Before fore thinking about conclusions - you will need to check the somatic cell counts across all treatments and the variation between animals for milk lactose concentration.  

I think If you increase the forage it will stimulate bacteria which produce more propionic acid. which is the precursor of lactose. 

Levels of nutritional components of milk other than fat and protein (i.e. , lactose, vitamins, and minerals) are rather constant and not subject to large changes through genetic or nutritional manipulation. You should check other interference factors, as suggested by Julian Hill.

The effect of diets on milk production and composition, and on lactation curves in pastured dairy goats.

Min BR1, Hart SP, Sahlu T, Satter LD.

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Abstract

A 2-yr study investigated effects of different levels of concentrate supplementation on milk production, composition, and lactation curves in pastured dairy goats. For both years, 44 Alpine goats (Capra hircus; 55 +/- 11 kg body weight) were randomly allocated to 4 groups. Animals were supplemented with 0.66 (treatments A and B), 0.33 (treatment C), or 0 kg of concentrate (treatment D) per kg of milk over 1.5 kg/d. Mixed vegetative forages were rotationally grazed by the goats (treatments B, C, and D), except that treatment A was confined and fed alfalfa hay. Individual milk production was recorded daily, and milk samples were collected once every 2 wk for the 7-mo period (March to September) and analyzed for fat, protein, lactose, urea-N, nonesterified fatty acids, and allantoin (second year only). Milk yield and composition varied among dietary treatments, with some measures affected by year. Average daily milk yield was lowest for treatment D. The increased level of concentrate supplementation in treatment A led to 22% greater milk yield compared with treatment D. Milk production increased by 1.7 and 0.9 kg for each additional kilogram of concentrate fed per day during the first and second years, respectively. Average peak yield, time of peak yield, and persistency were lower for treatment D than for other treatments. The percentage of milk fat was lower for treatment D than for other treatments. Concentration of milk protein was greater for treatments A and B during the first year, and was higher for treatment C than for other treatments during the second year. Average milk lactose concentration was higher for treatments B and C than for other treatments. However, milk urea-N concentration in treatment A was higher than other treatments. Milk allantoin, used to estimate microbial proteins synthesis, was 20 to 25% greater for treatment A than for other treatments. Averaged across year, plasma urea-N and nonesterified fatty acids concentration were lowest for treatment B. Average organic matter intake was similar among treatments during both years. Ratios of acetate and propionate concentrations for treatment A were lowest among treatments. In conclusion, milk production and composition were affected by the feeding treatment and year. Increased level of nutrition lead to an increase in daily milk yield, peak yield, time of peak yield, and persistency compared with treatment D. Alpine dairy goats grazing on fresh forages without concentrate supplementation can produce milk inexpensively, and response to concentrate supplementation is greater for low quality pasture.

In general, is unlikely that milk lactose increase or decrease in response to forage/concentrate ratio, mostly because lactose is a component closely related to water compound of milk. So, even if forage/concentrate ratio allow bacteria to produce more propionic acid, it will affect the amout of milk, far more than milk lactose.

You may need to check other extraneous factors because it is quite definite that your findings are not due to increasing forage level in the total mixed ration. Perhaps, you should follow up on the suggestion of Julian Hill .

Dea colleague Ali, in general, the ration composition usually change the rumen fermentation and that was why we pay so great attention to the diet composition when investigate the level of animal productivity and milk composition. If all experimental ocasions are equal and uppers health is normal, it is very ligical to try to find reasons for your finding in the nutrition.

It's a bit surprising, because, usually, C3/C2 decreases when forage proportion increases. What about milk production and days in milk, which could explain a dilution effect? Did you also see a decrease in milk protein? Is DMI equal in both treatments?

A possible explanation might be that the initial diet was high concentrate and that increasing forage proportion allowed a better rumen work. (see ratio fat/protein)

The main lipid component of forages is galactolipid, which involves glycerol, galactose, and unsaturated fatty acids (primarily linoleic and linolenic acid). plz check level of galactolipids in forage.

I found low levels of lactose in some circumstances:

- Low level of energy in the cow nutrition, with a low DMI intake.

- High levels of somatic cells counts.

- High intake of NaCl (grazing near the sea after windy weather). May be linked with a low forage:concentrate ratio or with the osmotic pressure in the milk.

I think that always is a correlation between high levels of lactose and good nutrition and rumen working fine.

Increase in the forage ratio may lead to proliferation of the rumen microbes which will leads to more propionic acid and eventually increase the concentration of the milk lactose content .

But, we know that increase in the forrage/concentrate ratio leads to increase in acetate ratio.

I agree with Julian Hill. Lactose is the most constant macro component in milk, particularly compared to protein or fat content.

By contrast, lactose content can considerably change in presence of mastitis even at subclinical level. For this, it's very important that milk from different treatments has the same somatic cell counts.  

I think must be checked the amount of NaCl in the concentrate. I have noticed than the NaCl in excess drops the content of lactose in milk.

If we feed less concentrate, high in NaCl, then the lactose in milk will increase.

The reason is the need to maintain the osmotic equilibrium in milk. If salts in milk are high, lactose will drop. For the same reason, when the lactose drops in a mastitis disease, the salts in milk will increase, and as a consequence, the conductivity increases. Ancient farmers had the habit of tasting the milk of the cows with subclinical mastitis suspect.

Entre los productos finales de la fermentación en el rumen se encuentran los acidos grasos volatiles (AGV). La composición de los AGV está influencia por la proporción de la fubra dietética. Esta afecta proporcionalmente  el aumento del acido acético, mientras que los almidones contenidos en los cereales incrementa la proporci 162n de propiónico. Los AGV además de ser una fuente de energía para el rumiante, constituyen importantes productos iniciales en la síntesis de diferentes compuestos orgánicos en el metabolismo intermediario. Por ej. al ác. acético le corresponde un papel primordial en la síntesis de la grasa de la leche, siendo relativamente reducidas las fracciones destinadas a la formación de la caseína y lactosa. En cambio, el ác. propiónico es el responsable en primer término de la síntesis de la lactosa.

It depends on the type of forage used for example maize silage has high starch content which in turns gives high propionate acid in the rumen which causes reduction in ph and build up of lactic acid when not absorbed in the ruminal wall. similarly if you graze your animal on good grass you can also get high lactose content in the milk because grass contains high sugar concentration which favours growth of lactic acid producing bacteria over the other micro organism which in turns give rise to high lactose content in the milk.