Yield and chemical composition of forages
In spite of being fully mature, the forage yield of the binary mixtures of small-grain cereals was similar to reports by González-Alcántara et al. (2020) for triticale silage and similar to the yield of rye with 3.5 and triticale with 4.3 ton DM/ha reported by Vega-García et al. (2023).
The pre-experimental results of chemical composition of the multi-species small-grain cereal crops were very similar to each other, where of OM, CP, IVDMD and ME contents were not statistically different among them (P > 0.05).
The pre-experimental results of chemical composition of the multi-species small-grain cereal crops treated with urea are very similar to each other, according to the variables of OM, CP, IVDMD and ME, where no statistically significant differences were observed (P > 0.05). Although it is worth mentioning that, in the last two variables, it could be considered that there was a numerically significant difference between treatments; because the treatments containing BLY seem to have a higher IVDMD compared to the one without and it could be because this small-grain cereal has a layer with a high level of ß-glucans and starch granules, which favours its digestibility (Newton et al., 2011) so it is commonly used in dairy cattle diets because it optimises rumen microbial performances (Baron et al., 2015); while the difference in ME between treatments is 1 to 2 MJ/kg which could translate into a difference of up to 20g/kg protein and 15 g/kg fat depending on weight and breed, according to Moran (2005), but they meet the energy needs of the cows to produce the amount of fat and protein reported in Table 4, because the selected cows were of the same breed and similar yields.
The treated forages met the nutrient requirements of the cows to produce the moderate milk yields, as well as the fat and protein reported in Table 4.
On the other hand, the variables that did show statistically significant differences (P < 0.05); DM, NDF and ADF; can be explained by the characteristics of each small-grain cereal. The DM reported in treatments containing RYE were higher probably because of the good performance of rye previously reported by other authors (Celis-Álvarez et al., 2017; Coblentz et al., 2018); and the fibre results, dependent on the phenological stage of the small-grain cereal, were also higher in the treatments containing RYE due to its morphology as it has a higher stem proportion that is reflected in a greater content of cellulose, hemicellulose and lignin (Baron et al., 2015). Given the full maturity of whole-crop forages herein reported, fibre contents are higher than those reported by Gómez-Miranda et al. (2020; 2022) and Vega-García et al. (2021) who evaluated small-grain cereal forages at an earlier phenological stage.
The results of the mature whole-crop cereal mixtures treated with urea showed there were no statistically significant differences (P > 0.05) for DM, OM, IVDMD and ME contents; where urea treated forages had higher digestibility and ME content than the pre-experimental untreated crops.
As for the rest of the variables, statistically significant differences (P < 0.05) were found among treatments for CP and ADF; and between periods for NDF. The treatment with the highest CP was that of TRT + BLY, which improved considerably compared to pre-experimental values; and the lowest increase in CP was that of TRT + RYE.
Results confirmed reports from several authors in different regions (Castejon and Leaver, 1994; Chenost, 1996; Deschard et al., 1987; García-Martínez et al. 2020; Kiangi et al., 1981) that showed how the urea treatment of low quality forages improved their nutritional value due to the effect of ammonia released from the urea on the cellulose and hemicellulose even at a late stage of maturation.
DM, OM and NDF variables are by far, different from the results reported for BLY silage by Gomez-Miranda et al. (2020) with 226.0 g/kg DM, 788 g/kg DM and 568 g/kg DM, respectively, although for CP (66 g/kg DM) and ADF (420 g/kg DM) the results were similar.
Results shown in Table 2 compare and are similar to the OM of RYE (934 g/kg DM) and TRT (926 g/kg DM) reported by Vega-García et al. (2021); the NDF (698 g/kg DM) content of maize straw treated with urea reported by Oji et al. (2007); the IVDMD (571 g/kg DM) of maize straw treated with urea reported by García-Martínez et al. (2020); and an ME content of 7.3 MJ EM/kg DM of wheat straw treated with urea reported by Kashongwe et al. (2014).
The results of DM, NDF and ADF of maize straw treated with urea from García-Martínez et al. (2009; 2020) (927 and 900 g/kg DM; 710 and 721 g/kg DM; 523 and 486 g/kg DM) and of maize stalks also treated with urea reported by Oji et al. (2007) (850 g/kg DM; 698 g/kg DM; 505 g/kg DM) are similar to those found in this study.
The OM (926, 934 and 929 g/kg DM) and CP (150, 136 and 135 g/kg DM) contents of the triticale and rye crops reported by Vega-García et al. (2021) and the 59 cm barley crop reported by Gómez-Miranda et al. (2022), respectively, were also similar to the small-grain whole-crop cereal mixtures treated with urea reported in Table 4.
On the other hand, the IVDMD and ME contents reported by García-Martínez et al. (2020) for maize straw treated with urea were similar results to the forages treated with urea of this work (571 g/kg DM and 9.2 MJ/kg), despite the fact that García-Martínez et al. (2020) worked with maize straw and not small-grain cereals.
Complementary pastures
Results from the KY grazed pasture were similar only in the OM and CP contents with 888 and 182 g/kg DM, respectively to those reported by Plata-Reyes et al. (2021); while Marín-Santana et al. (2020) showed differences in all the variables of chemical composition of the KY pasture in that experiment.
The cut pasture (PS) only presented similarities in OM and ADF content with 872 and 310 g/kg DM, respectively, compared to what reported by Gómez-Miranda et al. (2020) in the third period of their experiment in the rainy season. Resutls for the commercial concentrate were in accordance with the label statement.
Therefore, mature whole-crop cereal mixtures treated with urea did improve forage quality despite its advanced phenological stage, especially those containing triticale.
Supplements
The KY pasture and the cut pasture (PS) did not show differences between periods (P > 0.05), and the chemical composition of the concentrate was as stated in the label, so the feed supplements kept the same chemical during the experiment without representing an effect in the dairy cows’ diet; but the estimated ME content of the supplements was much higher than that of the treated with urea.
Animal production
Milk yields during the experiment were higher than pre-experimental yields and did not show differences among experimental periods (P > 0.05), which without differences in liveweight and body condition score (P > 0.05), indicated that the urea treated forages and supplements provided an improved and stable feeding in these systems during the dry season.
The mean of 20% refused treatment forages may have been due to the supplements that complemented the diet of experimental cows (concentrate, grazing and cut pasture). Urea treated forages may be rejected by cattle as it may be considered pungent due to ammonia accumulation and not very palatable especially at the beginning when the forage is just uncovered from the plastic covering (Deschard et al., 1987).
However, refusals of treated forages significantly decreased during the experiment, from 2.8 kg DM/cow/day in Period I, to 1.1 kg DM/cow/day in Period III, indicating adaptation of the cows to the treated forages, as well as a reduced reliance on the supplementary grazed and cut pasture. In any case, urea treated whole-crop small-grain cereal mixtures represented almost 50% of the estimated DM intake.
Regarding milk composition, mean fat yield (44 g/kg) was considerably higher than what is established in Mexican standards for raw milk (≥ 32 g/L), with a statistical trend (P = 0.06) for higher milk fat content in TRT + RYE.
Milk protein content was slightly below minimal content established in Mexican standards for raw milk. The content of milk components at a given time are affected by season, days in milk, pregnancy and genetic factors, so it is a complex process where there may be trade-offs between components (Moro et al., 2016). Jenkins and McGuire (2006) mentioned that the depression of proteins in milk may be an indirect effect of lower energy intakes in diets with a high forage proportion.
Results from Table 4 are similar to those reported by Vega-García et al. (2021) froms cows grazing small-grain cereals in the rainy season in milk yields with a mean of 13. 4 kg/cow/day, milk protein with 28.2 g/kg TRT), MUN (16.1 mg/dlL) and LW (469 kg /cow).
Regarding milk fat results, Sinclair et al. (2007) reported 39.9 g/kg milk for dairy cows fed urea-treated whole-crop wheat, similar to the milk fat content for experimental cows fed fully mature forage from binary mixtures of cereals treated with urea in the experiment herein reported.
Jonker et al., (2002) mentioned that MUN content indicates the nutritional status of dairy cows in terms of protein and energy balance, useful to evaluate diets and identify if it is necessary to modify the nutrients in the diet, as they are reflected in urinary N excretion which is directly related to milk excretion (Depeters and Ferguson et al., 1992), and in turn allows to avoid unnecessary emissions of N to the environment, thus reducing costs feeding costs and the environmental footprint.
Milk urea contents was within normal values and was not different among treatments (P > 0.05), but significantly decreased (P < 0.05) as the experiment progressed, from 17.8 mg/dL in Period I to 13.6 mg/dL in Period III (Table 4).
The observed MUN values were higher than levels reported by Vega-García et al. (2021) in cows grazing rye and triticale pastures, which report up to 16.1 mg/dL, respectively, in contrast to Gómez-Miranda et al. (2022), who reported values no higher than 12 mg/dL for cows grazing barley.
In conclusion, the treatment with urea of whole-crop mature small-grain cereal mixtures resulted in a viable complementary moderate quality salvage forage for feeding moderate yielding dairy cows in small-scale dairy systems during the dry season.