et al. 2013). The expected DMI for a dairy goat producing 1–2 L milk day− 1 on a high quality diet is about 1.4–1.9 kg day− 1 (AFRC 1993). Thus the TDMI levels across all the treatments (diets) were generally low and therefore low productivity in milk production. This indicated that there was a deficiency in critical nutrients in the diets (Leng et al. 2020), limiting the capacity of the diets to supply the correct amounts and balance of the nutrients required for milk production (Preston 1995), and the available feeds were therefore inefficiently utilized (Preston 1995; Leng et al. 2020), which in turn led to low voluntary intake (Migwi et al. 2013). The nutrient deficiency or imbalance, could have occured at the rumen microbial/enteric fermentation level or at the tissue metabolism level (Preston 1995; Migwi et al. 2013), which is the major limitation to animal production (Preston 1995). At the microbial fermentation level, the diets could have had low amounts of available glucogenic energy, protein, and starch required to ferment the feeds and produce adequate microbial protein (Preston 1995). At the tissue metabolism level, there could have been an imbalance in and/or inadequate end products of rumen fermentative digestion and feed components that escape rumen fermentation (Preston 1995) that are absorbed in the gut, including the VFA energy, amino acids, and long chain fatty acids (LCFAs) (Preston 1995), for meeting the animal requirements (Leng et al. 2020).
Usually, the nutrients absorbed from high fibre forages tend to be imbalanced in ratios of protein:energy and glucogenic:acetogenic substrates (Preston 1995; Leng et al. 2020), leading to inefficient utilization of acetate at tissue metabolism level (Migwi et al. 2013; Leng et al. 2020). Acetate accounts for 60–70% of VFA energy absorbed in gut, and when inefficiently metabolized through wasteful oxidation due to limitations in glucogenic substrates (propionate, amino acids) (Migwi et al. 2013), this leads to loss of energy as heat or heat increment, methane, and to heat stress in an animal (Migwi et al. 2013; Leng et al. 2020), resulting in decreased voluntary intake (Migwi et al. 2013). Heat increment from inefficient acetate metabolism is the major cause of heat stress (Leng 1990), and it results in low voluntary feed intake hence low animal productivity (Leng 1990; Migwi et al. 2013); reduces feed gross energy; and reduces animal ability to cope with increase in temperature (Migwi et al. 2013). Considering that ruminant productivity is primarily influenced by feed intake and the capacity of the diet to supply correct balance of nutrients required for various productive states (Preston 1995), supplementing nutrients (amino acids) at both the rumen and tissue metabolism levels is important (Preston 1995). The supplementation ensures an appropriate protein to energy ratio in the nutrients absorbed for optimal efficiency of feed or acetate utilization, and a reduction in heat increment (Preston 1995).
At the low levels of feeding observed as per the TDMI of 1.0 to 1.3 kg, which resulted in a DM intake of 2.9 to 3.9% of the goats’ live body weights, it is most likely that little dietary protein escaped rumen degradation other than microbial protein. The low feeding level could also have led to increased rumen retention time (Meenhan et al. 2021), resulting in the observed high DM digestibility coefficients of 0.77 to 0.79. The DM intake of dairy goats is about 3 to 7% of body WT (Devendra and Burns 1970; Steel 1996), with dairy goats in temperate environments consuming DM amount of about 5–7% of body WT compared to 3% in meat goats (Otieno et al. 2016). The breed-environment interaction is important in the total DM consumption (Otieno et al. 2016), whereby the indigenous dairy goat breeds in the tropics consume DM of about 3.3%, with the exotic breeds introduced in the tropics, and goats in the temperate regions, consuming about 3.6 and 5.0% body WT DM, respectively (Otieno et al. 2016). The relatively lower intakes for tropical dairy breeds and exotic breeds imported into the tropics are attributed to the high ambient temperature which tends to depress appetite (Otieno et al. 2016). It is noted that even when nutrients are formulated to meet animal requirements, the expected levels of animal performance may not be achieved unless a bypass protein is added (Preston 1995). Bypass nutrients are known to improve productivity of ruminants in the tropics (Preston 1995; Leng et al. 2020), with deficiency in dietary bypass protein being more serious than for energy that can come from cereals, and strategies to avail these are needed (Migwi et al. 2013). Bypass protein often increases feed intake of ruminants (Preston 1995; Leng et al. 2020), and improves the P/E ratio of the nutrients absorbed (Leng et al., 2020) thereby promoting milk production (Preston 1995). However, there may be a greater demand for glucogenic compounds than that provided via the ruminal digestion end products using bypass protein alone (Preston 1995). Therefore dietary supplementation to ruminants on low quality forages should ensure the supply and balance of nutrients needed by the microbial fermentative digestion system, and increasing the supply of absorbable amino acids, glucose precursors, and bypass protein in lower gut for an appropriate P/E ratio (Preston 1995; Leng et al. 2020). This is to enable conservation of surplus absorbed acetate to fat for storage in adipose tissue instead of being dissipated as heat and the resulting heat increment (Migwi et al. 2013). Excessive heat increment is a major problem for animals living in the tropics, especially during this time of climate change characterized by increase in global warming (Preston 1995; Leng et al. 2020). Developing and feeding a combination of the necessary supplements that ensure an appropriate P/E ratio in the absorbed nutrients, such as a source of N for rumen microorganisms, and bypass protein (Preston 1995; Leng et al. 2020) is needed, as this would result in a substantial increase in productivity beyond what may be projected (Preston 1995; Leng et al. 2020).
e.3 Milk yield
Supplemented goats under T2 and T3 produced significantly more milk by 37 and 55%, respectively, than the unsupplemented goats (T1). The increase in milk yield with increasing level of concentrate supplementation agreed with findings by Tolemariam et al. (2009); Ketto et al. (2014); and Otaru et al. (2020) using various goat types and levels of concentrate. The increase in milk production with increasing levels of concentrate is attributed to the improved availability of nutrients for the mammary glands during milk synthesis as energy levels improve (Tolemariam et al. 2009). Considering that the concentrate was maize-based (13.3%), the goats on T2 and T3 consumed more maize and therefore more dietary starch, which enhances more production of propionate in the rumen (Otaru et al. 2020). Propionate is converted to glucose in the liver and finally into lactose in the mammary glands, which triggers water movement into mammary glands secretory cells, resulting in greater volume of milk (Otaru et al. 2020).
For the unsupplemented (T1) animals on low quality forages, more acetate is produced in the rumen (Min et al. 2005), resulting in a high acetate to propionate ratio (A/P ratio). The high A/P ratio leads to low efficiency of using ME for milk production (Preston 1995; NRC 2007). Thus the supplemented goats on average produced more milk (412.5 mLd− 1) than the unsupplemented goats (306.8 mLd− 1), suggesting that concentrate amount as low as 150 g (0.14 kg DM) provided quite some additional amounts of energy for rumen microbes to produce N thereby improving the protein to energy ratio of absorbed nutrients, to improve milk production. However, the goats under T4 (450 g concentrate) had only an insignificant increase in milk production, of about 11% over the unsupplemented goats. So medium levels of concentrate supplementation improved milk yields but the high levels had minimal improvement in milk yield. It is suspected that supplementation with the higher level of concentrate reduced rumen pH, in agreement with observations by Min et al. (2005); Quang et al. (2015); Kholif (2020); and Elmhadi et al. (2022), which could have led to: subclinical ruminal acidosis (SRA) (Enemark et al. 2002; Ketto et al. 2014; Elmhadi et al. 2022); reduced cellulolytic bacterial activity resulting in decrease in fibre digestion (Otaru et al. 2020; Ketto et al. 2014); and it can also cause milk fat depression (Elmhadi et al. 2022). SRA, also called subacute ruminal acidosis (SARA) (Enemark et al. 2002) or chronic acidosis (Elmhadi et al. 2022), is a ruminal pH reduction state (Elmhadi et al. 2022), and it results in reduced production of acetate (Ketto et al. 2014; Kholif 2020) and butyrate which are important precursors of milk fat (Ketto et al. 2014), and reduced milk production (Enemark et al. 2002; Elmhadi et al. 2022). The predisposing factors for SRA include: too quick exposure to energy-rich feed rations (Enemark et al. 2002); feeding energy-rich diets after adaptation to forage diets (Elmhadi et al. 2022); and feeding grain and fibre separately compared to feeding as mixed diet (Elmhadi et al., 2022). All these factors were prevalent for the experiment that used animals that were initially under unsupplemented extensive grazing system, and this may explain why the milk yield for T4 (340 mL) was poorer than for T2 (420 mL). However, dairy goats have been fed higher levels of concentrate without adverse effects on digestion or milk production, such as the 700 gday− 1 (Otaru et al. 2020) and 1,200 gday− 1 (Lopez et al. 2019). Clinical acidosis on the other hand, results in diarrhoea or a marked fall in feed intake (Wangsness and Muller 1981), and these symptoms were not observed for the goats on T4. Further, the highest proportion of concentrate in a diet was 31%, which was below the 50–60% level above which rumen pH markedly decreases (Otaru et al. 2020) leading to reduced feed intake (Wangsness and Muller 1981). However, some authors reported decreased milk yield (Otaru et al. 2020) or non-significant differences in milk yield (Bhateshwar et al., 2018) with supplementation. The lack of significant differences could be attributed to the quality of the basal roughage used, as animals feeding on poor quality roughage show higher responses from concentrate supplementation compared to those on moderate to high quality forage (Otaru et al. 2020).
Considering that high NDF content in a diet can decreases milk yield (West et al. 1997), the observed mean NDF contents of 598.9–686.5 gkg− 1 DM for the diets were quite high, hence milk production was less than optimal. However, the generally low milk production was also based on the low initial milk production (589 mL d− 1) by the goats that were under free grazing system before the experiment. Generally, the low milk productivity of ruminants in the tropical regions relative to their production potential is mainly attributed to nutritional constraints, manifested as the imbalanced nutrients arising from digested available forages especially when not adequately supplemented (Leng et al. 2020). In addition, the low adaptation by the dairy goats imported from temperate regions (Otieno et al. 2016); as well as other environmental factors such as disease incidences and parasitism, and harsh climatic conditions may also play a role in reducing animals’ productivity (Leng et al. 2020). Supplementation of diets for lactation should therefore aim to correct imbalances of nutrients for milk production (Preston 1995). However, milk production level of dairy goats is also influenced by breed, genotype (Clark et al. 2017); environmental conditions (Park 2012); month of conception (Ali et al. 1983); and herd, doe, and sire (Iloeje et al. 1981). An interaction between dam grade and region for Alpine upgrades has been reported in Nyeri County in Central Kenya (Mburu et al. 2014). However, the genetic potential of the dairy goats in Kenya is low compared to the pure genotype of the exotic goats, as the available dairy goats are mainly the upgrades of the local goats using exotic bucks (Kiura et al. 2020). Furthermore, due to scarcity of the exotic bucks following a ban on buck importation in the 1990s, there could be some degree of inbreeding (Marete et al. 2011), and lower levels of management (Kiura et al. 2020) especially, in terms of forage feed supplementation. In addition, the dairy goats in Kenya are not yet selected for the milk production trait due to the low numbers of replacenent stock in the event of culling. Therefore, their milk productivity is low, and the maximum reported production in cross-sectional surveys is about 3.0 L/goat.day (Mburu et al. 2014) with many other such surveys reporting 0.6 to 2.1 L/goat.day (RDCOE 2011; Ogola et al. 2010; Kiura et al. 2015). Therefore, for experimental goats with data collection periods lasting four months and beyond, dairy goats in Kenya in the short term are not likely to give an average beyond 1.4 L/goat.day, the mean production obtained from a cross-sectional survey accompanied by a one month’s milk production recording from dairy goats (Kiura et al. 2020). There is therefore the need to lay emphasis on the nutrition and breeding aspects of the dairy goats in Kenya to improve their performance.
It was concluded that lactating dairy goats fed on a Napier grass basal diet require a daily supplementation level of 300 g dairy meal concentrate per goat to increase the total dry matter intake and thereby increase milk production. It is recommended that when lactating dairy goats fed on a Napier grass basal diet are supplemented with dairy meal concentrate, an additional supplement in form of rumen bypass protein be provided to increase the amount of amino acids absorbed in the intestines for an appropriate protein to energy ratio to meet animal requirements for improved milk production.